FILE: <bc-34.htm> GENERAL INDEX [Navigate to MAIN MENU ]
BIOLOGICAL CONTROL IN FORESTS
----CLICK on desired underlined categories [ to
search for Subject Matter, depress
Ctrl/F ]:
|
|
|
Overview Some unique ecological attributes are present in
relatively complex forest environments including a diversity of species,
ages, intraspecific genetic composition, spacing and stocking levels
(Dahlsten & Mills 1999). Intensively managed forests, even-aged stands,
plantations of single and mixed species and seed orchards resemble agriculture,
but even these usually exist in a variety of different conditions. It is
important to look at some of these ecological attributes in detail as the
opportunities for biological control vary depending on the environment and
species involved. In addition to timber production, forests serve as
wildlife refuges, recreation, watershed and grazing areas. Where in
agriculture the goal of management is to harvest a commodity one or more
times a year, in forestry pest management is further complicated by multiple
goals and competing interests, including sportsmen, environmentalists, bird
watchers, hikers, cattlemen, woodcutters and the Army Corps of Engineers. Forests tend to be extremely large continuous areas with
gradual boundaries, thus quantitative evaluation of controls becomes very
difficult and expensive. Control strategy in forests is also affected by the
length of time to harvest, which may be 20 to 30 years or more in warm
temperate areas to 50 to 100 years in colder areas. Compared to agricultural
ecosystems, forests are much more complex ecologically. Forests vary from
single species plantations to multistoried stands and plant diversity is
greater than in an agricultural field even in the simplest forest stand.
Researchers must often deal with stands trees exceeding 70 m in height, a
mixture of age and size classes, a mixture of tree species, numerous canopy
levels including herbaceous plants and different stocking levels or spacing.. In view of the ecological attributes of forest ecosystems,
the choice and evaluation of biological control tactics may vary. The
influence on the classical approach to biological control has been analyzed
by Pschorn-Walcher (1977). The vast, diverse, relatively less disturbed,
long-lived and highly stable in space and time ecosystem confers both
advantages and disadvantages for biological control. Diversity confers an
advantage for foreign exploration as a large complex of natural enemies are
available from which to choose (Pschorn-Walcher 1977). However, this could
also make it more difficult for colonization of new species of natural
enemies. There would be expected to be a greater chance for the introduced
natural enemies to be in competition with related native natural enemies
since there is a high probability that relatives would be present in the rich
forest fauna. The vastness and diversity create sampling and evaluation
problems but less disturbance allows long term evaluations to be more exact. The collector of natural enemies has an advantage in the
relatively uniform forested regions because only minor regional differences
are usually exhibited (Pschorn-Walcher 1977). However, any widely distributed
pest or a pest introduced in a number of locations in a large forest region
would make any colonization program long in term. Pschorn-Walcher (1977)
maintains that the great differences between forest and agroecosystems
dictate a different approach to biological control in forestry from
agriculture. The approach to biological control in agriculture, where there
is much less predictability because of continuous disturbance, can be faster
using trial and error releases until the best natural enemy is found. With
forest insects preintroduction studies are desirable in order to understand
the interrelationships of the various natural enemies and finally to select
the most likely natural enemies for success. Natural enemy complexes of
forest insects can be chosen with a higher degree of predictability for
successful introductions and therefore preintroduction studies are justified
(Pschorn-Walcher 1977). Studying the parasitoid complex in detail provides
information on those species that might be good colonizers, those that would
operate at low or high population levels, those that were monophagous or
polyphagous, those attacking early or late life stages, those that could
adapt to some degree of inbreeding and could then withstand initial low
number colonization, or prolonged laboratory rearing, and those that were
cleptoparasitoids and then could be selected out . Strategies in
Forest Biological Control A variety of approaches in biological control including
importation, augmentation and conservation have been used. The major efforts
have been in North America (Canada and the United States) and the classical
approach of importation has been the most commonly used. Undoubtedly this is
because the highest proportion of introduced forest pests occur in North
America (Pschorn-Walcher 1977). The majority of insects are lepidopteran and
hymenopteran defoliators (sawflies). Since these insects are relatively large
hosts it may explain why 9 of the 15 tachinid flies established in biological
control attempts were used in forests. It seems that Lepidoptera and
Hymenoptera are more commonly pests in the less disturbed, contiguous forest
regions. Also forests are not as intensively managed as agricultural
ecosystems and it may explain why Homoptera, which are common subjects for
biological control in agriculture, are not as common as forest pests. Importation of
Natural Enemies For Introduced
Pests.--The most
common approach in forestry has been the importation of natural enemies
against introduced pests (Turnock et al. 1976, Pschorn-Walcher 1977). This
has usually involved colonizing and establishing a relatively small number of
natural enemies for control of an exotic pest through direct inoculative
releases of newly imported parasitoids. With a few exceptions, parasitoids
have been the preferred natural enemies introduced in forestry. Dahlsten
& Mills (1999) gave some estimates of the numbers of importations of
parasitoids and predators and their success of establishment and control. The
data show that 78% of importations involved parasitoids (Hymenoptera or
Tachinidae). Only homopteran pests have attracted substantial importations of
predators and while the overall rates of establishment of these two groups of
natural enemies are equal, the parasitoids have on average been more than
twice as successful in achieving some degree of control of the target forest
pests. About 40 species predators were introduced against the
balsam woolly aphid, Adelges piceae (Tatz) in an unsuccessful
colonization program (Clark et al. 1971), there being no known parasitoids of
this species. Attempts to introduce predators against bark beetles have been
made on several occasions. Hopkins tried to introduce the clerid, Thanasimus
formicarius (L.) from Germany to West Virginia for control of the
southern pine beetle, Dendroctonus frontalis Zimm. in 1892-93.
Although a complete failure, it was the first attempt to import a natural
enemy of a forest insect into the United States (Dowden 1962). Other
unsuccessful attempts have been made using Rhizophagus spp. from
Britain both in Quebec, Canada in 1933-34 with one species against the
Eastern spruce beetle, D. obesus (Menn.) and in New Zealand in
1933 with three species against the European bark beetle, Hylastes ater
(Payk) (Clausen 1978). Success was reported in the Soviet Union with Rhizophagus
grandis Gyll. against the European spruce beetle, D. micans
Kugelann (Kobakhidze 1965, Grégoire et al. 1987). Both predator and host are
native and this is a good example of augmentation through periodic
inoculation. There are also recent projects in Britain and France with R.
grandis against D. micans (Evans & King 1987,
Grégoire et al. 1987). Several species of carabid beetles have been imported
for control of the gypsy moth, Lymantria dispar (L.), with one
species in particular, Calosoma sycophanta (L.) becoming well
established (Clausen 1978). Red wood ants were imported in North America on a few
occasions (Finnegan & Smirnoff 1981). Formica lugubris
Zett. was imported from Italy in 1971 and 1973 for forests in Quebec
(Finnegan 1975), and Formica obscuripes Forel was moved from
Manitoba to Quebec in Canada in 1971 and 1972 (Finnegan 1977). The 15 species
in the Formica rufa L. complex in North America are not well
known but F. obscuripes appeared to have potential and did not
occur in the east (Finnegan 1977). The effectiveness of these introductions
against defoliators such as the Swaine jack pine sawfly and the spruce
budworm is unknown as yet, but the ant populations are still encouraged so
that eventually they will be well established in a wide area. These ants have
been observed feeding on spruce budworm and other forest insects (McNeil et
al. 1978). One species of vertebrate, the masked shrew, Sorex cinereus
Kerr, was colonized in Newfoundland for control of the larch sawfly, Pristiphora
erichsonii (Hartig). In a rather unique situation there were no
insectivores and few small fossorial animals on the island. These shrews were
transported from northern New Brunswick, Canada in 1958 and subsequently
released. Shrews also feed on other insects and it is believed that the
importation was successful even though there was some public opposition to
the operation (Turnock & Muldrew 1971). Classical biological control using pathogens has not been
common in forestry. However, two exceptions are the accidental introduction
of a nuclear polyhedrosis virus of the spruce sawfly, Gilpinia hercyniae
(Htg.) into eastern Canada (McGugan & Coppel 1962). One nematode, Deladenus
siricidicola Bedding was imported for control of the woodwasp, Sirex
noctilio F. in Australia (Bedding & Akhurst 1974). Dahlsten & Mills (1999) noted four cases where mass
rearing and release programs were performed in the biological control of
forest insects: (1) propagation of 882 million Dahlbominus fuscipennis
(Zett.) at Belleville, Canada for Gilpinia hercyniae control
(McGugan & Coppel 1962); (2) release of 200 million D. fuscipennis
by the Maine Forest Service in the United States against G. hercyniae
(Clausen 1978); (3) mass rearing and release of several parasitoids of the
gypsy moth, Lymantria dispar, in the eastern United States
(Leonard 1974); and (40 the use of a nematode, Deladenus siricidicola
against Sirex noctilio in Australia (Bedding & Akhurst
1974). Importation of
Natural Enemies For Native
Pests.--As mentioned earlier in other sections, exotic natural
enemies may be used effectively against native organisms, even though the
procedure is sometimes controversial. This approach was evaluated by Hokkanen
& Pimentel (1984) who concluded that it ought to be the preferred
approach in biological control. This stems from the idea that through genetic
feedback mechanisms host-parasitoid systems evolve toward homeostasis and
because of this coevolved equilibrium parasitoids would be limited in their
effectiveness as biological control agents (Pimentel 1961, 1963). Generalists
would probably be preferable to specialists in the selection of candidate
agents. This approach must be done with extreme caution because the Pimentel
genetic feedback concept is not wholly acceptable (Huffaker et al. 1971) as
it is believed that natural enemies may become better adapted through
time in controlling their hosts. To support this are examples of long
standing and effective introduced natural enemies such as Rodolia cardinalis
and Cryptochetum for control of cottony cushion scale and many others.
