ENHANCING NATURAL ENEMY IMPACT
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Elzen & King (1999) discussed the manipulation of natural enemies for enhanced biological control. The successes in classical biological control have provided the background and encouragement for efforts in the manipulation of natural enemies. Such manipulations include conservation, augmentation, habitat management and genetic manipulation. During the 1980's there has been increased emphasis on the use of semiochemicals to manipulate natural enemies, especially Hymenoptera (Nordlund et al. 1981a). Also, insecticides are being stressed that minimize direct toxic and sub lethal effects to beneficial insects. The use of biological control together with insecticides is encouraging.
The enhancement of entomophage effectiveness has been reviewed in various ways by Ridgway & Vinson (1977) and Ables & Ridgway (1981). Propagation and release of entomophagous arthropods for use in augmentation was discussed by King et al. (1984) and this practice in the United States was reviewed by King et al. (1985a). Additionally the behavior of liberated parasitoids and predators was discussed by Weseloh (1984). Lewis & Nordlund (1985) stressed the importance of insect behavior in order to enhance natural enemy effectiveness. Literature on semiochemicals was discussed by Nordlund et al. (1981a,b; 1985) and Vinson (1975). Habitat manipulation to enhance parasitoid activity was reviewed by Powell (1986).
Entomophages may be potentially manipulated in many ways. The concepts of inundative and inoculative releases were first mentioned by DeBach & Hagen (1964). Inundative releases rely mainly on the agents released, not their progeny, whereas inoculative releases rely upon a buildup of the initial parasitoid populations so that immediate control is followed by additional control wrought by progeny (Li 1984). Augmentative releases have been described as supplemental releases, strategic releases, programmed releases, seasonal colonization, periodic colonization and compensatory releases (Ridgway et al. 1977, King et al. 1984, King et al. 1985a). Elzen & King (1999) give examples demonstrating the feasibility of controlling pests by augmentative releases of entomophages. Individual case studies were presented by King et al. (1985a).
Manipulation refers to those procedures that help the establishment and activity of natural enemies. Manipulation of a natural enemy or its environment may be justified if a definite need exists and a reasonable assurance of success is possible. Certain factors associated with the habitat, the host, or the natural enemy itself may render an entomophagous organism ineffective as a biological control agent, but still be subject to manipulation.
The habitat may have certain adverse climatic factors, such as heat, cold, low humidity or wind. Unattractive or otherwise unsuitable host plants may be present, or there may be a scarcity of food or water for adult natural enemies. Interspecific competition among natural enemies. Pesticides may be present, or cultural practices may not favor natural enemy activity.
The host may lack synchronization with parasitoid generations, or host plant resistance may not provide for hosts during critical times. There may be host strains resistant to natural enemy attack, or there may be periodic scarcities of suitable host stages.
The natural enemy may exhibit an annual ovarian diapause and migrate away from its hosts at certain times of the year (e.g., Coccinellidae), or its reproductive rate may be too low. It may exhibit an adverse tendency to disperse, coupled with an inability to find mates at the resulting low densities.
Generally, manipulation of a natural enemy should only be attempted if it involves some periodically occurring, unfavorable environmental factor, a lack of some easily supplied requisite, or some simple or minor, but correctable, intrinsic shortcoming.
Manipulative Methods Employed
Periodic Colonization involves periodic releases of mass-produced or field-collected natural enemies. Two types are inundative releases and inoculative releases. The first type, inundation, has largely been employed against the egg stage of univoltine pests. Control is largely the work of the insects released, not their progeny. It has been called a biotic insecticide since host mortality is more or less immediate, and there is no prolonged interaction between host and natural enemy populations. This method is best employed against pests of high value crops, against univoltine pests, or against multivoltine pests that reach injurious levels during but one generation annually.
The second, inoculation, is where the interaction between host and natural enemy populations persists through more than one generation of the natural enemy, and control is largely effected by the progeny of the beneficial forms released. Inoculative releases may take the form of accretive releases where small numbers of natural enemies are periodically released against low density pest populations. Entomophagous insects and their pest hosts may also be colonized concurrently in areas with a known history of pest invasions or where hosts are too scarce to support natural enemies, this in anticipation of pest invasions (e.g., Cryptolaemus on citrus mealybugs in California).
