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COLONIZATION OF NATURAL ENEMIES
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Overview Colonization refers to the field
release and manipulation of imported natural enemies for their establishment,
and to favor their spread and increase in a new environment. The natural
enemy must be permanently established in at least one locality for success to
be claimed. Then this serves as a locus for its natural spread, or as a field
colony and source of material for redistribution efforts (Etzel & Legner 1999). Insectary propagation
of imported natural enemies has been circumvented on occasion by repeated
introductions of the insects from abroad, followed by their direct and
periodic release in the field. Direct releases may be necessitated by
economic considerations, difficulties of culture, or by lack of adequate
insectary facilities. Direct releases are not encouraged by some biological
control workers who maintain that insectary propagation offers several
advantages: (1) it provides adequate numbers to insure the greatest latitude
in the timing and geographical coverage of releases, (2) insectary culture
insures vigorous stocks of natural enemies, and (3) insectary propagation
affords an excellent opportunity for detailed study of the biologies and host
relationships Usually a few specimens from initial
insectary stocks of an imported natural enemy are released in the field on
the chance that these limited numbers may be adequate to achieve
establishment. Such attempts usually fail to attain establishment. It is
worth a try, however, especially as it might preserve some genetic
variability that could be lost in culture. Ecological
Factors Influencing Success or Failure Failure of natural enemies to adapt to the
climate of the release area has accounted for the largest number of
unsuccessful colonizations. It may be the result of direct natural enemy
mortality. Sometimes it is the lack of synchronization between host and
natural enemy, in temperate climates especially. Initial releases of a new species should
cover as diverse a climatic area as possible to insure that climatic
conditions most suited to that particular species are encountered. A series
of strains of the species of natural enemy ought to be tried, since some
strains will be better adapted to different climates. Alternate hosts can be important in carrying the natural enemy over
unfavorable principal host seasons. Oligophagous parasitoids may exploit a
number of host species to maintain their populations during times of
principal host scarcity. Initial releases made under varied conditions can
insure that environments frequented by suitable hosts are encountered. Already-established entomophagous species,
although less effective as natural enemies, may compete for hosts and prevent
the limited numbers of individuals of a newly liberated species from
establishing a permanent colony. Releasing large numbers of a species at each
colonization sites can minimize this, or release sites can be chosen where
competitors are rare. Host insects may be protected with field cages until
they multiply sufficiently to hold their own. Predatory arthropods or insect pathogens may
decimate and prevent the establishment of a newly-colonized species, e.g.,
The scorpion fly, Harpobittacus nigriceps, caused very
high mortality among larvae of the cinnabar moth at colonization sites and
thus prevented establishment of this moth for the biological control of the
toxic weed, tansy ragwort, in Australia. This was despite a mass rearing
program where 500,000 larvae were liberated during the 1960-61 period. Other factors of lesser concern are the
unsuitability of certain host plants as shelter for the colonized natural
enemy; a host species may be physiologically unsuited to parasitoid
development; a highly developed dispersal habit may retard or prevent
establishment. Release
Numbers There are no reliable means of estimating
the minimum number of individuals necessary to establish imported natural
enemies. Theoretically, this number may be as few as a single mated female,
yet sometimes tens of thousands were required in past efforts. Excessive difficulty in the initial
establishment of a species indicates its lack of adaptability to the new
environment and its limited promise as a biological control agent in the area
released. Clausen (1951) after careful analyses of the
most successful cases of biological control achieved to the 1950's,
formulated what has become known as his three-generation, three-year
theory:1. an effective
parasitoid or predator can be expected to show evidence of control at the
point of release within a period of three host
generations or three years' time. 2. a fully effective parasitoid or predator is always easily and
quickly established. 3. failure of a parasitoid or predator to become established
easily and quickly indicates that it will not be fully effective after its
establishment is achieved. 4. colonization of an imported parasitoid may well be
discontinued after three years if there is no evidence of establishment. Clausen admitted that establishment might be
attained by further effort, but that a species that requires such efforts
will be of little real value, and its mere establishment will not compensate
for the additional costs and labor involved. Clausen's views have been
criticized for neglecting those importations that result in a partial degree
of biological control, which at least reduces the number and amounts of
chemical treatments required. After establishment in one locality, natural
spread of a natural enemy species is usually aided by distributing
field-collected adults or parasitized hosts to new locations. Recovery may take the form of field
observations of the natural enemy (especially in the case of predators.).