Nevertheless, the Hokkanen & Pimentel (1984) analysis
concluded that success in biological control was about 75% higher for the new
associations. These conclusions were disputed by Goeden & Kok (1986)
using biological control examples. They explain that the data used included
cacti, which are not representative of target weeds, and that there were
inaccuracies with some other examples. Dahlsten & Whitmore (1987)
analyzing the 286 examples of successful biological control used by Hokkanen
& Pimentel (1984) showed that there was a significant advantage for old
associations in terms of complete versus intermediate versus partial success.
The use of new associations as the preferred method for biological control is
also contradicted by the analyses of Hall & Ehler (1979) and Hall et al.
(1980), who found that the establishment rate of natural enemies was
significantly higher for introduced pests, the complete success of
importations against introduced pests was higher but not statistically
significant and the general rate of success for introduced pests higher than
for native pests. There appear to be some other misinterpretations in the
data of Hokkanen & Pimentel (1984) who used the reference by Clausen
(1978) for much of their information. These include the case of the elm leaf
beetle, Xanthogaleruca luteola (Müller) and some native Neodiprion
sawflies that were controlled by natural enemies in new associations (see
Clausen 1956, McGugan & Coppel 1962, DeBach 1964b, Bird 1971, McLeod
& Smirnoff 1971, Cunningham & DeGroot 1981, Finnegan & Smirnoff
1981, Laing & Hamai 1976, Clair et al. 1987). It is encouraging that there are examples of successful
introductions of natural enemies for control of both exotic and native pests.
Each approach has merit depending on the ecological circumstances. They state
that the sawfly examples of efforts against native species are good examples
of what can be done. Also the extremely successful project using a parasitoid
from a host in a different genus in North America against a native geometric
moth, Oxydia trychiata (Guenée), Colombia is a good case.
Biological control efforts against native species through the importation of
exotic natural enemies or by periodic inoculation of native natural enemies
has merit according to Carl (1982). Several ongoing (1996) examples of
careful evaluations for Canada are the Douglas-fir tussock moth, Orgyia
pseudotsugata (McDunn) (Mills & Schoenberg 1985), the spruce
budworm, Choristoneura fumiferana (Clemens) (Mills 1983a) and
bark beetles (Mills 1983b, Moeck & Safranyik 1984). Augmentation of
Natural Enemies.--As discussed in
an earlier section, the effects of natural enemies can be enhanced by various
manipulations of the organisms themselves or by alteration of their
environment, such approaches being extremely promising for native pests.
Although augmentation and conservation can be distinguished theoretically, it
is difficult to distinguish them in practice (Rabb et al. 1976). The two
tactics were defined by DeBach (1964c) as to manipulation of natural enemies
themselves (augmentation) or their habitat (conservation). Neither approach
has been used extensively in forestry, most literature being from agriculture
(DeBach & Hagen 1964, van den Bosch & Telford 1964, Rabb et al. 1976,
Stern et al. 1976, Ridgway et al. 1977). Augmentation is either by periodic
colonization or inoculation, development of adapted strains by artificial
selection or inundation (DeBach & Hagen 1964). The tactic may involve
either entomopathogens, parasitoids or predators. Attempts have been made with inoculation of several
parasitoids against forest pests in Europe and South America (Turnock et al.
1976). Inoculations were made of Rhizophagus against D. micans
in Russia, France and Britain and of the nematode Deladenus against S.
notilio in Australia. Red wood ants (Formica spp.) have been
moved and relocated in Europe where they are considered to be effective
predators on a number of forest pests. Otto (1967) reviewed a number of the
programs and concluded that good results were obtained primarily in pin
forests against dipterous and lepidopterous larvae. Ants are less effective
against sawflies and ineffective against beetles. Effective protection of
coniferous forests using ants has been achieved against five lepidopterans and
three sawfly pests in Germany, Switzerland, Italy, Russia, Poland and
Czechoslovakia (Otto 1967, Turnock et al. 1976). Three to eight species in
the Formica rufa complex in Europe are considered to be good
biological control agents, with identified species being F. polyctena
Forst, F. lugubris, and F. aquilonia Yarrow. Some examples of parasitoid inundation include Trichogramma
minutum Riley against the brown-tail moth, Euproctis chrysorrhea
L. in North America (DeBach 1964c, Howard & Fiske 1911); Trichogramma
spp. for control of various forest defoliators in Germany and Russia, and Telenomus
verticillatus Kieff. against the lasiocampid, Dendrolimus pini
(L.) in the Soviet Union (DeBach 1964c). In China inundative releases of Trichogramma
spp. are made routinely against various forest defoliators, which is
facilitated by a large and economic labor force (McFadden et al. 1981). Diprion pini
(L.) has been successfully controlled in Spain by the collection of sawfly
cocoons which were then either placed directly in special emergence cages or
exposed to Dahlbominus fuscipennis in the laboratory before
return to the field. Parasitoid emergence from these cages contributed about
3 million additional D. fuscipennis and ichneumonids, Exenterus
oriolus Htg to the forests, producing about 65% parasitism (DeBach
1964c, Ceballos & Zarko 1952). Various pheromones and kairomones have been identified for
hosts and natural enemies that are considered for implementation in natural
enemy release programs to enhance their performance (Haynes & Birch 1985,
Borden 1982, 1985, Vinson 1984). Mills (1983b) suggested the use of Dendroctonus
aggregation pheromones as a way of selecting useful European bark beetle egg
predators for introduction into Canada. Miller et al. (1987) have shown Thanasimus
undulatus Say to exhibit cross-attraction in field tests to other bark
beetle pheromones and Rhizophagus grandis to be attracted to
the frass of three North American Dendroctonus species in the
laboratory. Moeck & Safranik (1984) concluded that inundative releases of
native clerid beetles against low levels of D. ponderosae
offered a good potential. Bird encouragement programs have been used extensively in
Europe by providing nesting boxes in forests for cavity nesting spots (Bruns
1960). In California, Dahlsten & Copper (1979) demonstrated that
populations of the mountain chickadee can be increased two to three
fold with nesting boxes. It is speculated that bires operate in an inverse
density-dependent manner and their importance would be in preventing
outbreaks of forest pests rather than in suppressing them. Since 1980 entomopathogens have begun to play a dominant
role in forest biological control. The principal entomopathogens used are the
bacterium Bacillus thuringiensis Berl., and baculoviruses.
