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ENHANCING NATURAL ENEMY IMPACT
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Introduction Elzen &
King (1999) discussed the manipulation of natural enemies for enhanced biological
control. The successes in classical biological control have provided the
background and encouragement for efforts in the manipulation of natural
enemies. Such manipulations include conservation, augmentation, habitat
management and genetic manipulation. During the 1980's there has been
increased emphasis on the use of semiochemicals to manipulate natural
enemies, especially Hymenoptera (Nordlund et al. 1981a). Also, insecticides
are being stressed that minimize direct toxic and sub lethal effects to beneficial
insects. The use of biological control together with insecticides is
encouraging. The
enhancement of entomophage effectiveness has been reviewed in various ways by
Ridgway & Vinson (1977) and Ables & Ridgway (1981). Propagation and
release of entomophagous arthropods for use in augmentation was discussed by
King et al. (1984) and this practice in the United States was reviewed by
King et al. (1985a). Additionally the behavior of liberated parasitoids and
predators was discussed by Weseloh (1984). Lewis & Nordlund (1985)
stressed the importance of insect behavior in order to enhance natural enemy
effectiveness. Literature on semiochemicals was discussed by Nordlund et al.
(1981a,b; 1985) and Vinson (1975). Habitat manipulation to enhance parasitoid
activity was reviewed by Powell (1986). Entomophages
may be potentially manipulated in many ways. The concepts of inundative and
inoculative releases were first mentioned by DeBach & Hagen (1964). Inundative
releases rely mainly on the agents released, not their progeny, whereas
inoculative releases rely upon a buildup of the initial parasitoid
populations so that immediate control is followed by additional control
wrought by progeny (Li 1984). Augmentative releases have been described as
supplemental releases, strategic releases, programmed releases, seasonal
colonization, periodic colonization and compensatory releases (Ridgway et al.
1977, King et al. 1984, King et al. 1985a). Elzen & King (1999) give
examples demonstrating the feasibility of controlling pests by augmentative
releases of entomophages. Individual case studies were presented by King et
al. (1985a). Manipulation refers to those procedures that help the establishment and
activity of natural enemies. Manipulation of a natural enemy or its
environment may be justified if a definite need exists and a reasonable
assurance of success is possible. Certain factors associated with the habitat, the host, or the natural
enemy itself may render an entomophagous organism ineffective as a
biological control agent, but still be subject to manipulation. The habitat may have certain adverse
climatic factors, such as heat, cold, low humidity or wind. Unattractive or
otherwise unsuitable host plants may be present, or there may be a scarcity
of food or water for adult natural enemies. Interspecific competition among
natural enemies. Pesticides may be present, or cultural practices may not
favor natural enemy activity. The host may lack synchronization with parasitoid
generations, or host plant resistance may not provide for hosts during
critical times. There may be host strains resistant to natural enemy attack,
or there may be periodic scarcities of suitable host stages. The natural enemy may exhibit an annual
ovarian diapause and migrate away from its hosts at certain times of the year
(e.g., Coccinellidae), or its reproductive rate may be too low. It may
exhibit an adverse tendency to disperse, coupled with an inability to find
mates at the resulting low densities. Generally, manipulation of a natural enemy
should only be attempted if it involves some periodically occurring,
unfavorable environmental factor, a lack of some easily supplied requisite,
or some simple or minor, but correctable, intrinsic shortcoming. Manipulative Methods Employed Periodic Colonization involves periodic releases of mass-produced or field-collected
natural enemies. Two types are inundative releases and inoculative releases. The
first type, inundation, has largely been employed against the egg stage of
univoltine pests. Control is largely the work of the insects released, not
their progeny. It has been called a biotic insecticide since host
mortality is more or less immediate, and there is no prolonged interaction
between host and natural enemy populations. This method is best employed
against pests of high value crops, against univoltine pests, or against
multivoltine pests that reach injurious levels during but one generation
annually. The second, inoculation, is where the
interaction between host and natural enemy populations persists through more
than one generation of the natural enemy, and control is largely effected by
the progeny of the beneficial forms released. Inoculative releases may take
the form of accretive releases
where small numbers of natural enemies are periodically released against low
density pest populations. Entomophagous insects and their pest hosts may also
be colonized concurrently in areas with a known history of pest invasions or
where hosts are too scarce to support natural enemies, this in anticipation
of pest invasions (e.g., Cryptolaemus
on citrus mealybugs in California). Selective Breeding is not a practical method to date, but offers a challenging
field for research. Environmental Manipulation may supply artificial structures that serve as shelters or as
nesting sites for natural enemies. Supplemental food for adult natural
enemies may be supplied. Alternate hosts may be supplied for beneficial insects
or their phytophagous hosts may be offered alternate host plants.