Parasitoids may be reared from field-collected hosts. Dissection of
field-collected hosts may reveal parasitism, and sweep-net or suction machine
sampling for adult parasitoids and predators can reveal the species' presence
in an area. Prediction
of Success The colonization of entomophagous and
phytophagous natural enemies largely remains a matter of empirical trial and
error. Data from past efforts suggest that the probability of a newly
colonized entomophagous species becoming permanently established averages one in three. Predictive data gathered at the point of origin of the
natural enemy may require a decade of labor intensive, costly effort. Most
projects do not have adequate funds to support such studies, nor may control
be delayed for that long a time. Nevertheless, in certain cases, such as in
the biological control of weeds and medically important arthropods, lengthy
pre-introduction studies are required to preclude the introduction of harmful
species. There is continued effort being made in biological
control to devise techniques for quantitatively evaluating the effect of
natural enemies on pest populations in the field. Evidence for the occurrence
of biological control is of three major types: (1) data showing a reduction
in the pest population density invariably followed the introduction of the
natural enemy, time after time, in place after place; (2) data showing that
following the establishment of a natural enemy, the pest population remained
at a much lower average density than before the establishment of the natural
enemy; and (3) data showing a decidedly higher survivorship of the pest when
it was protected from attack by the natural enemy. Some newer approaches that have resulted in
variable success are: (1) attempts to correlate host and natural enemy
population dynamics; (2) analyses of life table data; (3) experimental
methods; (4) mechanical and chemical exclusion; (5) trap-method; (6) hand
removal exclusion method; (7) biological check method (= use of ants to
interfere with natural enemies); and (8) naturally-occurring exclusion. Methods.--The ease of insectary culture cannot be correlated with
ease of establishment. In analyzing the successful biological control of the
alfalfa blotch leafminer, Agromyza frontella (Rondani)
in the northeastern United States, Drea & Hendrickson (1986) noted that
none of the most abundant European parasitoids became established.
Successfully introduced parasitoids were obtained by laboriously collecting
30-40,000 host puparia in Europe, and subjecting them to specially developed
laboratory recovery techniques in order to obtain healthy individuals for
field release. Working with the same leafminer, Harcourt et
al. (1988) directly field released a genetically diverse group of 586 adults
of the braconid Dacnusa dryas (Nixon) in eastern
Ontario. The release site was then used as a field nursery for parasitoid reproduction, with specimens
collected and released at various other sites. Within three years the
parasitoid had reduced leafminer populations 50-fold, followed by a general
collapse to noneconomic levels. This parasitoid had a high dispersal
capacity, host specificity and adaptability to diverse environmental
conditions and synchronized well with the host life cycle. Complete biological control of the citrus
mealybug was obtained in southern India by introducing the encyrtid
parasitoid Leptomastix dactylopii Howard (Krishnamoorthy
& Singh 1987). Field colonization was repeated 9-24 times over a short
period of 2-4 months. Two orchards received 11,394 and 26,380 adults of the
parasitoid. The discovery of the citrus blackfly in
Barbados in 1964 prompted quick biological control importations in the same
year before the fly reached problematic levels. The aphelinid parasitoids Eretomocerus
serius Silvestri and Prospaltella clypealis
Silvestri increased rapidly and controlled the blackfly within nine months
(Bennett 1966, Bennett & van Wherlin 1966, Clausen 1977). Another example of rapid control was that of
the southern green stinkbug, which first appeared in Hawaii in late 1961. The
parasitic scelionid Trissolcus basalis and the tachinid Trichopoda
pennipes pilipes (Fab.) were imported in 1962 and
controlled the pest by 1965 (Clausen 1977). Field releases may consist of immature
rather than mature entomophages. Katsoyannon & Argyriou (1985) released
the aphelinid Prospaltella perniciosi Tower against the
San Jose scale, Quadraspidiotus perniciosus Comstock, by
suspending squash fruit infested with parasitized scales in almond orchards.