Selective Breeding is not a practical method to date, but offers a challenging field for research.
Environmental Manipulation may supply artificial structures that serve as shelters or as nesting sites for natural enemies. Supplemental food for adult natural enemies may be supplied. Alternate hosts may be supplied for beneficial insects or their phytophagous hosts may be offered alternate host plants. Artificially supplying suitable host stages when these are unavailable in the field, and eliminating honeydew-feeding ants may also be effective. The habitat may be modified to eliminate or reduce the adverse effects of cultural practices, pesticides, dust deposits, etc.
Various examples in DeBach (1963) and Rabb (1962) describe how the construction of nesting shelters encouraged high local populations of Polistes wasps in cotton fields in the West Indies and in tobacco fields in North Carolina, increasing the total predation of injurious lepidopterous larvae.
Nesting boxes provided for insectivorous birds in some intensively managed European forests also resulted in increased predator densities and protection from defoliating insects. Many adult natural enemies utilize exudates from floral or extra-floral nectaries, as well as pollen, as sources of nutrients and water. The culture or conservation of plant food sources in the proximity of cropland and orchards has been found to enhance the effectiveness of various natural enemies. Pollen is known to be an important supplementary food for adult, aphid-feeding Syrphidae and Coccinellidae as well as certain predacious mites.
The long-practiced method of clean cultivation for weed control may be undesirable from the standpoint of removing wild plants infested with honeydew-producing insects or containing nectaries. Colonization of alternative insect hosts may improve synchronization between a pest and its natural enemies. Several benefits that may be derived from this technique are: (1) the damping of extreme oscillations in natural enemy and host population densities; (2) maintaining functional natural enemy populations by providing a continuous food supply during periods of low pest densities; (3) providing suitable overwintering hosts; (4) promoting maximum distribution of the natural enemy; and (5) reducing intra- and interspecific competition among natural enemies (cannibalism and combat).
Modifications of adverse cultural practices can improve natural control because cultivation may kill soil-inhabiting beneficial insects or pupating, non-subterranean natural enemies. Reduced or delayed cultivation may reduce this mortality and also dust. Dust is especially known to harm parasitoids and predators; it can be minimized by sprinkling, by planting cover crops, by paving access roads or by holding cultivation to a minimum. Properly timed irrigation may promote epidemics of fungal pathogens of insect pests by providing the proper conditions of humidity in the microenvironment. Improperly timed irrigation, on the other hand, may drown or drive away beneficial insects.
Trichogramma spp. have been extensively researched for inundation since Flanders (1930) suggested that they offer a possible alternative to insecticides. Comprehensive reviews on the use of Trichogramma were presented by Ridgway & Vinson (1977), Ridgway et al. (1977) for use in the Western Hemisphere, by Huffaker (1977) for China, by Belyarov & Smetnik (1977) for the Soviet Union, and for augmentation in cotton by King et al. (1985). By 1985 Trichogramma spp. were the most widely used entomophages for augmentation (King et al. 1985). Lepidopterous pest control by mass rearing and release of Trichogramma spp. has carried out for many decades. The pioneering research of Howard & Fiske (1911) and Flanders (1929, 1930) in the United States stimulated research with Trichogramma spp. worldwide, and a number of successes in reducing insect populations by augmentation with Trichogramma have been reported. Hassan (1982) and Bigler (1983, 1984) reported 65-93% reduction in larval infestations of the European corn borer following Trichogramma releases during the 1970's in Germany and Switzerland. Voronin and Grinbert (1981) reported positive reductions of pest such as Loxostege spp, Agrotis spp., and Ostrinia species following Trichogramma releases. In China a significant reduction in populations were reported for Ostrinia spp., Heliothis spp. and Cnaphalocrocis spp., crop damage being reduced (Li 1984).