Artificially supplying suitable host stages when these are unavailable in the
field, and eliminating honeydew-feeding ants may also be effective. The
habitat may be modified to eliminate or reduce the adverse effects of
cultural practices, pesticides, dust deposits, etc. ADDITIONAL
WAYS TO ENHANCE IMPACT Various examples in DeBach (1963) and Rabb
(1962) describe how the construction of nesting shelters encouraged high
local populations of Polistes
wasps in cotton fields in the West Indies and in tobacco fields in North
Carolina, increasing the total predation of injurious lepidopterous larvae. Nesting boxes provided for insectivorous birds in some intensively managed
European forests also resulted in increased predator densities and protection
from defoliating insects. Many adult natural enemies utilize exudates from
floral or extra-floral nectaries, as well as pollen, as sources of nutrients
and water. The culture or conservation of plant food sources in the proximity
of cropland and orchards has been found to enhance the effectiveness of
various natural enemies. Pollen is known to be an important supplementary
food for adult, aphid-feeding Syrphidae and Coccinellidae as well as certain
predacious mites. The long-practiced method of clean
cultivation for weed control may be undesirable from the
standpoint of removing wild plants infested with honeydew-producing insects
or containing nectaries. Colonization of alternative insect hosts may improve
synchronization between a pest and its natural enemies. Several benefits that
may be derived from this technique are: (1) the damping of extreme
oscillations in natural enemy and host population densities; (2) maintaining
functional natural enemy populations by providing a continuous food supply
during periods of low pest densities; (3) providing suitable overwintering
hosts; (4) promoting maximum distribution of the natural enemy; and (5)
reducing intra- and interspecific competition among natural enemies
(cannibalism and combat). Modifications of adverse cultural practices
can improve natural control because cultivation may kill soil-inhabiting
beneficial insects or pupating, non-subterranean natural enemies. Reduced or
delayed cultivation may reduce this mortality and also dust. Dust is
especially known to harm parasitoids and predators; it can be minimized by
sprinkling, by planting cover crops, by paving access roads or by holding
cultivation to a minimum. Properly timed irrigation may promote epidemics of
fungal pathogens of insect pests by providing the proper conditions of
humidity in the microenvironment. Improperly timed irrigation, on the other
hand, may drown or drive away beneficial insects. Trichogramma spp. have been extensively researched for inundation since
Flanders (1930) suggested that they offer a possible alternative to
insecticides. Comprehensive reviews on the use of Trichogramma were presented by Ridgway & Vinson
(1977), Ridgway et al. (1977) for use in the Western Hemisphere, by Huffaker
(1977) for China, by Belyarov & Smetnik (1977) for the Soviet Union, and
for augmentation in cotton by King et al. (1985). By 1985 Trichogramma spp. were the most
widely used entomophages for augmentation (King et al. 1985). Lepidopterous
pest control by mass rearing and release of Trichogramma spp. has carried out for many decades. The
pioneering research of Howard & Fiske (1911) and Flanders (1929, 1930) in
the United States stimulated research with Trichogramma spp. worldwide, and a number of successes in
reducing insect populations by augmentation with Trichogramma have been reported. Hassan (1982) and Bigler
(1983, 1984) reported 65-93% reduction in larval infestations of the European
corn borer following Trichogramma
releases during the 1970's in Germany and Switzerland. Voronin and Grinbert
(1981) reported positive reductions of pest such as Loxostege spp, Agrotis
spp., and Ostrinia species
following Trichogramma
releases. In China a significant reduction in populations were reported for Ostrinia spp., Heliothis spp. and Cnaphalocrocis spp., crop
damage being reduced (Li 1984). Oatman & Platner (1985) found that two
common lepidopterous pests of avocado in southern California, Amorbia cuneana Walsingham and the omnivorous looper, Sabulodes aegrotata (Guenée), could be effectively controlled by
liberations of 50,000 Trichogramma
platneri in each of four
uniformly spaced trees per acre. At least three weekly releases were required
for control of S. aegrotata, while two were
necessary for A. cuneana. Considerable success has been achieved in
California with the periodic introduction of cichlid fish and some
invertebrate predators of mosquitoes and midges in connection with biological
control of aquatic weeds and pestiferous insects [Please refer to
Research #1, #2, #3 ] Hassan (1982) obtained 65-93% reduction in
larval infestations of Ostrinia
nubialis (Hübner) after four
years of releases in Germany. Reduction in insect density and crop damage in
several agroecosystems was also reported from China and the Soviet Union (Li
1982, Voronin & Grinberg 1981). Oatman & Planter (1971, 1978)
demonstrated the feasibility of augmenting Trichogramma pretiosum
to reduce damage in tomatoes caused by the tomato fruitworm, cabbage looper
and Manduca spp., although
they found that chemical control was also necessary for pests not susceptible
to T. pretiosum. Oatman et al. (1983) reported on an integrated control program for the
tomato fruitworm and other lepidopterous pests on summer plantings of fresh
market tomatoes in southern California in 1978-1979. Twice weekly
applications of DipelR (delta-endotoxin of Bacillus thuringiensis
Berlinger var. kurstaki),
plus twice weekly releases of T.