Kfir et al. (1985) suspended small logs heavily infested with black-pine
aphids that were parasitized by the aphelinid Pauesia sp., in
trees at a height of 1.5-2 m for field colonization. They found that spread
and establishment were rapid due to the high dispersal rate and searching
ability of Pauesia. In addition to its utility in classical
biological control a field insectary
is particularly useful for inoculative augmentation, where early season
releases of small numbers of entomophages at key location achieve effective
biological control. This is advisable when the entomophage has a short life
cycle, high fecundity and great vagility, yet cannot persist year-round. An
example is found in Pediobius foveolatus on Mexican bean
beetles in small areas of snap beans, Phaseolus vulgaris,
from which they spread to adjacent soybean fields (Stevens et al. 1975, King
& Morrison 1984). Inoculative releases of this same parasitoid protected
urban gardens from damage by the Mexican bean beetle (Barrows & Hooker
1981). Similarly, the parasitoid Aphidius smithi was
reared in field cages on the pea aphid, and the progeny were allowed to
spread to adjacent alfalfa (Halfhill & Featherston 1973). Stinner (1977) reviewed the efficacy of
inundative releases, and Goodenough (1984) improved packaging and
distribution equipment, materials and procedures for releasing the egg
parasitoid Trichogramma praetiosum (also see Reeves
1975, Jones et al. 1977, Jones et al. 1979, Bouse et al. 1980, 1981). Aircraft
liberations of entomophagous parasitoids have occurred with Trichogramma
spp. (Ridgway et al. 1977, Bouse et al. 1981), with Lixophaga diatraea
(Ridgway et al. 1977) and Chelonus spp. in cotton fields (E. F.
Legner, unpub.). Aerial release technology has also been developed for
liberations of the cassava mealybug parasitoid, Epidinocarsis lopezi,
and of cassava green mite predators, since ground release would be a major
obstacle to controlling the pests in the huge African cassava belt (Herren
1987). Notable features of these systems are an automatic acceleration of the
parasitoids in the release device prior to ejection to reduce effects of
deceleration outside the aircraft, and a streamered container for predaceous
mites that is retained in the cassava plant canopy for effective mite
dispersal (Herren et al. 1987). Inoculative and inundative releases of
biological control agents are now rather common in glasshouses. Hansen (1988)
showed that cucumbers grown in glasshouses could be effectively protected
from the onion thrips, Thrips tabaci Lindeman, by 3-4
releases of the predatory phytoseiid mite Amblyseius barkeri
(Hughes), at rates of 300-600/m2. Establishing the predator before
the thrips were found enhanced success. Periodic colonization of the aphelinid Encarsia
formosa Gahan was successful against the greenhouse whitefly, Trialeurodes
vaporariorum (Westwood), in Canada (Clausen 1977). In Australia
this parasitoid has become permanently established both in glasshouses and
outdoors, in some areas. King & Morrison (1984) noted that E. formosa
is extensively used in Europe in augmentive control of the greenhouse
whitefly. Gerling (1966) determined that temperatures above 24°C were
necessary for the parasitoid to control the whitefly. Sampling
& Dissemination In order to increase the distribution of Praon
palitans and Trioxys utilis Muesebeck on
their host the spotted alfalfa aphid, alfalfa cuttings and mechanical sweeper
collections were utilized (van den Bosch et al. 1959, Clausen 1977). One of the largest collection and
distribution programs in the history of biological control occurred in Mexico
in 1950-1953, when several species of parasitoids from the Indian Peninsula
were imported against citrus blackfly. A special gasoline tax was levied to