These agents have been tested against a wide variety of forest defoliators in
the form of inundative treatments and have the advantage of reduced impact on
other groups of natural enemies and non target organisms. Morris et al. (1986)
point out that microbial insecticides are likely to receive as wide an
application in forestry as in agriculture for several reasons. Forest
protection is of much greater concern to the general public due to the more
extensive areas covered by forest pests. Forest pest problems also tend to
involve only single target species rather than a complex of pests, which
requires the development of only a single microbial product. The forest crop
is also better able to withstand the slower action of microbial treatments in
comparison with agricultural crops. The spruce budworm, Choristoneura fumiferana
in North America and the gypsy moth both in Europe and North America have
been the main targets of extensive development of Bacillus thuringiensis
as a means of inundative biological control. More consistent success has been
attained against the spruce budworm and guidelines have been formulated
(Morris et al. 1984). Baculoviruses, which include the nuclear polyhedrosis
viruses (NPV) and granulosis viruses (GV), have been widely tested in field
trials against forest insects (Cunningham 1982). They show a marked degree of
specificity for their phytophagous hosts and have no impact on non target
organisms. Natural epizootics of NPV are often responsible for the
termination of outbreaks of major forest insect pests, particularly among the
Diprionidae and Lymantriidae. The diprionid sawflies provide some of the most
striking examples of the use of NPVs in biological control (Cunningham &
Entwistle 1981). The virulence of the diprionid NPVs is appreciably greater
than that of other host groups (Entwistle 1983) and the gregarious habit of
the diprionid larvae promotes the spread of virus through the larval
population. Virus production cannot be achieved on artificial media
and for sawflies, in contrast to Lepidoptera which can be reared on
artificial diets, foliage fed host larvae are required for mass production of
the virus. Host larvae must either be collected from the field for infection
in the laboratory (Rollinson et al. 1970) or a heavily infested plantation
may be sprayed with virus and the infected larvae harvested as they die
(Cunningham & DeGroot 1981). The periodic inundation of the virus can be
carried out either by distribution of host cocoons containing infected
eonymphs in forest stands or by more conventional aerial or ground spraying
machinery. The former methods has potential for Neodiprion swainei
(Smirnoff 1962) which has an NPV that spreads rapidly from epicenters, while
the latter has been widely used for N. lecontei and N. sertifer
(Geoff.) (Cunningham & Entwistle 1981). The NPV of N. sertifer
has been successfully used in 12 countries and is the most operationally used
of the sawfly NPVs. One factor that contributes to this success is the more synchronous
hatching of the larvae of N. sertifer, as a result of
overwintering as eggs rather than as eonymphs, which facilitates the timing
of spraying to infect the younger more susceptible larval instars. Conservation of
Natural Enemies.--Conservation of natural enemies should be considered a
part of all silvicultural systems and treatments. In addition there are
measures that can be taken directly to conserve natural enemies. However, studies of existent predators are
few (see Legner & Moore 1977 ). There has been much more done in agriculture to conserve
natural enemies (van den Bosch & Telford 1964), including strip
harvesting and habitat diversification (Stern et al. 1976). Pesticide
disturbances should be avoided as much as possible, which includes the forest
floor where Syme (1977) has shown that a parasitoid of the European pine
shoot moth, Rhyacionia buoliana (Schiff.) requires the flowers
of small herbaceous plants for nourishment. The judicious use of chemical
insecticides is important for conserving natural enemies. There are
undoubtedly many naturally occurring biological controls in forests where
often the importance of a natural enemy is not known until their effect on
the host is disrupted (Hagen et al. 1971). Secondary outbreaks have been
known in forestry, but an extensive outbreak of the spruce spider mite, Oligonychus
ununguis (Jacobi) following the application of DDT for western budworm
control in Montana and Idaho has been documented (Johnson 1958). Outbreaks of the pine needle scale, Chionaspis pinifoliae
(Fitch) occurred in California on Jeffrey and lodgepole pines near Lake Tahoe
when an area was fogged with Malathion to control adult mosquitoes (Luck
& Dahlsten 1975). The importance of natural enemies was shown in this
study as the collapse of the scale population after spraying was halted,
occurred over a three-year period and was shown to be due to a small complex
of predators and parasitoids. Other insecticide induced outbreaks have been reported
for the target insects. The elimination of parasitoids and virus diseases of
the European spruce sawfly, Gilpinia hercyniae, after three
years of spraying with DDT in New Brunswick, Canada, resulted in an outbreak
of the sawfly (Neilson et al. 1971). In Texas an increase in an infestation
of southern pine beetle, Dendroctonus frontalis, was attributed
to the deleterious effects of chemical insecticides on the natural enemies
(Williamson & Vité 1971). Swezey & Dahlsten (1983) have documented
the effects of lindane on the emergence of natural enemies of the western
pine beetle, D. brevicornis (LeConte). The physical environment in forests may be changed to
favor natural enemies. Parasitoids and predators can be benefitted by
encouraging specific plants for food, shelter and protection from their
natural enemies (Bucklner 1971, Sailer 1971). The effectiveness of natural
enemies in Poland in 1958 was increased by applying fertilizers, planting
deciduous trees and shrubs and providing nectar plants (Burzynski 1970,
Koehler 1970). The presence of wild carrot, Daucus carota L.,
in pine plantations in Canada increases control of the European pine shoot
moth, Rhyacionia buoliana (Syme 1981). Longevity and fecundity
of the most effective introduced parasitoid, Orgilus obscurator
(Nees), was increased due to its feeding on the nectar of several flowers
(Syme 1977). In efforts to conserve natural enemies of bark beetles,
Bedard (1933) recommended examination of infested trees for high degrees of
parasitism prior to control in order to conserve parasitoids. The disruption
of old infestations of mountain pine beetle in lodgepole pine should be
avoided since the braconid Coeloides rufovariegatus (Prov.) is
very abundant in old infestations (DeLeon 1935). Wind thrown western white
pines should not be disturbed because of the high populations of mountain
pine beetle parasitoids (Bedard 1933). Because the parasitoid Coeloides
vancouverensis (D.T) is more abundant in small diameter Douglas fir
infested with the beetle D. pseudotsugae, such trees ought to
be left in place (Ryan & Rudinsky 1962). Clerid predators of the western
pine beetle eventually move to the lower portions of the bole of infested
trees and thus the lower sections of trees should not be treated with
insecticide during control projects (Berryman 1967). Clerids associated with
the southern pine beetle emerged later than the bark beetles and it was urged
that infested trees not be removed until after clerid emergence (Moore 1972). Biological
Control Organizations in Forestry There are various world organizations devoted to
biological control of forest pests. They indicate that activity has been most
prevalent in temperate and Mediterranean regions, but that there are no
organizations devoted solely to the biological control of forest insects.
References pertaining to organization are Clausen (1956), Beirne (1973),
Greathead (1980), Taylor (1981), Embree & Pendrel (1986), Detailed
Examples of Biological Control of Forest Pests Dahlsten & Mills (1999) provide detailed case
histories of biological control projects in forest environments; the
following being for the most part from their account: LARCH CASEBEARER, Coleophora laricella
Hübner--Coleophoridae The larch casebearer is native to central Europe and is
relatively innocuous in the alpine area on its normal host, Larix decidua
Mill. (Jagsch 1973). A fairly rich complex of parasitoids is thought to
maintain the casebearer at lower densities in its endemic region (Ryan et al.
1987). It is a defoliator of Larix species and becomes a pest in
Europe and Asia wherever larch is planted. This insect was probably
introduced on nursery stock into North America from Europe and was first
found at Northampton, Massachusetts in 1896 and in Canada at Ottawa in 1905
(Otvos & Quednau 1981). They spread rapidly on tamarack, Larix laricina
(Du Roi) K. Koch, in eastern Canada so that by 1947 it was in Newfoundland,
the Maritimes, and Ontario and in the United States, Maine, Michigan and
Wisconsin (McGugan & Coppel 1962). It is currently widely distributed in
the eastern United States and Canada. In 1957 the casebearer was discovered
on western larch, Larix occidentalis Nutt, in Idaho (Denton
1958) and in 1966 in British Columbia (Moinar et al. 1967). It is now widely
distributed over the range of western larch including British Columbia,
Montana, Idaho, Washington and Oregon (Clausen 1978). The casebearer has one generation per year. The adults
begin appearing in late May and lay eggs on either side of the needles. The
larvae hatch and burrow directly down into the needles. In the late summer
the larvae emerge from the mined needles and form overwintering cases. They
feed for a while and then move to branches and twigs to pass the winter. In
the early spring the larvae with their cases move and begin feeding on the
young buds and foliage. Pupation occurs within the enlarge case, which is
commonly attached to a branch on a leaf whorl. The larval feeding, when
extensive, causes a loss of growth that is its greatest impact on larch (Ryan
et al. 1987). The biological control program had its beginning in 1928
in western Canada with a request to the Farnham House Laboratory of CIBC for
information on the parasitoid complex of the casebearer in Europe (McGugan
& Coppel 1962). Importation and field releases of 5 species of
parasitoids occurred in eastern Canada between 1931 and 1939 as follows:
1,037 Agathis pumila (Ratz.)--Braconidae, 29,664 Chrysocharis
laricinellae (Ratz.)--Eulophidae, 506 Cirrospilus pictus
(Nees)--Eulophidae, 3,283 Dicladocerus westwoodii
Steph.--Eulophidae, and 97 Diadegma laricinellum
(Strobl)--Ichneumonidae (Clausen 1978). All species were subsequently
recovered at release sites in Ontario but only two became well established
and spread rapidly, A. pumila and C. laricinellae.