pretiosum, were compared
with weekly applications of methomyl. There were no significant differences
in fruit yield or size between the two control regimes. Methomyl adversely
affected predator populations, host eggs, and egg parasitization by T. pretiosum, whereas Dipel did not. In The Netherlands, Trichogramma spp. have been utilized to develop biological
control of Lepidoptera. Two approaches there were (1) the selection of the
best species and strains of Trichogramma
(van Lenteren et al. 1982) and studies of the manipulation of Trichogramma behavior. The
first approach has been studied extensively, especially in Brassica spp. Inundative
releases of Trichogramma
were feasible for control of Mamestra
brassica on Brussels
sprouts, but control was not very effective at low host densities (van der
Schaaf et al.a 1984). Glas et al. (1981) reported reduction in larval infestations of Plutella xylostella in cabbage crops. Van Heinigen et al. (1985) summarized several years of work with Trichogramma releases. The
second approach (2) to Trichogramma
manipulation in The Netherlands involved examination of semiochemical
mediated behavior. These studies indicated that kairomones and volatile
substances released by adult female hosts (sex pheromones) were important in
foraging behavior of Trichogramma
(Noldus & van Lenteren 1983, Nodlus et al. 1986, 1987). Preintroductory
evaluation using the methods outlined by Wackers et al. (1987) may improve
prospects for augmentative release of specific strains of Trichogramma in The
Netherlands. It is evident that control can occur through
augmentation with Trichogramma
spp. under certain conditions. However, there have been variable results and
cases of insufficient pest control reported (King et al. 1985b). Trichogramma pretiosum was tested in
augmentative releases in Arkansas in 1981-82 and in North Carolina in 1983
for management of H. zea and H. virescens
in cotton. These releases failed to provide adequate control in 1981-82, but
in 1983 cotton fields treated by seven augmentative releases of T. pretiosum at 306,000 emerged adults/ha./release yielded
significantly more cotton than control fields which were not treated with
insecticides. Insecticidal control fields yielded more cotton than did
control of T. pretiosum release fields, which
led these researchers to conclude that management of Heliothis spp. in cotton by augmentative releases with
this parasitoid was not economically feasible (King et al. 1985b). However,
the greater yields obtained in North Carolina in 1983 in the T. pretiosum release fields supported the use of Trichogramma spp. In order to obtain consistent results, large
numbers of Trichogramma spp.
should be released. However, in addition the effectiveness of Trichogramma may certainly be
influenced by such factors as, (1) the density and/or phenology of the pest,
(2) the species or strain of Trichogramma,
(3) vigor of the parasitoids, (4) method of distribution, (5) crop phenology,
(6) number of other natural control agents present, and (7) the proximity to
crops receiving insecticides and drift of insecticides into Trichogramma release fields
(King et al. 1985b). Trichogramma
spp. seem highly susceptible to most chemical insecticides, with lethal
effects resulting from direct exposure to spray applications, drift or posttreatment
contact with pesticide residues on foliage (Bull & Coleman 1985). It has
been suggested that the inconsistent results in Arkansas and North Carolina
was due to chemical insecticides (King et al. 1985b). In order to effectively manipulate entomophages
there must be a thorough knowledge of the biology and host associations of
the organisms. Although such information may be gained through laboratory
studies, it is necessary that such data be followed by studies in the field.
Ideally a comparison of laboratory, field cage and field studies can provide
useful information which could be used to predict the impact of the natural
enemy on pest populations. In augmentation programs, the level of
control achieved may be influenced by many interacting factors. Primary
factors include the availability of hosts and host/parasitoid synchrony;
conditions of weather during release of entomophages, including the effect of
environmental factors on foraging; influence of habitat type; chemical
pesticide usage, either concurrent or not or adjacent; the fitness of
laboratory reared parasitoids. The use of augmentative releases is very
complex, and the environmental effects acting on released entomophages may be
highly variable. Therefore, studies must be planned that will be used to
predict when and under which specific situations biological control by
augmentation may work. For example, Microplitis
croceipes (Cresson)
efficiency appears to be greater during summer on agricultural crops than in
spring on wild host plants. Fewer M.
croceipes adult females were
required in a summer study as compared to an early spring study to achieve
comparable control of tobacco budworm in field cage experiments. All
parameters, biotic and abiotic should be explored in evaluating augmentation
release results. Augmentation of entomophages of row crop pests may be
implemented only after considerable effort has been expended to prove the
feasibility of this approach. Therefore, the efficiency and financial
benefits must be determined. Reliance on entomophages to control pests should
be limited to those situations where scientifically, environmentally and
economically sound procedures are available. Theory predicts that predator/parasitoid
effectiveness can be increased through propagation and liberation.