support this program, which reached a peak employment of 1,600 workers
(Clausen 1977). Difficulties with mass production make
collection and distribution programs particularly desirable. Harris &
Okamoto (1983) reported that the braconid fruitfly parasitoid Biosteres
oophilus (Fullaway) could not be reared in large numbers because
of sex ratio problems in culture. A method was developed for parasitoid
distribution utilizing existing field populations. Papaya fruits exposed in
the laboratory to oriental fruit flies, Dacus dorsalis
Hendel, were subsequently exposed in the field for 24 hours to effect
parasitization. The fruit fly larvae were placed on a diet in the laboratory,
and resulting puparia were recovered for parasitoid emergence. This method allowed
one technician to process over 11,000 parasitoids per day. In medical entomology special sampling
devices have been developed (CLICK HERE). Small scale collection and distribution can
also be effect, however. Campbell (1975) developed a simple technique for
citrus growers to distribute Aphytis melinus for red
scale control. A basket with scale-infested oranges was placed in an orchard
where the parasitoid was active. Two weeks later half the oranges were
replaced and taken to new orchards for colonization. Native beneficial arthropods may also be
successfully redistributed. The predaceous phytoseiid Euseius hibisci
(Chant) was easily colonized in citrus orchards against the citrus thrips by
transferring orange branch terminals infested with the predator to six
centrally located trees per 4 ha. The mite readily dispersed aerially among
groves within one season, resulting in a dramatic reduction of insecticide
treatments (Tanigoshi & Griffiths 1982, Tanigoshi et al. 1985). Another
method of field colonizing this predator was to place bundles of lima bean
seedlings containing the laboratory reared mite, into crotches of citrus
trees. Certain caution must be exercised in the distribution of established
entomophages in order to avoid the simultaneous dispersal of pest insects,
hyperparasitoids and other unwanted organisms. In the Australian biological
control program against black scale, many indigenous species were transferred
around the country, including predaceous coccinellids and lepidopterans.
Unfortunately, the native hyperparasitoids Quaylea whittieri
(Girault) and Myiocnema comperei Ashmead were
distributed as well (Clausen 1977). Principal
Factors Influencing Establishment Species and Strains.--It is not unusual for an entomophagous species to have
strains which vary in characteristics such as climatic or host population
adaptation. For example, strain differences between populations of the
tachinid Lixophaga diatraeae were demonstrated by King
et al. (1978). Consequently the same species of entomophage may be sought
from many different areas and the different collections reared separately to
maximize biological control. Obrycki
et al. (1987) reported
that there are two observed biotypes of the eulophid Edovum puttleri
Gressell, an egg parasitoid of the Colorado potato beetle. It was believed
that matching biotypes to the agronomic and climatic conditions of the
release areas would be important in achieving maximum control. Harrison et al. (1985) stressed the
importance of precise taxonomic identification and biological testing of Trichogramma
spp. before mass production for inundative releases. They found that T.
pretiosum was preferable to T. exiguum Pinto
& Platner for augmentive control of Heliothis spp. on
cotton in the central Mississippi delta area because T. pretiosum
could develop at the 35°C temperatures common in that area. Climate and Weather.--Researchers generally make every effort to obtain
entomophages from areas with climates similar to those at the release sites.