Between 1942 and 1947 large scale redistribution releases were made at a
number of sites in eastern Canada. The parasitoids were obtained at
established colony sites at Millbridge, Ontario (Clausen 1978). By 1948
populations of the casebearer were low on the original release sites. The
parasitoids followed the spread of the casebearer to the west assisted by
occasional releases (Ryan et al. 1987). This can be cited as an example of a
successful biological control program (Webb & Quednau 1971). A separate, extensive parasitoid importation program was
also conducted between 1932 and 1936 in the eastern United States in New
England and New York (Clausen 1978). Four of the same parasitoids as released
in Canada were used in the U.S. (Clausen 1978) as follows: 8,141 A. pumila,
24,671 C. laricinellae, 231 D. westwoodii, and
3,580 D. laricinellum (Strobl). Although there is little
information to go on, the results were apparently the same in the eastern United
States with the establishment of A. pumila and C. laricinellae
followed by high parasitization rates particularly by A. pumila
(Dowden 1962). Releases of the two established parasitoids were also made in
1937, 1950 and 1952 in Michigan and Wisconsin. In the western United States, the first releases of A.
pumila were made in 1960 with 2,360 adult parasitoids that were
collected in Rhode Island (Clausen 1978). These were released at 5 locations
in Idaho. Recoveries were made at 3 sites in 1962. Between 1964 and 1969
field rearing of A. pumila in whole tree cloth cages permitted
the release of this parasitoid at 400 sites in Idaho, Montana, Washington and
British Columbia (Ryan et al. 1987). The parasitoid became established and
built up at some sites but at other sites it either didn't become established
or it didn't build up. In addition, significant defoliation still occurred
throughout much of the area by 1970 and the program was rated as a failure
(Turnock et al. 1976, Ryan et al. 1987). Between 1971 and 1983 a new strategy was used as C.
laricinellae and five other species of parasitoids from Europe and
Japan were released over a period of several years. C. laricinellae
became widely established but the other species don't appear to be very
important for control of the casebearer though isolated recoveries have been
made (Ryan et al. 1987). In an effort to properly evaluate the effect of the
parasitoids, the larch casebearer was sampled at sites in Oregon where the casebearer
had recently invaded. The populations were followed to the point of severe
defoliation from 1972 to 1978 and then parasitoids were released between 1979
and 1985 (Ryan 1983, 1986; Ryan et al. 1987). The first parasitoid to be
released was C. laricinellae followed by A. pumila.
Parasitoids increased and the casebearer steadily declined and this trend has
continued in all plots through 1987 (R. B. Ryan, personal communication).
Although the prospects are good for a complete success, Ryan et al. (1987) feel
it is too soon to make the claim. In British Columbia the larch casebearer biological
control program was reviewed in 1974 due to the successes in eastern Canada
(Otvos & Quednau 1981). Four parasitoids have been released: A. pumila,
C. larcinellae, Diadegma laricinellum, and Dicladocerus
japonicus Yshm. The story is much the same as with the other release
programs--A. pumila and C. laricinellae have
become well established and the other two have not been recovered. It is too
early to evaluate the effects of the two parasitoids but C. laricinellae
is fairly common in British Columbia and may be responsible for the reduction
of larch casebearer and less tree mortality (Otvos & Quednau 1981). The larch casebearer is a successful biological control
program in eastern Canada and may shortly be successful in the northwestern
United States. It is an example of a classic introduction program with the
subsequent redistribution of the parasitoids from areas of establishment to
new areas. It is interesting because the two parasitoids complement one
another in their action against the casebearer. Agathis is
extrinsically superior at low host densities and Chrysocharis is
effective at high host densities. Quednau (1970) hypothesized that Agathis
can only give partial control on its own and that success is only possible
through cooperative interaction with Chrysocharis. Ryan (1985)
hypothesizes that Agathis may not be detected in successive samples
since parasitized larvae commonly descend to understory vegetation. Samples
could be biased toward Chrysocharis due to the absence of Agathis
in the foliate that is sampled. There has been no success in establishing
other parasitoid species. This program also is an example of one where there
was a rigorous attempt to evaluate efficacy of the parasitoids (Ryan 1986,
Ryan et al. 1987). WINTER MOTH, Operophtera
brumata (L.)--Geometridae This polyphagous defoliator of hardwoods is native to most
of Europe and parts of Asia, where it is particularly frequent on fruit trees
and oak. It was first recognized as an accidental introduction on the south
shore of Nova Scotia in 1949 and eventually extended its range to the whole
of this region together with small isolated parts of New Brunswick and Price
Edward Island by 1958. In the first few years after its appearance in Nova
Scotia, damage was evident in apple orchards, shade trees and oak forests.
However, at this time hardwoods were not commercially exploited in the
Province and so the winter moth was not considered a serious pest (Embree
1971). Consequently it was possible to initiate a biological control program
rather than a program of insecticide eradication. The general research policy
in the early 1950's was directed towards population dynamics of forest insect
populations and thus the biological control program was initiated in 1954
with a view to population studies of the host and introduced parasitoids. Prior to the introduction of parasitoids from Europe, the
winter moth fluctuated erratically at high population densities. These
fluctuations resulted from the coincidence of hatching of the overwintering
eggs and bud burst in early spring (Embree 1965). This same key mortality
factor was also found to be responsible for changes in population levels of
winter moth in Britain (Varley & Gradwell 1968). Three tachinid and three ichneumonid parasitoids were
obtained in sufficient quantity for introduction into Nova Scotia from
Europe. The parasitoids were collected and shipped to Canada by staff of the
Belleville Laboratory and the CIBC and field releases were made during the
period 1954-62. These included releases of over 22,000 individuals of the
tachinid Cyzenis albicans (Falk.) and a total of 2,261
individuals of the ichneumonid, Agrypon flaveolatum (Grav.),
the only two species that became established. C. albicans is
very fecund and oviposits microtype eggs around the edge of damaged foliage
where they are ingested by late-instar host larvae. The egg hatch in the
midgut of the host and the larvae bore through the gut wall to develop
rapidly after the host has pupated. The tachinid pupates and overwinters
within the host pupal case in the ground. The biology of A. flaveolatum
is similar but its oviposits directly into the host larvae and has larger
eggs and much lower fecundity. Following the establishment of these two parasitoids,
parasitism by C. albicans increased rapidly to 50% in 1960 and
life table data showed that a considerable increase in prepupal mortality was
responsible for the collapse of the winter moth population in the main study
site (Embree 1965). Parasitism by A. flaveolatum increased only
following the initial decline of the host outbreak and while it may have
enhanced the depression of the winter moth density, population models
indicate that the efficiency of C. albicans alone is sufficient
to account for successful biological control (Hassell 1980). However, a more
recent analysis of the life table data from Nova Scotia and Britain (Roland,
pers. comm.) indicates that the increased pupal mortality may have arisen
only indirectly from the introduction of C. albicans. Increased
parasitism by C. albicans is closely followed by an increase in
the activity of soil predators, perhaps sustained on overwintering C. albicans
puparia through late summer and early spring when prey are generally more
scarce. Thus predation rather than parasitism may be more directly
responsible for the observed increase in winter moth pupal mortality. Recent
unpublished work in British Columbia indicates that staphylinid predators are
especially important in regulation and that C. albicans puparia
are avoided because they are too large for the predators. More recently, between 1976 and 1978, winter moth has been
noted in Oregon, Washington and British Columbia on various hardwood and
fruit trees. Both C. albicans and A. flaveolatum
were relocated to these areas between 1979 and 1982 and recoveries were made
in many regions (Kimberling et al. 1986). However, it is too early to
determine the success of these releases. But in contrast to the earlier
program in Nova Scotia, the western program has been conducted at a time when
research policy has moved away from population dynamics toward practical application
of pest control and thus no detailed monitoring of the winter moth before and
after parasitoid release has been made. This program is often considered a good example of
biological control in which, in contrast to earlier multiple introduction
programs, selective introduction were made. These led to the establishment of
a high host density specialist (Cyzenis), with high fecundity to bring
about the collapse of an outbreak, and a low host density specialist (Agrypon),
that has good searching ability to maintain the collapsed population at a low
level of abundance. However, the main reason for the release of a smaller
number of parasitoid species was the relatively meager size of collections in
Europe, where winter moth abundance was not high at the time. Thus the only
conscious selection process was of parasitoid species obtained in sufficient
quantity for meaningful release (Mesnil 1967), although once the two
established parasitoids were becoming effective in the early 1960's a
decision was made to curtail releases of other species (Embree 1966). The end
results was the successful establishment of two particularly suitable
parasitoids and the program provides one of the best examples of the detailed
evaluation of a biological control project. Also as was pointed out in
earlier sections, the development of a detailed model in England prior to the
importations tended to show very little regulatory impact by Cyzenis,
which might have precluded its importation into North America. COLOMBIAN DEFOLIATOR, Oxydia trychiata
(Guenée)--Geometridae A successful example of the use of an exotic parasitoid to
control a native forest pest was the importation of the egg parasitoid, Telenomus
alsophilae Viereck, from North America to Colombia in South America
against a geometrid defoliator (Bustillo & Drooz 1977, Drooz et al.