Host/entomophage interactions must be thoroughly studied, however before any
program can be relied upon. Such interactions may be assessed through
studying the functional response of the predator/parasitoid to host density
and how this relates to dispersal of the organism. Field evaluations must
provide the data necessary for defining the number of entomophages required
for release per unit area, and this together with mass production technology
determines the economic feasibility of the approach. Such fundamental
knowledge as searching rate, functional response, and efficiency could
significantly add to the predictability of success in augmentation efforts. Flight Chambers are useful tools for examining flight responses and foraging
patterns of parasitoids. Most designs presently in use are similar to the
wind tunnel of Miller & Roelofs (1978), which was used to study moth
flight. Nettles (1979, 1980) did a lot of work with the wind tunnel flight
responses of parasitoids in studies of Eucelatoria
spp. These examined response of the parasitoid to volatiles from the host and
host habitat, and suggestions were made regarding the use of Eucelatoria attractants to
increase parasitoid populations in the vicinity of Heliothis hosts. Drost et al. (1986) examined the flight behavior mediated by airborne
semiochemicals in M. croceipes and emphasized the
importance of preflight conditioning to the plant-host complex on positive
searching responses of M. croceipes. Other research
showed the M. croceipes reared on hosts fed
cowpea seedling leaves instead of artificial diet had an increased percentage
of oriented flights to odors of a cowpea seedling--H. zea
complex in a flight tunnel. The increased response was much stronger after
adult females had searched a fresh host plant complex (Drost et al. 1988).
Elzen et al. (1986) evaluated the effects of cotton, Gossypium spp., cultivars and species on the flight
responses of Campoletis sonorensis (Cameron) in a study
which found higher innate searching on glanded versus glandless varieties. It
was implied that volatile chemicals present in the glanded varieties had a
positive effect on parasitoid foraging in the wind tunnel that was not
produced by glandless cottons or Old World species. Additionally, Elzen et al.
(1987a) found strong innate responses by M.
croceipes to cotton and
further suggested that the parasitoid responses represented fixed action
patterns. Herard et al. (1988a)
conducted experiments with M.
demolitor which were similar
to those of Drost et al. (1986). Herard et al. (1988b) also described rearing methods suitable for
semiochemical studies. The wind tunnel flight chamber has been further
refined with the development of a novel system for injection of semiochemical
volatiles directly into the moving air (Zanen et al. in press). Wind tunnels
may aid in efforts to solve the mysteries of parasitoid host habitat location
and host location and provide insights which may allow manipulation of
parasitoid behavior. Wind tunnels are also ideal for the early isolation of
semiochemicals and for use in bioassay directed fractionation and
confirmation of synthetic chemical activity. Of course, laboratory assessments of
entomophages must be supported by field experiments. For example, field
surveys have shown that parasitization of Heliothis
spp. larvae varies greatly in space and time (Lewis & brazzel 1968,
Graham et al. 1972, Roach 1975, 1976; Smith et al. 1976, Burleigh &
Farmer 1978, Puterka et al. 1985, King et al. 1985). A summary of suggested
methods and steps in manipulation of semiochemical-mediated foraging behavior
is given in Nordlund et al. (1981a). Parasitization can vary spatially due to variation in
parasitoid host plant detection, search rate, or retention of parasitoids on
host plants. Host plant species and stage, host density and weather are
likely to affect all three processes. Parasitization can vary temporally
because of variation in host detection, searching, retention, parasitoid
natality or mortality. Research designed to gather information to predict
distribution of parasitization across host plants under varying conditions
could yield important information on the population dynamics of hosts and
parasitoids. These predictions are crucial to rational conservation and
augmentation of parasitoids. For example, the search rate of M. croceipes in field cages was higher on Gossypium hirsutum L. in summer than on Geranium dissectum
L. in spring (Hopper & King 1986). However, this difference may arise
from different temperatures and not from different host plant species. In
field cages M. croceipes parasitized more
hosts on G. hirsutum than on Phaseolus vulgaris and more hosts on P. vulgaris
than on Lycopersicon esculentum (Mueller 1983).
However, it is unclear if these differences arose from differences in host
plant attraction or from differences in search rate. In field cage
experiments it was found that M.
croceipes parasitized a
significantly lower proportion of H.
virescens larvae on Geranium dissectum than on either Trifolium incarnatum
or Vicia villosa . The attraction of Compoletis sonorensis varies with host plant species (Elzen et al.
1983) cotton variety (Elzen et al. 1986), and the attraction to cotton
correlated with volatile chemical profile of the varieties (Elzen et al.
1984, 1985). The host plant species on which H. zea
has been feeding affects the response of M.
croceipes to nonvolatile
kairomones from its host. These data suggest that variation in parasitization
found in host plant surveys may arise from variation in attraction or
retention of wasps by semiochemicals directly or indirectly derived from the
host plants. Studies of host habitat preference may provide clues to the best
habitat in which to release parasitoids in augmentation. The effects of
kairomones on searching of pink bollworm parasitoids were studied by Chiri
& Legner (1983, 1986), but no effective means of deploying these chemicals for
enhanced biological control was found. In fact, their application may
actually reduce parasitoid effectiveness by confusion. Screening of the biological characteristics of entomophages has been
advocated (Sabelis & Dicke 1985, van Lenteren 1986). An example of a
predator currently used in IPM in Dutch orchards is Typhlodromus pyri
Scheuten. As note by Dicke (1988) despite the use of this predator, its
biology has not been thoroughly studied. Based on its response to volatile
kairomones it was later determined that T.