The importation of two climatic strains of the parasitic braconid Trioxys
pallidus (Haliday) to control the walnut aphid, Chromaphis
juglandicola (Kaltenbach) discussed earlier is a classic example. Current weather is likewise important in
parasitoid releases. Laboratory experiments by Gross (1988) determined that
unfavorable temperatures, relative humidities and levels of free water at
eclosion could have pronounced adverse effects on emergence of the egg
parasitoid Trichogramma pretiosum. He noted the
importance of identifying these effects for Trichogramma
emergence at field liberation sites. The commonly erratic results of Trichogramma
releases might well be due to inattention to such factors (Gross (1988). Releases of the coccinellid Chilocorus
bipustulatus L. against the white date scale, Parlatoria
blanchardi (Targioni-Tozzetti) in date palm oases at 700-1600 m
elevation in northern Niger, were most successful during the rainy season (Stansly
1984). Smith (1988) considered the effect of wind and other factors on the
fate of Trichogramma minutum released inundatively
against the spruce budworm. Yu & Luck (1988) referred to the use of
temperature-dependent, stage-specific developmental rates for timing
parasitoid releases. Habitats.--If a pest insect attacks a variety of plants, both economic
and noneconomic, it is well to attempt to establish natural enemies on as
many of the alternative plant hosts as possible to increase reservoir populations
where they will be unaffected by pesticides (Argyriou 1981). Adaptation.--Poor adaptation of parasitoids to specific host races can
cause failure in field colonization. Such was the case when the encyrtid Metaphycus
luteolus (Timberlake) from California would not adapt to the brown
soft scale in Texas (Clausen). In inundative release programs especially it
must first be determined if the released entomophage is well suited to
attacking the intended host. For example, in South Africa the egg parasitoid Trichogramma
pretiosum was mass-produced and liberated against Heliothis
armigera, but with poor results caused at least in part by a
generally unsuitable host (Kfir 1981). Dispersal.--Entomophage dispersal varies greatly between species.
However, even entomophages that disperse slowly can be effective biological
control agents, as shown earlier with the Rhodesgrass scale parasitoid, Neodusmetia.
Also the red wax scale, Ceroplastes rubens Maskell,
which is serious on citrus in Japan, was controlled successfully by the
encyrtid Anicetus beneficus Ishii & Yasumatsu,
although its spread naturally at the rate of only one mile in two years
(Clausen). Their hosts can considerably assist the
dispersal of some parasitoids. As Clausen (1977) noted, the occurrence of
alate females in many species of aphids can greatly facilitate dispersal of
early parasitoid stages carried in their bodies. Praon palitans
is rapidly dispersed because it frequently parasitizes the winged adult of its
host, the spotted alfalfa aphid, and is carried as an immature form for long
distances during aphid migratory flights. Trioxys utilis,
on the other hand, depends mainly on its own locomotion for dispersal since
it usually kills its host before the aphid can reach the winged stage
(Schliner & Hall 1959). The encyrtid Anagyrus indicus
Shafee dispersed as much as 61 km in one year after it was released in Jordan
against the spherical mealybug, Nipaecoccus viridis
(Newstead), a citrus pest (Meyerdirk et al. 1988). Releases of 41,054 had
been made over an 18-month period, some directly into the trees and some into
organdy sleeves that were tied around infested branches. Problems of dispersion can occur with
releases of entomophages in field augmentation programs as well as in
glasshouses. It is well known that augmentive releases of the coccinellid Hippodamia
convergens Guérin in California field crops are useless because
the beetles immediately leave the release sites (DeBach & Hagen 1964). In
commercial glasshouses problems of obtaining even dispersion of coccinellids