Oatman & Platner (1985) found that two common lepidopterous pests of avocado in southern California, Amorbia cuneana Walsingham and the omnivorous looper, Sabulodes aegrotata (Guenée), could be effectively controlled by liberations of 50,000 Trichogramma platneri in each of four uniformly spaced trees per acre. At least three weekly releases were required for control of S. aegrotata, while two were necessary for A. cuneana.
Considerable success has been achieved in California with the periodic introduction of cichlid fish and some invertebrate predators of mosquitoes and midges in connection with biological control of aquatic weeds and pestiferous insects [Please refer to Research #1, #2, #3 ]
Hassan (1982) obtained 65-93% reduction in larval infestations of Ostrinia nubialis (Hübner) after four years of releases in Germany. Reduction in insect density and crop damage in several agroecosystems was also reported from China and the Soviet Union (Li 1982, Voronin & Grinberg 1981). Oatman & Planter (1971, 1978) demonstrated the feasibility of augmenting Trichogramma pretiosum to reduce damage in tomatoes caused by the tomato fruitworm, cabbage looper and Manduca spp., although they found that chemical control was also necessary for pests not susceptible to T. pretiosum. Oatman et al. (1983) reported on an integrated control program for the tomato fruitworm and other lepidopterous pests on summer plantings of fresh market tomatoes in southern California in 1978-1979. Twice weekly applications of DipelR (delta-endotoxin of Bacillus thuringiensis Berlinger var. kurstaki), plus twice weekly releases of T. pretiosum, were compared with weekly applications of methomyl. There were no significant differences in fruit yield or size between the two control regimes. Methomyl adversely affected predator populations, host eggs, and egg parasitization by T. pretiosum, whereas Dipel did not.
In The Netherlands, Trichogramma spp. have been utilized to develop biological control of Lepidoptera. Two approaches there were (1) the selection of the best species and strains of Trichogramma (van Lenteren et al. 1982) and studies of the manipulation of Trichogramma behavior. The first approach has been studied extensively, especially in Brassica spp. Inundative releases of Trichogramma were feasible for control of Mamestra brassica on Brussels sprouts, but control was not very effective at low host densities (van der Schaaf et al.a 1984). Glas et al. (1981) reported reduction in larval infestations of Plutella xylostella in cabbage crops. Van Heinigen et al. (1985) summarized several years of work with Trichogramma releases. The second approach (2) to Trichogramma manipulation in The Netherlands involved examination of semiochemical mediated behavior. These studies indicated that kairomones and volatile substances released by adult female hosts (sex pheromones) were important in foraging behavior of Trichogramma (Noldus & van Lenteren 1983, Nodlus et al. 1986, 1987). Preintroductory evaluation using the methods outlined by Wackers et al. (1987) may improve prospects for augmentative release of specific strains of Trichogramma in The Netherlands.
It is evident that control can occur through augmentation with Trichogramma spp. under certain conditions. However, there have been variable results and cases of insufficient pest control reported (King et al. 1985b). Trichogramma pretiosum was tested in augmentative releases in Arkansas in 1981-82 and in North Carolina in 1983 for management of H. zea and H. virescens in cotton. These releases failed to provide adequate control in 1981-82, but in 1983 cotton fields treated by seven augmentative releases of T. pretiosum at 306,000 emerged adults/ha./release yielded significantly more cotton than control fields which were not treated with insecticides. Insecticidal control fields yielded more cotton than did control of T. pretiosum release fields, which led these researchers to conclude that management of Heliothis spp. in cotton by augmentative releases with this parasitoid was not economically feasible (King et al. 1985b). However, the greater yields obtained in North Carolina in 1983 in the T. pretiosum release fields supported the use of Trichogramma spp.