pyri prefers the European
red spider mite, Panonychus ulmi (Koch) to the apple rust
mite, Aculuc schlechtendali (Nalepa), which
was confirmed by electrophoretic diet analysis (Dicke & Dejong 1988). Field experiments may also provide insights
into the efficiency of a particular entomophage. For example, in field cages
containing Gossypium hirsutum or G. dissectum, M.
croceipes searching rate for
Heliothis zea and H. virescens
larvae did not depend on host density (Hopper & King 1986). In field
experiments on G. hirsutum, M. croceipes
parasitized a higher proportion of Heliothis
larvae in plots where host density was higher. Also, parasitoid aggregation
but not increased searching rate, caused the increased parasitization at high
host density which supports the linear functional response reported by Hopper
& King (1986). Some parasitoids have been shown to aggregate in areas of
high host density in laboratory experiments (Legner 1969, Hassell 1971,
Murdie & Hassell 1973, T-Hart et al. 1978, Collins et al. 1981, Waage
1983). Since host plant species vary in attraction and suitability for Heliothis, which can cause
variation in larval density, the spatial variations in Heliothis parasitization observed in field surveys may in
part result from parasitoid aggregation at high host densities. Several
parasitoid species are more attracted to plants on which hosts have fed than
to undamaged plants (Thorpe & Caudle 1938, Monteith 1955, 1964, Arthur
1962, Madden 1970< and mechanically damaged plants increase parasitoid
searching (Vinson 1975). Damaged terminals of G. hirsutum
attract more C. sonorensis than do undamaged
terminals (Elzen et al. 1983). Microplitis
croceipes is attracted to
wind borne odor of H. virescens frass and larvae
(Elzen et al. 1987), and M. croceipes responds to
nonvolatile kairomones produced by H.
zea (Jones et al. 1971,
Gross et al. 1975), H. virescens, and H. subflexa (Lewis & Jones 1971). Luck et al. (1988) suggested criteria for evaluating entomophages that are scheduled for
introduction. Experimental evaluation through life table analysis,
examination of percent parasitization, key factor analysis, and the use of
simulation models, may provide insights into the probability of success in
augmentation. Evaluation of entomophages may include introduction and
augmentation, and techniques using cages and barriers, removal of
entomophages, prey enrichment, direct observation and biochemical evidence of
entomophage feeding, and quantified experiments to gauge the impact of the
entomophages. Entomophages may be ineffective due to a
lack of host synchrony, temporal displacement in ephemeral systems, lack of
protected sites, lack of alternate hosts, adverse environmental conditions or
influence of pesticides, etc. Temporal synchronies between entomophage and
pest, and oscillations in populations have been documented by Varley &
Gradwell (1974). The influence of weather on parasitoid searching has,
however, received little attention. Often when natural control is not
achieved it is due to the lack of synchrony of entomophage and host in time.
These complex relationships make intervention at any one level difficult and
less likely to produce desirable results. An understanding of the effects of the
pesticide component is important (Croft & Brown 1975). Pesticide
resistance in entomophages was discussed by Croft & Morse (1979), and
recommendations for changing control practices to preserve entomophages were
listed. Insecticide use in cotton and the value of predators and parasitoids
for managing Heliothis was
reviewed by King (1986), and results on Pectinophora
gossypiella were given by
Legner & Medved (1979 , 1981 ). The detrimental effects of pesticides on
entomophages are well documented, and it may be important to note that an
underlying problem in practical implementation of augmentation is the use of
pesticides. Unexpected problems may be encountered, even from pesticide drift,
so that basic toxicological studies may be required to determine if the
entomophages intended for use in augmentation have some degree of tolerance
to the effects of insecticides, especially as resurgence of primary and
secondary insect pests has been documented in some heavily sprayed
monocultures (Huffaker 1971). Actions of insecticides on entomophages include
not only those causing direct mortality, but also those that act in indirect
ways, or that alter entomophage biology adversely. First and foremost, there
are the obvious direct lethal actions of broad spectrum insecticides, such as
organophosphates, on entomophages. Because entomophages have more specific
enzymes evolved for handling the toxins of their hosts they are much more
susceptible to broad spectrum insecticides than their hosts which have an
array of plant chemicals with which to contend (Krieger et al. 1971). The
occurrence of primary pest release and resurgence of a previously innocuous
secondary pest have been widely reported where insecticides selectively
destroy entomophages (DeBach & Bartlett 1951, Michelbacher et al. 1946,
Doutt 1984, Lingren & Ridgway 1967). For example, azinphosmethyl, a broad
spectrum organophosphate, selectively destroys entomophages in apple orchards
(Falcon 1971). On the other hand, chlordimeform was found less toxic than
some other insecticides to several species of entomophages (Platt &
Vinson 1977, Platt & Bull 1978). Sometimes insecticides do not kill
entomophages, but they may so affect them that normal behavior or
reproduction is encumbered. Press et al. (1981) found the permethrin and
pyrethrin reduced the number of adult Bracon
hebetor Say produced when
parental females were exposed to hosts and insecticides simultaneously.