and chrysopids make them unsuitable for augmentive biological control
(Chambers 1986). Augmentive releases of the tachinid Lixophaga
diatraeae against the sugarcane borer, the parasitoid resulted in
rapid dispersal from the release sites, which negated the effects expected
from releasing mated females at a different rate (King et al. 1981). There
was some indication that parasitoids remained more confined to sugarcane
fields that were surrounded by woodlands. Trichogramma pretiosum was
found to produce significantly higher parasitization rates on corn earworm
eggs on field peas and cotton when they had prerelease exposures to corn
earworm eggs in the laboratory (Gross et al. 1981). This led to discussions
of the possible use of kairomones when parasitoids were released to improve
their efficiency. Numbers & Generation Time.--There is a general desire to release as many entomophages
at a site as possible. Beirne (1975) declared that biological control
projects in Canada were much more successful when >800 individuals were
released per liberation. However, large numbers of some entomophages are
difficult to obtain, which invariably makes establishment more burdensome. Laricobius
erichsonii (Rosenhauer), a derodontid predator of the balsam
woolly adelgid, Adelges piceae (Ratzeburg), not only has one
generation a year but also a very slow annual dispersal rate. Considerably
more effort was therefore required for its establishment (Clausen 1977). Some entomophages that are released in small
numbers have become rapidly established. Such was the case with the encyrtid
parasitoids Metaphycus stanleyi (Compere), M. helvolus
(Compere) and M. lounsbury (Howard), and the pteromalid Scutellista
cyanea Motschulsky, against the black scale in southern
California. However, large numbers of the encyrtid Diversinervus elegans
Silvestri had to be released before recoveries were made, which was then
followed by rapid spread (Clausen 1977). The braconid Apanteles pedias
Nixon was established on the spotted tentiform leafminer, Phyllonorycter
blancardella (F.), in Ontario by releasing only two females in
May of 1978. By autumn of 1979 parasitization at the original site had
reached 25.7% and the parasitoid was recovered 43 km away (Laing & Heraty
1981). Females were placed in a fine mesh sleeve cage over susceptible hosts
on apple branches for parasitization. High reproductive rate and dispersal
were two factors that enabled establishment from such a small release. Drea & Hendrickson (1988) attributed
successful control of the alfalfa blotch leafminer in the northeastern United
States with a colonization procedure that emphasized timing, environmental
conditions and parasitoid numbers. Periodic releases throughout the growing
season were achieved by scheduling removal of groups of parasitized puparia
from diapause to an emergence environment. When parasitoids were released in
an area where alfalfa harvesting was staggered, susceptible hosts were always
present. Adequate numbers is
nevertheless a vague term. The case of the alfalfa blotch leafminer required
releases of very small numbers of the two parasitoid species that became the
most important in regulation. During 1977-78, only 3,307 Chrysocharis
punctifacies Delucchi and 5,207 Dacnusa dryas
were liberated at the original release fields. Drea & Hendrickson (1986)
used a dribble release
technique in which only a few dozen parasitoids were released weekly. They
felt that repeated releases were more important than large numbers at any one
time. Fabre & Rabasse (1987) obtained
establishment of the aphidiid Pauesia cedrobii Stary
& Leclant by inserting 225 adults per sleeve cage placed on cedar
branches with colonies of the cedar aphid, Cedrobium laportei
Rem. Furuhashi & Nishino (1983) released 100
adults of the aphelinid Aphytis yanonensis DeBach &
Rosen on each of three trees in citrus groves on two occasions to combat the
arrowhead scale, Unaspis yanonensis Kuwana. Within six