In order to obtain consistent results, large numbers of Trichogramma spp. should be released. However, in addition the effectiveness of Trichogramma may certainly be influenced by such factors as, (1) the density and/or phenology of the pest, (2) the species or strain of Trichogramma, (3) vigor of the parasitoids, (4) method of distribution, (5) crop phenology, (6) number of other natural control agents present, and (7) the proximity to crops receiving insecticides and drift of insecticides into Trichogramma release fields (King et al. 1985b). Trichogramma spp. seem highly susceptible to most chemical insecticides, with lethal effects resulting from direct exposure to spray applications, drift or posttreatment contact with pesticide residues on foliage (Bull & Coleman 1985). It has been suggested that the inconsistent results in Arkansas and North Carolina was due to chemical insecticides (King et al. 1985b).
In order to effectively manipulate entomophages there must be a thorough knowledge of the biology and host associations of the organisms. Although such information may be gained through laboratory studies, it is necessary that such data be followed by studies in the field. Ideally a comparison of laboratory, field cage and field studies can provide useful information which could be used to predict the impact of the natural enemy on pest populations.
In augmentation programs, the level of control achieved may be influenced by many interacting factors. Primary factors include the availability of hosts and host/parasitoid synchrony; conditions of weather during release of entomophages, including the effect of environmental factors on foraging; influence of habitat type; chemical pesticide usage, either concurrent or not or adjacent; the fitness of laboratory reared parasitoids. The use of augmentative releases is very complex, and the environmental effects acting on released entomophages may be highly variable. Therefore, studies must be planned that will be used to predict when and under which specific situations biological control by augmentation may work. For example, Microplitis croceipes (Cresson) efficiency appears to be greater during summer on agricultural crops than in spring on wild host plants. Fewer M. croceipes adult females were required in a summer study as compared to an early spring study to achieve comparable control of tobacco budworm in field cage experiments. All parameters, biotic and abiotic should be explored in evaluating augmentation release results. Augmentation of entomophages of row crop pests may be implemented only after considerable effort has been expended to prove the feasibility of this approach. Therefore, the efficiency and financial benefits must be determined. Reliance on entomophages to control pests should be limited to those situations where scientifically, environmentally and economically sound procedures are available.
Theory predicts that predator/parasitoid effectiveness can be increased through propagation and liberation. Host/entomophage interactions must be thoroughly studied, however before any program can be relied upon. Such interactions may be assessed through studying the functional response of the predator/parasitoid to host density and how this relates to dispersal of the organism. Field evaluations must provide the data necessary for defining the number of entomophages required for release per unit area, and this together with mass production technology determines the economic feasibility of the approach. Such fundamental knowledge as searching rate, functional response, and efficiency could significantly add to the predictability of success in augmentation efforts.
Flight Chambers are useful tools for examining flight responses and foraging patterns of parasitoids. Most designs presently in use are similar to the wind tunnel of Miller & Roelofs (1978), which was used to study moth flight. Nettles (1979, 1980) did a lot of work with the wind tunnel flight responses of parasitoids in studies of Eucelatoria spp. These examined response of the parasitoid to volatiles from the host and host habitat, and suggestions were made regarding the use of Eucelatoria attractants to increase parasitoid populations in the vicinity of Heliothis hosts. Drost et al. (1986) examined the flight behavior mediated by airborne semiochemicals in M. croceipes and emphasized the importance of preflight conditioning to the plant-host complex on positive searching responses of M. croceipes. Other research showed the M. croceipes reared on hosts fed cowpea seedling leaves instead of artificial diet had an increased percentage of oriented flights to odors of a cowpea seedling--H. zea complex in a flight tunnel. The increased response was much stronger after adult females had searched a fresh host plant complex (Drost et al. 1988). Elzen et al. (1986) evaluated the effects of cotton, Gossypium spp., cultivars and species on the flight responses of Campoletis sonorensis (Cameron) in a study which found higher innate searching on glanded versus glandless varieties. It was implied that volatile chemicals present in the glanded varieties had a positive effect on parasitoid foraging in the wind tunnel that was not produced by glandless cottons or Old World species. Additionally, Elzen et al. (1987a) found strong innate responses by M. croceipes to cotton and further suggested that the parasitoid responses represented fixed action patterns. Herard et al. (1988a) conducted experiments with M. demolitor which were similar to those of Drost et al. (1986). Herard et al. (1988b) also described rearing methods suitable for semiochemical studies. The wind tunnel flight chamber has been further refined with the development of a novel system for injection of semiochemical volatiles directly into the moving air (Zanen et al. in press). Wind tunnels may aid in efforts to solve the mysteries of parasitoid host habitat location and host location and provide insights which may allow manipulation of parasitoid behavior. Wind tunnels are also ideal for the early isolation of semiochemicals and for use in bioassay directed fractionation and confirmation of synthetic chemical activity.