1977). There are a number of interesting facets to the program since the
normal geometrid host of the parasitoid in North America, the fall
cankerworm, Alsophila pometaria (Harris), is in a different subfamily
and genus than the target pest, Oxydia trychiata, in South
America. The Colombian geometrid, O. trychiata, has a wide
distribution extending from Costa Rica to most of the countries in South
America. The moth has 3 generations per year and apparently is capable of
normal development on introduced tree species (citrus, coffee, pine and
cypress). There has been an attempt to establish exotic conifer species in
Colombia for the production of pulp and paper. This previously unimportant
insect became a pest in these pine and cypress plantations (Drooz et al.
1977). The egg parasitoid, T. alsophilae
(Scelionidae) has several biological attributes that are well worth noting
since they may have influenced this unique cross genus introduction. First,
its normal host, the fall cankerworm, feeds on several broad leaved trees but
its host in South America feeds on conifers. This indicates that host plant
odors or other differences between conifers and broad leaved trees are
unimportant in host egg finding. There may have been a clue to this because
the fall cankerworm feeds on several genera of deciduous hardwoods. The
parasitoid is apparently easily to handle as changes in photoperiod and lack
of cold in the winter did not interfere with development (Drooz et al. 1977).
The climate at the origin of the parasitoid in Virginia (30°
N. Lat., el. 370 m, mean winter temperature 2°C
and mean summer temperature 24°C) compared to
that of the release site in Colombia (6° N. Lat., 2340
m, temperature range 6° - 26°C
with annual mean of 16°C) shows a shift
from a temperate to a tropical climate although the extremes are about the
same. The rainfall patterns in the two regions also differ. The ecological
plasticity of this parasitoid is thus demonstrated, and in addition it is
long-lived (>6 months) (Drooz et al. 1977). The parasitoid may be easily reared, which is important to
a biological control project (Drooz et al. 1977), and eggs of another species
of geometrid, Abbottana clemataria (J. E. Smith) are used
because it could be propagated on artificial diet. Around 18,000 parasitoids
were sent to and released in a pine plantation in Colombia between October
and December in 1975 (Bustillo & Drooz 1977, Drooz et al. 1977).
Parasitization rates on O. trychiata eggs were very high and by
the time the parasitoid had undergone three generations in April of 1976 few
adults could be found at normal emergence time. Only 13 egg masses of O.
trychiata could be found and these were 99% parasitized. By May the
outbreak was controlled when larvae could not be found in the area (Drooz et
al. 1977). It is speculated that the parasitoid maintains itself on any of
the four species of Oxydia or other geometrids in Colombia. EUROPEAN PINE SHOOT MOTH, Rhyacionia buoliana
(Schiff.)--Tortricidae This species occurs throughout Europe and parts of Asia
where it is a major pest of pine plantations. It was first discovered in
North America at New York in 1914 and was later also found on imported
nursery stock in Canada in 1925. While its distribution extended throughout
the northeastern United States and eastern Provinces of Canada, as well as in
British Columbia and the northwestern United States, it was considered an
important pest only in the red pine plantations in the northeastern United States
and southern Ontario. In 1927, the Commonwealth Institute of Biological Control
was engaged to collect parasitoids in Great Britain for introduction into
Canada and this led to the release of eight species during the period 1928-43
and an additional five species from material collected in continental Europe
during 1954-58 (McGugan & Coppel 1962). Two additional species were
released during 1968-74, one from Germany and one from Argentina (Syme 1981).
A similar program of parasitoid introductions was carried out in the New
England states from 1931-37 (Dowden 1962). This program is another example of
the multiple introduction approach where emphasis is placed on the need to
provide rapid results without detailed preintroduction studies. Of the 15
species of parasitoids released in New England and in southern Ontario, only
three larval parasitoids, the braconid Orgilus obscurator
(Nees), and the ichneumonids Eulimneria rufifemur (Thoms.) and Temelucha
interruptor (Grav.), became firmly established. However, it was not
until the early 1960's that T. interruptor was disclaimed as a cleptoparasitoid
detrimental to the potential impact of O. obscurator (Arthur et
al. 1964). Orgilus obscurator
is a specific larval parasitoid with a high fecundity and an efficient host
finding ability that permits it to avoid both superparasitism and very low
host density situations (Syme 1977). In contrast, T. interruptor
is a more general parasitoid of Microlepidoptera and while it also has a high
fecundity it is inefficient at host finding and oviposits most successfully
in host larvae previously attacked by O. obscurator. Both
parasitoids attack young host larvae and only develop further when the host
larvae approach maturity. However, the first instar larva of T. interruptor
is competitively superior to that of O. obscurator, which is
killed at an early stage to ensure the successful development of the
cleptoparasitoid (Schroeder 1974). Although the biological control program against pine shoot
moth in North America is considered to be unsuccessful, there are isolated
reports of high levels of parasitism by O. obscurator followed
by the collapse of shoot moth populations at Dorcas Bay in Ontario (Syme
1971) and near Quebec City (Béique 1960). The occurrence of wild carrot, Daucus
carota (L.) at Dorcas Bay where parasitism by O. obscurator
reached 92% prompted further investigations on the influence of this nectar
and pollen source on parasitism in Ontario. Syme (1977) demonstrated the
beneficial influence of flowers on the longevity and fecundity of O. obscurator
and was able to show increased rates of parasitism and elimination of pine
shoot moth populations when the parasitoid was released into plantations
where D. carota was plentiful (Syme 1981). GYPSY MOTH,
Lymantria dispar (L.)--Lymantriideae This insect is native to the Palearctic region where it is
a pest of broadleaf forests in eastern and southern Europe. It was brought to
North America and accidentally released in Massachusetts in 1868. Since then
it has become a serious pest of hardwoods throughout the northeastern United
States and has a continually expanding range which currently extends into
Ontario, Quebec and southward into Virginia with isolated infestations in
Minnesota, Oregon and occasionally California. A biological control project was organized by the U. S.