months the scale had declined markedly and parasitism reached 80%. The time of year can affect parasitoid
release numbers. Campbell (1976) reported that successful establishment of
the California red scale parasitoid Aphytis melinus in
the Riverland district of South Australia required colonizing a minimum of
100 adult wasps into ever third tree in every third row of a citrus orchard
in summer and early autumn; but in cooler weather in later autumn,
establishment required the release of 1,000 adults per tree. Widespread
establishment of the same parasitoid in the Sunraysia district of New South
Wales was easily achieved by about 50 small number releases, each consisting
of only 100-300 parasitoids per tree, or by placing pumpkins covered with
parasitized hosts in citrus trees. Augmentive inoculative releases of small
numbers of parasitoids or predators can also be successful. Releases of the
phytoseiid mite Metaseiulus (Typhlodromus) occidentalis
(Nesbitt) at the rate of only 64/tree early in the season resulted in
effective control of the spider mite Tetranychus mcdanieli
McGregor (Croft & McMurtry 1972, McMurtry et al. 1984). However, releases
of nine species of phytoseiid mites at rates of 1,200 mites/tree over four
weeks to control the avocado brown mite, Oligonychus punicae
(Hirst), were unsuccessful ((McMurtry et al. 1984). Pickett & Gilstrap
(1986) controlled Banks grass mites, Oligonychus pratensis
(Banks), and two-spotted spider mite on corn in Texas by making early season
inoculative releases of the phytoseiid mites Phytoseiulus persimilis
and Amblyseius californicus (McGregor). However, they
noted that the cost of production and application of the predaceous mites
would have to be reduced to make the procedure commercially feasible. In a
glasshouse environment Rasmy & Ellaithy (1988) effectively controlled
two-spotted mite on cucumbers by releasing 10 predatory Phytoseiulus
persimilis per plant at the first sign of spider mite damage. In general the releases of most entomophages
used in augmentive biological control require large numbers. While field-testing
the effectiveness of mass produced Trichogramma strains, Hassan
et al. (1988) released 400-9,000 parasitoids in four to six treatments per
apple tree to control the codling moth and the summer fruit tortrix, Adoxophyes
orana. When Trichogramma pretiosum
was used against Heliothis spp. on cotton, Johnson (1985) was
unable to increase field parasitism by three low-level releases, two at
12,500 per ha. followed by one at 37,500 per ha, at 7-day intervals. Meadow et al. (1985) noted that augmentive
releases of the predaceous cecidomyiid Aphidoletes aphidimyza
(Rondani) had only been done on a large scale in glasshouses in Finland and
the Soviet Union. They experimented with control of the green peach aphid, Myzus
persicae (Sulzer). In small plots of tomatoes and peppers in
glasshouses and the field, effective control was achieved at varying rates. Stenseth & Aase (1983) investigated the
numbers of Encarsia formosa required to control
greenhouse whitefly on cucumbers in Norwegian glasshouses. Three introductions
of five parasitoids per plant at fortnightly intervals would result in
adequate control of an initial number of 10-30 adult whiteflies per 100
plants, whereas at lesser host densities only three parasitoids per plant
were required. It was noted that parasitoid introduction before March 1st in
Norwegian glasshouses was not successful on account of the deleterious effect
of low light intensity on parasitoid reproduction. Van de Veire & Vacante (1984) released
the same parasitoid on glasshouse tomatoes by hanging 40 paper discs, each
with ca. 110 parasitized whitefly pupae at intervals in an area of 1,500 m2.
This suggested rate was in accordance with the recommendation of Woets (1978)
for greenhouse whitefly control (also see Woets 1973, and Woets & Van
Lenteren 1976). Rutz & Axtell (1981) reported that
weekly releases of a native strain of Muscidifurax raptor
caused a significant reduction in the house fly population at a poultry farm.
