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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. This can be minimized by releasing large numbers of a species at each
colonization sites, 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. Success was enhanced by establishing the predator
before the thrips were found. 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). The dispersal of some parasitoids can be considerably
assisted by their hosts. 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 was 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 house flies 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 ] |