Department of Agriculture, Bureau of Entomology in 1905 and extensive foreign
exploration for parasitoids and predators was carried out in Europe, Japan,
North Africa and Asia at various intervals since that time (Doane &
McManus 1981). This was the first major classical biological control project
against a forest insect. The gypsy moth project has revealed that (1) insect
disease was recognized as an important biological control factor, (2) the
sequence theory of natural enemies was introduced by W. F. Fiske, (3) a
number of future important contributors to biological control were trained on
the project (H. S. Smith, W. R. Thompson and W. D. Tothill), (4) sleeve cages
were invented as well as other equipment and techniques that are still in use
today and (5) L. O. Howard and W. F. Fiske were the first to clearly
distinguish between those causes of mortality that act in relation to the
density of the population and those that do not. L. O. Howard also stimulated
the Canadian interest in biological control in the early 1900's by making
available facilities and scientific assistance from the Melrose Highlands
Parasite Laboratory of the U. S. Bureau of Entomology. Early importations of natural enemies occurred between
1905-14 and again between 1922-33. While some collections were made in Japan,
attention focused on Europe where temporary field laboratories were placed
wherever gypsy moth outbreaks were sufficient to permit the rearing of
parasitoids from a large number of hosts. Frequent shipments of parasitoids
and predators were made to the gypsy moth laboratory at Melrose Highlands,
Massachusetts and this resulted in the liberation of >690,000 living
insects of more than 45 species during this period (Dowden 1962). The
enormous importation and multiple release program enabled two larval/pupal
predators, two egg parasitoids, six larval parasitoids and one pupal
parasitoid to become established in the New England states. The two egg
parasitoids were also subject to either large scale rearing releases in the
case of Ooencyrtus kuwanae (How.), or to large scale relocation
releases in the case of Anastatus disparis Ruschka. Most of the
establishments occurred rapidly after the initial field releases but the
tachinids Parasetigena silvestris (R.-D.) and Exorista larvarum
(L.) were not recovered until 1937 and 1940 respectively and the chalcidid Brachymeria
intermedia (Nees) was only recovered in 1965. Biological control by established parasitoids and
predators in New England was limited and large scale aerial applications of
DDT were used until the early 1960's. Since 1960 renewed interest in the
search for additional natural enemies has extended explorations in Europe,
Japan, Morocco, India, Iran and Korea (Doane & McManus 1981). Since 1963
the USDA Agricultural Research Service Beneficial Insects Research Laboratory
has continued to receive gypsy moth natural enemies in their quarantine
facilities and have been able to distribute more than 200,000 individuals of
about 60 species to other State and Federal facilities for culture, study and
field release. From 1966 until 1971, the Gypsy Moth Methods Improvement
Laboratory at Otis Air Force Base in Massachusetts was charged with the
development of rearing procedures for the imported natural enemies. From
1963-71 in conjunction with the New Jersey Department of Agriculture about 7
million parasitoids of 17 species were reared and released in the forests of
New Jersey and Pennsylvania. Then from 1971-77 a Gypsy Moth Parasite Distribution
Program was established in which the New Jersey Dept. of Agriculture and the
University of Maryland reared and released an additional two million
parasitoids of 18 species throughout the New England states. Since the late
1970's more new parasitoids and a predator from Japan and Korea and from the
Indian gypsy moth, Lymantria obfuscata Walk., have been
imported (Coulson et al. 1986). More than 100,000 individuals of nine new
species or strains have been released in the field in Delaware, Massachusetts
and Pennsylvania. Although much knowledge of the biology and rearing methods
of the imported parasitoids was gained during this massive program of
importation, propagation and release, it has resulted in the addition of only
a single pupal parasitoid, Coccygomimus disparis (Vier.) to the
complex of 10 species established during the initial importation program.
This has prompted Tallamy (1983) to compare the establishment of gypsy moth
parasitoids with island biogeography theory, suggesting that a dynamic
equilibrium now exists between further introductions and the extinction of
established parasitoids. In the last 30 years two of the parasitoids that
were initially established, Anastatus disparis and Exorista
larvarum have become very rare, while two pupal parasitoids Brachymeria
intermedia and C. disparis have become established.
However, the main reasons for the failure to establish additional parasitoids
in recent years are the parasitoids' requirements for suitable alternative
overwintering hosts for their second generation each year and the fact that
several of the parasitoid species released during the 1960's were not closely
associated with gypsy moth as a principal host in their areas of origin. The failure of the established natural enemies to control
expanding outbreaks of the gypsy moth encouraged attempts during the 1970's
to augment the impact of previously established species. Through inundative
releases of Cotesia melanoscelus (Ratz.), Weseloh &
Anderson (1975) were able to show significantly increased rates of parasitism
but this had little influence on foliage protection or egg mass counts for
the following generation. On the other and several other inundative releases
of this and other species failed to provide any evidence of increased
parasitism in comparison to control plots (Doane & McManus 1981). The
combined release of parasitoids and pathogens has been used as a method of
augmentation. Wollam & Yendol (1976) were able to show a synergistic
effect of the release of C. melanoscelus in plots treated with
a double application of low concentration Bacillus thuringiensis
over plots treated with each of these natural enemies alone. The resultant
reduction in defoliation and subsequent egg mass densities has more recently
been attributed to the retarding effect of B. thuringiensis on
host larval growth which exposes the younger larvae to parasitism for a
longer period of time (Weseloh et al. 1983). A similar effect of C. melanoscelus
in conjunction with viral treatments is unlikely to occur since this
parasitoid avoids oviposition in moribund host larvae (Versoi & Yendol
1982). Augmentation through use of microbial pathogens has been
of considerable importance against gypsy moth with significant advances in
recent years. Early trials with B. thuringiensis in the 1960's
were not effective in providing foliage protection; but the discovery of
improved strains (Dubois 1985b) and successive improvements in formulation
and application technology during the late 1970's and early 1980's led to
greater success. The results of aerial applications during the 1970's
remained highly variable but a recommendation of double application of low
concentrations was developed and used operationally for the first time on a
large scale in 1980. This also met with limited success but further
experimental work in the early 1980's (Dubois 1985a) indicated that the use
of higher concentrations and acrylamide stickers could provide not only good
foliage protection but also could reduce subsequent egg mass densities
significantly with a single application. This development reduced the cost of
B. thuringiensis applications and has been used operationally
with success on 40-70% of the 1.3-1.5 million ha. of hardwood forest treated
since 1983. Many field trials have been conducted with virus sprays
against gypsy moth both in North America and Europe (Cunningham 1982). An NPV
virus strain (Hamden standard) isolated from a natural epizootic in
Connecticut in 1967 forms the basis for the commercially produced "Gypchek"
that was registered for use against gypsy moth in North America in 1978.
However, early trials of the baculovirus produced erratic results and while
continued improvements in formulation and application have produced more
positive results, it has never been accepted for operational use (Podgwaite
1985). Reasons for this are the relatively low virulence of the virus, its
rapid degradation on foliage in the field and the more recent successes with
the use of B. thuringiensis. The gypsy moth program has been spectacular in both the
scale and the continued enthusiasm with which it has been conducted, but that
the results have been disappointing and serve as a good example of the
failure of classical biological control in situations where the introduced
pest is also severe in its region of origin. Therefore the search for natural
enemies in areas where gypsy moth is not a pest, in non-outbreak populations
or from related non-pest Lymantria species may prove to be a better
strategy. HYMENOPTERA: SYMPHYTA EUROPEAN SPRUCE SAWFLY, Gilpinia hercyniae
(Hartig)--Diprionidae A spruce (Picea spp.) feeding insect native to most
of Europe, the European spruce sawfly was first noted as an accidental
introduction in Canada in 1922. By 1930 a severe outbreak was causing concern
in the Gaspe Peninsula and by 1936 the sawfly threatened to devastate the
spruce forests of eastern Canada by extending its range across all eastern
Provinces and adjacent United States and causing severe damage over an area
of more than 10,000 sq. miles (McGugan & Coppel 1962). One of the most extensive projects undertaken in classical
biological control was begun against European spruce sawfly in 1933. Gilpinia
hercyniae was not at first distinguished from G. polytomum
(Htg.) and the Farnham House Laboratory in England (now known as CIBC) was
engaged to make large scale parasitoid collections from the latter species in
Europe. Initial studies revealed that apart from the egg parasitoids, all
other parasitoids develop so as to overwinter in the host cocoon. This
simplified parasitoid collections in Europe to those stages of development. A
team of about 30 persons collected >1/2 million cocoons of G. polytomum
in Europe for shipment to Canada during 1932-40. Additionally more >1/2
million eggs and 31 million cocoons of other spruce and pine feeding sawflies
were shipped to supplement the numbers of the less host specific parasitoid
species available for field release (Morris et al. 1973, Finlayson &
Finlayson 1958). There were 96 species of primary and secondary parasitoids
obtained from these cocoon collections at the Belleville Laboratory in Canada
and a multiple introduction program involving two egg parasitoids and 25
larval and cocoon parasitoids was initiated in 1933-51. The importation of a
wide variety of parasitoids from diverse hosts permitted the inclusion of
several sawfly pests as additional targets for some of the releases (McGugan
& Coppel 1962). The addition of an elaborate controlled environment
quarantine building was made at Belleville in 1936 allowed the mass rearing
of several of the imported European parasitoids. Dahlbominus fuscipennis,
a gregarious ectoparasitoid of prepupae, readily attacked cocoons in the
laboratory and was selected along with several other species for a massive
program of mass rearing for release. The mass-rearing peaked in 1940 when a
total of 221.5 million D. fuscipennis was released and by the
end of the program in 1951 a total of 890 million directly imported or
laboratory reared parasitoids had been liberated (McGugan & Coppel 1962). Only 5 species of parasitoids out of 27 released became
established over more than several generations, although four additional
species were recovered during the years shortly after release. Three of the
five species, D. fuscipennis, Exenterus amictorius
(Panz.) and E. confusus Kerr, were widely established only
during the outbreak and have since not been recovered from G. hercyniae.