Releases were made at the rate of five parasitoids per bird per week (150,000
parasitoids per week) by placing parasitized house fly pupae at 10 to 15
spots on the manure in each poultry house. Kfir (1981) noted that the common practice
of citing the total number of Trichogramma released per unit of
crop is meaningless without specifying the sex ratio. Biotic Interactions.--Several methods are developed to enhance the interaction
between a pathogen, parasitoid or predator and the organism it attacks. It is
usually advantageous to release a beneficial organism at a time when the
susceptible stage of its host is present in greatest numbers. For example,
Nechols & Kikuchi (1985) recommended that field releases of the encyrtid Anagyrus
indicus Shafee should be made when the third nymphal stage of the
host, the spherical mealybug Nipaecoccus vastator
(Mask.), is the most numerous in order to provide the longest exposure period
for the most suitable host stages. In augmentive biological control efforts it
may actually be desirable to release host material along with the beneficial
organism to increase the beneficial population. In a laboratory experiment
Nickle & Hagstrum (1981) successfully increased numbers of the braconid Bracon
hebetor Say in a simulated peanut warehouse by releasing the
parasitoid together with preparalyzed host individuals of the almond moth. In
a glasshouse system Parr (1972) placed spider mites on cucurbits to allow the
predaceous phytoseiid Phytoseiulus persimilis to
increase its population in time to control the increase of the endemic spider
mit population. For the control of filth flies in dairies, Petersen (1986)
made early season releases of unparasitized freeze-killed house fly pupae, as
well as house fly pupae that were parasitized with the pteromalid Muscidifurax
zaraptor. The freeze-killed pupae, which remained suitable as
hosts for four weeks in the spring, apparently provided substrate for
sufficient parasitoid population increase to effective control houseflies and
stable flies in the dairies. As another example of this technique, releases of
a field crop insect, the imported cabbageworm, together with two parasitoids
early in the growing season, successfully reduced pest damage (Parker &
Pinnell 1972). Field colonization of exotic parasitoids may
be complicated by competition with native parasitoids, as could have been the
case with parasitoid releases against the beet leafhopper, Circulifer
tenellus (Baker) in the Imperial Valley of California (Clausen
1977). McMurtry et al. (1984) suggested that competition
with or interference by the native predator Euseius hibisci
may have limited the abundance of nine species of phytoseiids that were
augmentively released at 1,200 mites/tree to control the avocado brown mite,
since average densities of the brown mite and of the total phytoseiids were
not significantly affected by the releases. However, in an orchard with few
phytoseiids, Penman & Chapman (1980) were able to control the European
red mite, Panonychus ulmi (Koch) with releases of the
phytoseiid Amblyseius fallacis (Garman) at 300/tree. Other interactions such as predation and
cannibalism can also pose problems. Dreistadt et al. (1986) reported that efficacy of inundatively releasing eggs
of the common green lacewing to suppress the tuliptree aphid, Illinoia
liriodendri (Monell), was prevented by ant predation, cannibalism,
highly variable viability of the commercially produced green lacewing eggs
and lacewing larval entrapment on the sticky release tapes used. Effects of host plants on entomophages
constitute another factor in the success of a project, For example, Ekborn
(1977) noted that methods for using Encarsia formosa for
controlling the greenhouse whitefly were more effective on tomatoes than on
cucumbers. Gould et al. (1975) discussed techniques for using E. formosa.
Host plants can affect entomophages
indirectly through determination of the phenology of the host. For example,
Schaefer et al. (1983) colonized Pediobius foveolatus
against the Mexican bean beetle by placing parasitized larval mummies in
nurse plots near soybean fields. The nurse plots were planted with locally
adapted snapbeans, or with mixtures of snapbeans and soybeans, prior to
normal planting dates to provide early reservoirs for bean beetle population
buildup and the subsequent early increase of parasitoids. Autoparasitism.--Complications in field colonization can be caused by the
habit of autoparasitism, as was illustrated earlier in the aphelinids with
hyperparasitic males to control armored scales. Special colonization
procedures, such as successive releases of mated and unmated females, are
required (Clausen 1977). Exercise 30.1--What numbers are generally sought for in efforts to
establish a newly imported
entomophage? Exercise 30.2--Give an example of where field colonization of hosts
enhances entomophage multiplication. Exercise 30.3--How can weather affect entomophage establishment
during liberations? Exercise 30.4-- Compare direct releases of a natural enemy species
with insectary reared material. Exercise 30.5-- Discuss some ecological factors that influence
success or failure of colonization. Exercise 30.6-- How many individuals of a natural enemy species
should be released during colonization attempts? Exercise 30.7-- How may recoveries of a natural enemy species be
made? Exercise 30.8-- How might you predict the outcome of colonization
attempts? Exercise 30.9-- Following the successful colonization of an imported
natural enemy, how may the degree
of control be evaluated? REFERENCES: Please refer to <bc-30.ref.htm> [ Additional references may be found
at MELVYL
Library ] |