COLONIZATION OF NATURAL ENEMIES
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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.
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
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
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?
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