Although E. amictorius had little impact, the other two species
achieved variable but appreciable levels of parasitism and have been credited
with the decline of the outbreak in at least some areas. Two other
parasitoids, Exenterus vellicatus Cush. and Drino bohemica
Mesn., never became important until the collapse of the outbreak but have
replaced the three species present during the outbreak to maintain host
population at low, non-damaging densities. The epizootic of European spruce sawfly began to decline
in 1939-40, which coincided in the southern part of the range with the
occurrence of a nuclear polyhedrosis virus, Borrelinavirus hercyniae.
This virus is thought to have been accidentally imported and released in
Canada along with the parasitoid. It spread rapidly to produce virus
epizootics throughout most of the outbreak range and by 1943 host population
densities had declined to very light infestations. Unlike other diprionid
sawflies, G. hercyniae larvae are not gregarious and the rapid
spread and subsequent impact of the virus was attributed to its virulence
(Bird & Elgee 1957). More recent studies in the Great Britain, where G.
hercyniae was accidentally introduced from the European continent in
1968, indicate that birds play an important role in virus transmission
(Entwistle 1976). The importance of D. bohemica, E. vellicatus
and the NPV virus in maintaining the spruce sawfly at low population
densities in Canada has been inadvertently demonstrated through chemical
spray treatments aimed against spruce budworm. Both in the early 1960's and
again in the 1970's sawfly population levels increased immediately following
the cessation of a 2-3 year spray treatment, due to the detrimental effects
of the spray on natural enemies, but declined after several generations as a
result of increased parasitism and the reappearance of the virus (Neilson et
al. 1971, Magasi & Syme 1961). There are several interesting features of this successful
biological control program. First the success of the accidental introduction
of the virus provides to date the most outstanding example of the use of a
pathogen in classical biological control. Its ability to control the sawfly
population in the absence of parasitoids has been demonstrated (Bird &
Burk 1961, Entwistle 1976) and in Canada it has persisted in the forest
environment since the initial introduction despite the low host densities
(Magasi & Syme 1981). The multiple introduction program of parasitoids
resulted in the establishment of the two more effective and specific species,
despite the release of a wide range of potential competitors. However, the
continuous and large scale release of poorly adapted parasitoids, which were
later recovered only from other sawfly hosts, was successful in inducing
significant levels of mortality prior to the introduction of the virus. LARCH SAWFLY, Pristiphora
erichsonii (Hartig)--Tenthredinidae A comparatively rare insect in Europe, the larch sawfly
was first generally recognized as established in larch forests throughout the
eastern Provinces of Canada in 1884. Several short lived but severe
infestations were observed in 1906-16 in which hugh quantities of tamarack (Larix
laricina) were destroyed (McGugan & Coppel 1962). Ever since the
sawfly has been found throughout the range of larch in North America but
remains more important on tamarack than on western larches. It is unknown whether
the sawfly was a recent introduction in the late 19th Century or of much
older origin in North America (Ives & Muldrew 1981). But the lack of
native parasitoids prompted a classical biological control program in
1910-13, 1934 and 1961-64. Collections were made in Great Britain during the early
phase of introductions (McGugan & Coppel 1962). They were shipped to
Canada for quarantine, screening and direct release. This led to the
establishment of the specific ichneumonid larval parasitoid Mesoleius tenthredinis
Morley, which in Manitoba was found in 20% of sawfly cocoons in 1960 and had
parasitized over 80% of the population by 1927 (Criddle 1928). Subsequently a
tachinid Zenillia nox (Hall), was collected in Japan in 1934 by
the U. S. Dept. of Agriculture and released both in New Brunswick and British
Columbia but failed to establish. The success of parasitism by M. tenthredinis
prompted an extensive relocation program to distribute this parasitoid
throughout Canadian larch forests. Rapid establishment was reported with
subsequent reductions in sawfly populations and reduced timber losses. This appeared to be another example of the success of
classical biological control in Canada, but in the late 1930's larch sawfly
defoliation again became prevalent in Manitoba. Parasitism by M. tenthredinis
appeared to have dropped to low levels, so 75,000 parasitoids were
transferred from British Columbia across central Canada. While the
parasitoids' range increased, levels of parasitism remained low due to the
encapsulation of parasitoid eggs by host larvae (Muldrew 1953). The
appearance of a resistant European strain of the sawfly, capable of
encapsulating M. tenthredinis eggs, appears to have resulted
from the parasitoid introduction program in 1913, when imported larch sawfly
cocoons were placed directly in the field. The resistant strain has since
spread across Canada and into neighboring states of the United States,
becoming predominant in most regions (Wong 1974). Renewed efforts were made in 1957 to obtain more parasitoids
from Europe and Japan, and long term study plots were chosen in Manitoba to
better evaluate the dynamics of the larch sawfly populations and the impact
of introductions. These studies (Ives 1976) indicated that mortality in the
cocoon and adult stages determined population trends and that high water
tables and predation by small mammals were largely responsible for the
erratic population abundance. The native tachinid, Bessa harveyi
(Tns.), considered the most important parasitoid in the renewed outbreaks,
had little impact. The CIBC collected 11 parasitoids in Europe and Japan and
shipped them to Canada between 1959-65. Five of the more abundant species
were selected for release and >200 adult were liberated. A separate
introduction of the masked shrew, Sorex cinereus Kerr from New
Brunswick to the island of Newfoundland was made in 1958 in order to fill the
vacant niche for an insectivore and to increase cocoon predation. The shrew
as successfully established as well as two of the parasitoids. One of these
parasitoids, the ichneumonid Olesicampe benefactor Hinz.,
attacks young sawfly larvae, the second, a Bavarian strain of M. tenthredinis,
was shown to be only weakly encapsulated by the resistant sawfly strain and
was able to pass its characteristics on to the progeny of mixed (Britain X
Bavarian) crosses (Turnock & Muldrew 1971). Parasitism by M. tenthredinis
initially increased following the release of the Bavarian strain but O.
benefactor became the dominant parasitoid influencing cocoon survival.
Parasitism by the latter at the release point in Manitoba attained levels of
ca. 90% between 1967-72 (Ives & Muldrew 1981) and was the dominant factor
for the collapse of the sawfly epizootic (ives 1976). Olesicampe benefactor
was relocated from Manitoba to most other Provinces in Canada (Turnock &
Muldrew 1971) as well as to Maine (Embree & Underwood 1972), Minnesota
(Kulman et al. 1974) and Pennsylvania (Drooz et al. 1985). Effects of the masked shrew on larch sawfly cocoon
survival in Newfoundland has never been adequately estimated. Predation of
cocoons is thought to have increased, but outbreaks have continued through
the 1970's (Ives & Muldrew 1981). Therefore, O. benefactor
seems to offer the greatest potential for controlling larch sawfly in Canada.
However in 1966 a hyperparasitoid, Mesochorus globulator Thunb.
began to attack this parasitoid in Manitoba. The polyphagous hyperparasitoid
is common in Europe and may also have been accidentally introduced during the
initial 1910-13 introductions. It has spread throughout the region and into
Wisconsin, although it hasn't been reported from Pennsylvania (Drooz et al.