Of course, laboratory assessments of entomophages must be supported by field experiments. For example, field surveys have shown that parasitization of Heliothis spp. larvae varies greatly in space and time (Lewis & brazzel 1968, Graham et al. 1972, Roach 1975, 1976; Smith et al. 1976, Burleigh & Farmer 1978, Puterka et al. 1985, King et al. 1985). A summary of suggested methods and steps in manipulation of semiochemical-mediated foraging behavior is given in Nordlund et al. (1981a).
Parasitization can vary spatially due to variation in parasitoid host plant detection, search rate, or retention of parasitoids on host plants. Host plant species and stage, host density and weather are likely to affect all three processes. Parasitization can vary temporally because of variation in host detection, searching, retention, parasitoid natality or mortality. Research designed to gather information to predict distribution of parasitization across host plants under varying conditions could yield important information on the population dynamics of hosts and parasitoids. These predictions are crucial to rational conservation and augmentation of parasitoids. For example, the search rate of M. croceipes in field cages was higher on Gossypium hirsutum L. in summer than on Geranium dissectum L. in spring (Hopper & King 1986). However, this difference may arise from different temperatures and not from different host plant species. In field cages M. croceipes parasitized more hosts on G. hirsutum than on Phaseolus vulgaris and more hosts on P. vulgaris than on Lycopersicon esculentum (Mueller 1983). However, it is unclear if these differences arose from differences in host plant attraction or from differences in search rate. In field cage experiments it was found that M. croceipes parasitized a significantly lower proportion of H. virescens larvae on Geranium dissectum than on either Trifolium incarnatum or Vicia villosa . The attraction of Compoletis sonorensis varies with host plant species (Elzen et al. 1983) cotton variety (Elzen et al. 1986), and the attraction to cotton correlated with volatile chemical profile of the varieties (Elzen et al. 1984, 1985). The host plant species on which H. zea has been feeding affects the response of M. croceipes to nonvolatile kairomones from its host. These data suggest that variation in parasitization found in host plant surveys may arise from variation in attraction or retention of wasps by semiochemicals directly or indirectly derived from the host plants. Studies of host habitat preference may provide clues to the best habitat in which to release parasitoids in augmentation. The effects of kairomones on searching of pink bollworm parasitoids were studied by Chiri & Legner (1983, 1986), but no effective means of deploying these chemicals for enhanced biological control was found. In fact, their application may actually reduce parasitoid effectiveness by confusion.
Screening of the biological characteristics of entomophages has been advocated (Sabelis & Dicke 1985, van Lenteren 1986). An example of a predator currently used in IPM in Dutch orchards is Typhlodromus pyri Scheuten. As note by Dicke (1988) despite the use of this predator, its biology has not been thoroughly studied. Based on its response to volatile kairomones it was later determined that T. pyri prefers the European red spider mite, Panonychus ulmi (Koch) to the apple rust mite, Aculuc schlechtendali (Nalepa), which was confirmed by electrophoretic diet analysis (Dicke & Dejong 1988).