Topical application of carbaryl on adult female Bracon hebetor
results in reduced numbers of eggs that develop from vitellogenic oocytes,
and resorption of mature ova (Grosch 1975). Residues of pyrethrin
significantly reduce parasitization rates of T. pretiosum
(Riley) on H. zea eggs (Jacobs et al. 1984).
Formamidines are especially recognized for their ability to disrupt pest
mating, reproductive and feeding behavior (Knowles 1982, O'Brien et al.
1985). Whether or not these compounds have such effects upon entomophages is
not well known. Parasitized hosts have been found to be more susceptible to
insecticides than nonparasitized hosts, thus preventing normal development of
immature parasitoids. Lymantria
dispar (L.) larvae
parasitized by Apanteles melanoscelus (Ratzburg) are
significantly more susceptible to carbaryl than nonparasitized larvae, and
more time is required for surviving parasitoids to develop (Ahmad &
Forgash 1976). Fix & Platt (1983) found that H. virescens
larvae parasitized by C. nigriceps are 1.42X more
susceptible to methyl parathion, and 2.5X more susceptible to permethrin than
are treated unparasitized larvae. These cases show how insecticides can have
an additional, indirect action on entomophages. Entomophage abundance may also be reduced when their hosts have been
decimated by insecticides. This was the case of the predator Orius insidiosus (Say) feeding on the cotton leaf perforator Bacculatrix thruberiella Bush in cotton
treated with chlordimeform. Numbers of O.
insidiosus steadily declined
when populations of the leaf perforator was reduced by chlordimeform sprays
(Lingren & Wolfenbarger 1976). Direct mortality is the most severe way
that insecticides can impact, chlordimeform may become important in certain
pest management strategies, due to the property of controlling pests
behaviorally and physiologically at low sublethal doses. Lo doses of
chlordimeform significantly decrease fecundity and egg viability of adult
female tobacco bollworms, and prevent moths from separating after mating
(Phillips 1971). Although chlordimeform decreased the number of eggs laid by
Lepidoptera, it is not clear whether reduced fecundity was caused by
interference with ovarian development or with oviposition behavior
(Hollingworth & Lund 1982). Chlordimeform also reduces fecundity in the
cotton aphid, Aphis gossypii Glover (Ikeyama &
Maekawa 1973) and in the cattle tick, Boophilus
microplus (Masingh &
Rawlins 1979). Feeding behavior is upset by chlordimeform in larval tobacco
cutworms, Spodoptera litura F. (Antoniosus &
Saito 1981), armyworm, Leucania
separata Walker (Watanabe
& Fukami 1977) and in cockroaches, Periplaneta
americana L. (Matsumura
& Beeman 1982). But the effects of chlordimeform vary with species and
there is much selectivity between species and stages for actual acute
toxicity; some insects are very sensitive and others are immune, as in the
case of adult boll weevils (Wolfenbarger et al. 1973). Additional evidence
for the suitability of chlordimeform for some pest management strategies is
given by Platt & Vinson (1977), who found that chlordimeform is ca. 100X
less toxic to the parasitoid C.
sonorensis than
organophosphates similar to azinphosmethyl. By controlling pests at sublethal
doses, the problem of pest resistance may be lessened. Dittrich (1966) found
that chlordimeform is effective against some pests that have already become
resistant to organophosphate and carbamate insecticides. The work with Trichogramma augmentation may provide clues for other
species of entomophages. From 1981-83 King et al. (1985) collected Heliothis larvae from
insecticide treated and untreated cotton fields and found 1/3rd of the larvae
were parasitized, particularly by the braconid M. croceipes.
These levels of parasitism of M.
croceopes were greater than
any reported in cotton since the advent of organochlorine insecticides in the
1940's (King 1986). As M. croceipes has been commonly
found in cotton in the SE United States (King et al. 1985) and due to the
apparent tolerance of this parasitoid to some commonly used insecticides
(King et al. 1985, Powell et al. 1986, Bull et al. 1987), augmentation
releases of the parasitoid are anticipated in the future (King 1984). This
parasitoid was recently exposed to insecticides commonly used in cotton using
a spray tower (Elzen et al. 1987b). Direct treatment with the pyrethroid
fenvalerate, a mixture of the formamidine chlordimeform plus fenvalerate, and
the carbamate thiodicarb resulted in nearly 100% survival of both sexes at
both the lowest and highest field rates recommended for these insecticides.
The organophosphate acephate and the carbamate methomyl were extremely toxic
to adult M. croceipes, causing 100%
mortality at the lowest recommended field rates. Marking studies have shown that lady beetles, lacewings, syrphid flies and parasitic wasps
fed on nectar or pollen provided by borders of flowering plants around farms.
Many insects were shown to have moved 250 ft. into adjacent field crops. The
use of elemental marker rubium also showed that syrphid flies, parasitic
wasps and lacewings fed on flowering cover crops in orchards and that some
moved 6 ft. high in the tree canopy and 100 fleet away from the treated area.