1985). While hyperparasitism attained very high levels (80-90%) in Manitoba
during 1970's, sawfly populations continue to remain low in abundance, and
thus control may be achieved by O. benefactor despite the
occurrence of the hyperparasitoid. The larch sawfly program gives further evidence of the
value of the more specific and well adapted parasitoids in classical
biological control. As in the case of the European spruce sawfly, while a
wide range of parasitoids was released, only the more specific species became
established. However, while in the absence of hyperparasitism O. benefactor
may have been an ideal control agent, its competitive superiority over the
Bavarian strain of M. tenthredinis may have prevented the
latter from establishing and spreading more widely. This and the known
occurrence of various geographic strains of M. tenthredinis
differing in ability to avoid encapsulation by the host, emphasizes the value
of detailed studies of parasitoid biologies prior to introduction. Also, the
accidental introduction of both a parasitoid resistant strain of the host and
probably also a hyperparasitoid indicates the critical need for quarantine
handling of imported material to avoid unnecessary liberations. EUROPEAN WOOD WASP, Sirex noctilio F.--Siricidae Biological control attempts against the woodwasp is one of
the very few large projects directed against a wood boring insect. Woodwasps
usually are considered secondary pests and attack dead or dying trees. Sirex
noctillio occurs in Canada and throughout Europe but is most common in
the Mediterranean area. It is somewhat specific to Pinus species
(Spradbery & Kirk 1978), and is unique among the siricids in Europe in
that it is able to kill standing green trees. Under the right circumstances,
as occurred in New Zealand and Australia, this insect was able to cause
serious losses to Monterey pine, Pinus radiata D. Don.,
plantations. The pest was first discovered on the North Island of New Zealand
about 1900 but it was not until 1927 that it was abundant enough in exotic
pine plantations for control to begin (Taylor 1981). High mortality occurred
in P. radiata plantations between 1940-49 in New Zealand, and S.
noctilio reached Australia in southern Tasmania in 1952 and Victoria
in 1961 (Taylor 1976). There is a special relationship of S. noctilio
to a symbiotic fungus, Amylostereum areolatum (Fr.) Boidin,
that serves as a kairomone for the parasitoids of the woodwasp. Also the
parasitic nematode, Deladenus siricidicola Bedding, is wholly
dependent in nature on the woodwasp and the fungus (Bedding 1972). Adults of S.
noctilio emerge from midsummer to late fall and mate in the upper
foliage of trees. Female wood wasps oviposit by drilling holes through the
bark into the sapwood of trees that are usually predisposed or damaged. At
the time of oviposition the symbiotic fungus is introduced (Taylor 1981).
Adults live only a few days in nature. The eggs hatch when the surrounding
area has been invaded by the fungi and this occurs when some drying has taken
place to favor the fungi. First and second instar larvae feed exclusively on
fungus and third and fourth instars begin to tunnel into the wood. The larvae
turn back toward the bark to about 5 cm from the bark surface to enter the
prepupal stage. Pupation may not occur until the second or third year after
hatching, depending on the weather. After pupation adults emerge in about
three weeks, and each generation emerges over a period of two to three years
with the proportion of individuals emerging in the first, second and third
year varying by site (Taylor 1981). Biological control was initiated in New Zealand in 1927
(Taylor 1981). During 1929-32 the ichneumonid, Rhyssa persuasoria
L. was introduced but the control was not satisfactory (Turnock et al. 1976).
Then Ibalia leucospoides (Hochenw.) (Ibalidae) was colonized in
1954-58 which resulted in improved control (Zondag 1959). The two parasitoids
were then colonized in Tasmania. A large scale biological control effort did
not begin until 1961 following the discovery of S. noctilio in
Victoria, Australia. A National Sirex Fund was established, which consisted
of a consortium of federal, state and private agencies, and a committee was
formed to coordinate research and control in Victoria (Taylor 1981). A world
wide search for natural enemies was begun by the Division of Entomology,
CSIRO in 1962. The search for parasitoids in the northern hemisphere was
completed by 1973, and during the 11-year period 21 species of parasitoids
were sent to Tasmania for culture (Taylor 1976). The plan was to obtain all
the available parasitoids of siricids in conifers and as many strains as
possible from different climatic zones with emphasis on the Mediterranean
area. This included collections of siricids in conifers other than Pinus
and from genera and species other than Sirex noctilio. Ten
different parasitic species were released in Tasmania and Victoria, six
having become established and one additional species, the ichneumonid Rhyssa
hoferi Roher, probably established (Taylor 1981). Of the seven species
two are holarctic (R. persuasoria and I. leucospoides),
two are palearctic (I. rufipes drewseni Borries and the
ichneumonid Odontocolon geniculatus Kreichbaumer) and three are
nearctic [the stephanid Schlettererius cinctipes Cresson and
the ichneumonids Megarhyssa nortoni (Cresson) and R. hoferi]. These species tend to be complementary, although there
might be some competition within the guild attacking larger larvae. The Ibalia
species attack first or second instar siricid larvae and the two species have
different emergence times so that they do not compete directly. The
ichneumonids attack the more developed larvae of their host and there may be
differential preference based on tree diameter (Taylor 1981). Schlettererius
cinctipes emerges after the peak emergence of the ichneumonids, while
the other two are also complementary as O. geniculatus is
small, emerges in springtime and attacks late hatching larvae that are still
closer to the bark surface. Rhyssa hoferi is adapted to drier
areas and could do well in drier climates (Taylor 1981). A parasitic nematode, Deladenus siricidicola,
was found in New Zealand in 1962 (Zondag 1969). It causes female wood wasps
to lay infertile eggs. Additional nematodes wee sought during 1965-75 without
success (Bedding & Akhurst 1974). Different strains of the nematode have
also been released throughout wood wasp infested areas in Tasmania and
Victoria and it is well established throughout. This nematode also affects
the reproduction of some of the female parasitoids (Bedding 1967), which
apparently does not adversely affect biological control. The nematode is
credited with reductions of wood wasp populations to very low levels in
certain areas. The Sirex noctilio biological control
program is significant for several reasons. A large group of organizations
cooperated in a well funded, extensive worldwide search for parasitoids as
well as a research program that examined many aspects of the host tree/Sirex/fungus/parasitoid
relationships (Taylor 1981). As with Gilpinia hercyniae there
was a fortuitous introduction (the nematode). Sirex noctilio
was introduced from the northern to the southern hemisphere and attacked an
exotic host plant Pinus radiata (native to California). The
search for parasitoids in the north was made from S. noctilio and
its host trees to siricids in other genera and species in Pinus as
well as other conifers. The project was well planned with attention given to
colonizing strains of parasitoids suited to different climatic zones and
developmental stages of the host. It is believed that this biological control project will
eventually be completely successful (Turnock et al. 1976). It has been
thought that the combination of parasitoids and nematodes along with sound
forest management should hold S. noctilio down to the level
where losses are not serious (Taylor 1976). GREATER EUROPEAN SPRUCE BEETLE, Dendroctonus micans
(Kugelmann)--Scolytidae This bark beetle, probably native to coniferous forests of
eastern Siberia, is one of only two Dendroctonus species occurring in
the palearctic region. Dendroctonus micans is primarily a pest
of spruce, Picea spp., but will occasionally attack Pinus sylvestris
L. The beetle has been expanding its range for many years and is still
spreading. About 200,000 ha are currently suffering from epizootics and
recently invaded areas include Great Britain, France, The Georgian S.S.R. and
Turkey (Grégoire et al. 1987, Evans 1985). In the inner parts of its range
where the beetle has been established for a long time populations remain at
low densities and it is not a pest. This bark beetle differs from the more aggressive North
American Dendroctonus species in that it attacks its host tree in low
numbers, killing the bark in patches. Successive attacks over a period of
five to eight years may be necessary to kill a tree except during beetle
outbreaks (Grégoire 1985). The beetle shows kin-mating, gregarious
larvae and apparently lacks associated pathogenic fungi that are
characteristic of many Scolytidae. Dendroctonus micans has very
few natural enemies which may be due to its unique biology that seems to
protect the beetles from competitors and generalist natural enemies by the
defenses of its living host (Everaerts et al. 1988). One specific predator Rhizophagus grandis
Gyllenhal is very abundant in areas where the bark beetle has been present
for long periods of time. This rhizophagid beetle is believed to be
responsible for maintaining the low, stable D. micans
population in these areas (Kobakhidzi 1965, Grégoire 1976, Moeck &
Safranyik 1984). A massive biological control project was initiated against
D. micans in Georgia S.S.R. in 1963 (Kobakhidze 1965). The
spruce beetle had extended its range into Georgia following World War II in
timber imported from the north. A predator relocation program was planned as
the predator did not follow its host. Rhizophagus grandis was
released in large numbers as larvae and adults on spruce trees infested by D.
micans (Kokakhidze et al. 1968). Effective control apparently has been
achieved (Grégoire et al. 1987). First observed in the Massif Central of France in the
early 1970's, D. micans was targeted for biological control in
a program funded by the European Economic Community in 1983. Its main thrust
was to establish the predator, R. grandis (Grégoire et al.
1987). A similar program was initiated in 1983 in Great Britain (Evans 1985,
Evans & King 1987). Evaluations are still in progress, but knowledge that
the predator is attracted to the frass of three North American Dendroctonus
species (Miller et al. 1987) suggests its possible use against species other
than D. micans. REFERENCES: Please refer to: |