The use of nectar or pollen by beneficial insects helps them to survive and
reproduce. Thus, planting flowering plants and perennial grasses around farms
may lead to better biological control of pests in nearby crops (Long et al.
1998). The
effectiveness of resident
insect predators as biological control agents of peach twig borer was tested
in a series of field experiments. It was shown that the native gray ant, Formica
aerata was the most common and effective generalist predator. Treatments
with native gray ant present had significantly lower peach twig borer
abundance and peach shoot damage. Ant population densities were studied in
seven commercial orchards. However, results showed that although this ant is
found in most peach and nectarine orchards, its abundance was not clearly
associated with any single cultural practice and may be difficult to manipulate
(Daane & Dlott 1998). Spray tower treatment of Microplitis croceipes with insecticides
applied directly to the insects was followed in another study by exposure of
parasitoids to plants which were sprayed in the spray tower. Parasitoids were
then caged on these plants and mortality observed after 24 hrs. The
fenvalerate/chlordimeform mixture caused 10-23% mortality, with thiodicarb
causing a similar percent mortality, whereas methomyl caused significantly
high mortality, ranging from 23-70%. It is probable that the use of
thiodicarb as an ovicide and larvicide for Heliothis control will increase in the future because of
resistance to pyrethroids, and fortunately M. croceipes
seems relatively tolerant to this insecticide. Useful information for models to predict the
impact of entomophages on reducing herbivore induced damage or plant stress
is obtained by monitoring and sampling entomophages that are indigenous or
released by augmentation. Modelling of population interactions requires
accurate tools to determine absolute densities of entomophages and pests.
Monitoring entomophage populations, particularly parasitoids, may be
complicated by factors such as lack of a stable sex ratio, movement (females
must forage for often patchily distributed hosts), weather, and lack of
synchrony with host populations. However, monitoring methods to evaluate
parasitoid populations have been suggested. The most reasonable approach to
this problem would involve estimating population numbers from captures of
males or females in traps baited with an appropriate attractant, such as sex
pheromone. Powell (1986) suggested that monitoring systems be explored using
some volatile host or host habitat attractant to trap female arthropods,
thereby capturing the agent responsible for doing the parasitizing, and
perhaps obviating any problems which may arise from an unstable sex ratio. There has been no system developed whereby a
parasitoid can be monitored with sex pheromone for decision making in an
agricultural crop. Although much effort has been expended in the field of
insect sex pheromones, few studies have resulted in identification of
parasitoid sex pheromones. Robacker & Hendry (1977) identified neral and
geranial from female Itoplectis
conquisitor (Say), and
demonstrated that these chemicals were attractive to males in the laboratory.
Eller et al. (1984)
identified and demonstrated the field effectiveness of ethyl palmitoleate, a
female sex pheromone of Syndipnus
rubiginosus Walley, a
parasitoid of the yellowheaded spruce sawfly, Pikonema alaskensis
(Rohwer). Powell & King (1984) showed that males
of M. croceipes were attracted to virgin females in the field,
and diurnal activity
of males and females was found to differ. From these observations it was
believed that knowledge of parasitoid activity periods would be important in
developing techniques for sampling parasitoid populations in the field.
Subsequently it was determined that SentryR wing traps (Albany
International) were more effecting in capturing M. croceipes
males than were Pherocon II traps (Zoecon). Studies in unsprayed cotton in
1984 revealed that wing traps baited with living virgin females could be used
to estimate parasitoid populations, and recently M. croceipes
mating behavior and sex pheromone response were reported by Elzen &
Powell (1988), and a tentative identification of the female-produced sex
pheromone has been made. Control guidelines often recognize the
impact of entomophage populations on pest populations (Rude 1984). But,
explicit instructions for using entomophages in decision making are lacking,
and where present are used with reservation. Two exceptions are Michelbacher
& Smith (1943) who recommended that insecticide control decisions in
alfalfa for Colias eurytheme Boisduval be made
only after determining that the number of Apanteles
medicaginis Muesebeck
present was capable of maintaining the pest under control and Croft (1975)
reported on a decision making index for predicting the probability of adequate
control of a phytophagous mite that would occur depending ont the
predator/prey ratio per apple leaf. Vertebrates & invertebrates used in pest
aquatic insect control are more easily monitored because of their size and
confinement [ Please refer to Research #1, #2, #3 ]. Pheromone traps baited with living virgin females attracted large numbers of
male cereal aphid parasitoids when placed in cereal fields (Powell &
Zhang 1983). Aphidius rhapalosiphi DeStef and Praon volure Haliday males were caught in separate traps baited
with females. Monitoring may be useful in this situation to achieve maximum
impact when the parasitoid/aphid ratio is particularly high, especially early
in the season (Powell 1986). Methods for isolating sex pheromones (Golub
& Weatherston 1984), as well as bioassay directed fractionation,
identification (Heath 7 Tumlinson 1984), and synthesis (Sonnet 1984) are detailed.
These methods are adaptable for identification of parasitoid pheromones
(Elzen & Powell 1988). Entomophage culture is treated extensively in a different section , as previously
discussed. It is obvious that efficient and cost effective methods of rearing
entomophages must be developed if augmentative releases are to be feasible.
Large numbers of beneficial insects may be employed in greenhouses, field
cages and laboratory studies. Thousands of entomophages available at
unpredictable times, may be required for commercial augmentation.
Considerable attention has been devoted to development of techniques to
produce quality entomophages in large numbers (King & Leppla 1984). The
genetic implications of long term laboratory rearing of insects are vast
(Bouletreau 1986, Mackauer 1976). Powell & Hartley (1987) described
techniques for producing large numbers of parasitoids efficiently. These
researchers adapted a multicellular host rearing tray technique (Hartley et
ala. 1982) to rear M. croceipes and some other
parasitoids. Techniques reduced parasitoid harvest time by 1/2 and
simultaneous release of nearly 17,000 wasps was possible using low
temperature storage. Powell & Hartley (1987) also noted several factors
that were important for maintaining this large scale rearing program, and
which may be applicable to other programs. Included were (1) a continuous
host supply, (2) use of environmental chambers to alter developmental rates
of hosts and parasitoids, (3) constant appropriate environmental conditions,
(4) sanitary rearing conditions with flash sterilization of diet, (5) use of
laminar flow hoods, (6) autoclaving reusable supplies, (7) disinfecting work
areas, (8) acid or antibiotics in water or food, (9) adequate technical
support, space and equipment. Elzen & King (1999) show a list of
beneficial insects that have been reared for augmentative purposes by the U.
S. Department of Agriculture. Costs.--King et al. (1985) cited costs for release of Trichogramma pretiosum to control Heliothis spp. at US$7.68/ha.
per application. This cost compared well with the cost of a commonly used
pyrethroid, fenvalerate, at US$16.18/ha when applied at 0.11 kg/ha. The
pyrethroid had to be applied only once for every two to three parasitoid
applications. However, the development of resistance to pyrethroids by Heliothis (Luttrell et al.
1987) and to dimethoate by Lygus
lineolaris Paisot de
Beauvois (Snodgrass & Scott 1988), and in general, possible development
of resistance, makes augmentation attractive. In Vitro Rearing.--Augmentation may be commercialized only for selected
organisms for which suitable diets and storage methods are developed.
Artificial rearing would offer possibilities, but as is discussed in the
section on Entomophage Nutrition, this technology is poorly developed.
Various groups have made progress in in
vitro rearing of parasitoids
nevertheless. Over 22 entomophage species have been reared in vitro. Several Hymenoptera (1 ectoparasitoid, 4 pupal
parasitoids and 4 species of Trichogramma)
and 3 species of Diptera have been cultured with varying success (King et al.
1984). Predators have been reared on artificial diets, notably Chrysopa carnea Stephens (Vanderzant 1973, Martin et al. 1978).
While there have been numerous successes in oviposition stimulant
identification or partial rearing (Nettles & Burks 1975, Nettles 1982),
definite development of a feasible in
vitro rearing system for
entomophages has yet to be developed. Presently most parasitoids are
expensive to rear, and the costs involved would preclude mass rearing in vivo preparatory to thrifty augmentation efforts. Although
considerable advances have been made in in
vivo rearing, the advances have
not been achieved with in vitro rearing to such an
extent. The work of Wu et al. (1982) illustrates an instance in which a
completely synthetic artificial host egg was produced which contains no
insect derivatives, and supports Trichogramma
oviposition and development. Greany et al. (1984) suggested that mass rearing
of Trichogramma using
completely artificial hosts would soon become economical, however. Hymenopterous larval endoparasitoids have
not been successfully reared to the adult stage on artificial diet. However, Cotesia marginiventris and M.
croceipes have been reared
on artificial media through the first instar. Larval endoparasitoids have
evolved complex mechanisms that interact with the host's internal dynamics
and organs without damaging this environment or causing untimely death of the
host. The function of these interacting factors must be understood for in vitro rearing of larval endoparasitoids to become a
reality. Developments in artificial rearing of entomophages on artificial
diet may allow production of sufficient numbers of individuals to practically
implement the further evaluation of entomophages in biological control. Parasitoids which have been reared to the
adult stage on artificial media include the larval ectoparasitoid Exeristes roborator (F.) (Thompson 1982), the endoparasitoids of
eggs: T. pretiosum Riley (Hoffman et al.
1975) and T. dendrolimi (Wu et al. 1982); of
larvae: Lixophaga diatreae (Towns) (Grenier et
al. 1978) and Eucelatoria bryani Sabrosky (Nettles et al.
1980), and of pupae: Brachymeria
lasus (Walker) (Thompson
1983), Pachycrepoideus vindemiae Rondani (Thompson et
al. 1983), Itoplectis conquisitor (Say) (House 1978),
and Pteromalus puparum L. (Hoffman &
Ignoffo 1974). Exercise 48.1-- When should manipulation be attempted to
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