Field experiments may also provide insights into the efficiency of a particular entomophage. For example, in field cages containing Gossypium hirsutum or G. dissectum, M. croceipes searching rate for Heliothis zea and H. virescens larvae did not depend on host density (Hopper & King 1986). In field experiments on G. hirsutum, M. croceipes parasitized a higher proportion of Heliothis larvae in plots where host density was higher. Also, parasitoid aggregation but not increased searching rate, caused the increased parasitization at high host density which supports the linear functional response reported by Hopper & King (1986). Some parasitoids have been shown to aggregate in areas of high host density in laboratory experiments (Legner 1969, Hassell 1971, Murdie & Hassell 1973, T-Hart et al. 1978, Collins et al. 1981, Waage 1983). Since host plant species vary in attraction and suitability for Heliothis, which can cause variation in larval density, the spatial variations in Heliothis parasitization observed in field surveys may in part result from parasitoid aggregation at high host densities. Several parasitoid species are more attracted to plants on which hosts have fed than to undamaged plants (Thorpe & Caudle 1938, Monteith 1955, 1964, Arthur 1962, Madden 1970< and mechanically damaged plants increase parasitoid searching (Vinson 1975). Damaged terminals of G. hirsutum attract more C. sonorensis than do undamaged terminals (Elzen et al. 1983). Microplitis croceipes is attracted to wind borne odor of H. virescens frass and larvae (Elzen et al. 1987), and M. croceipes responds to nonvolatile kairomones produced by H. zea (Jones et al. 1971, Gross et al. 1975), H. virescens, and H. subflexa (Lewis & Jones 1971).
Luck et al. (1988) suggested criteria for evaluating entomophages that are scheduled for introduction. Experimental evaluation through life table analysis, examination of percent parasitization, key factor analysis, and the use of simulation models, may provide insights into the probability of success in augmentation. Evaluation of entomophages may include introduction and augmentation, and techniques using cages and barriers, removal of entomophages, prey enrichment, direct observation and biochemical evidence of entomophage feeding, and quantified experiments to gauge the impact of the entomophages.
Entomophages may be ineffective due to a lack of host synchrony, temporal displacement in ephemeral systems, lack of protected sites, lack of alternate hosts, adverse environmental conditions or influence of pesticides, etc. Temporal synchronies between entomophage and pest, and oscillations in populations have been documented by Varley & Gradwell (1974). The influence of weather on parasitoid searching has, however, received little attention. Often when natural control is not achieved it is due to the lack of synchrony of entomophage and host in time. These complex relationships make intervention at any one level difficult and less likely to produce desirable results.
An understanding of the effects of the pesticide component is important (Croft & Brown 1975). Pesticide resistance in entomophages was discussed by Croft & Morse (1979), and recommendations for changing control practices to preserve entomophages were listed. Insecticide use in cotton and the value of predators and parasitoids for managing Heliothis was reviewed by King (1986), and results on Pectinophora gossypiella were given by Legner & Medved (1979 , 1981 ).
The detrimental effects of pesticides on entomophages are well documented, and it may be important to note that an underlying problem in practical implementation of augmentation is the use of pesticides. Unexpected problems may be encountered, even from pesticide drift, so that basic toxicological studies may be required to determine if the entomophages intended for use in augmentation have some degree of tolerance to the effects of insecticides, especially as resurgence of primary and secondary insect pests has been documented in some heavily sprayed monocultures (Huffaker 1971). Actions of insecticides on entomophages include not only those causing direct mortality, but also those that act in indirect ways, or that alter entomophage biology adversely. First and foremost, there are the obvious direct lethal actions of broad spectrum insecticides, such as organophosphates, on entomophages. Because entomophages have more specific enzymes evolved for handling the toxins of their hosts they are much more susceptible to broad spectrum insecticides than their hosts which have an array of plant chemicals with which to contend (Krieger et al. 1971). The occurrence of primary pest release and resurgence of a previously innocuous secondary pest have been widely reported where insecticides selectively destroy entomophages (DeBach & Bartlett 1951, Michelbacher et al. 1946, Doutt 1984, Lingren & Ridgway 1967). For example, azinphosmethyl, a broad spectrum organophosphate, selectively destroys entomophages in apple orchards (Falcon 1971). On the other hand, chlordimeform was found less toxic than some other insecticides to several species of entomophages (Platt & Vinson 1977, Platt & Bull 1978).
Sometimes insecticides do not kill entomophages, but they may so affect them that normal behavior or reproduction is encumbered. Press et al. (1981) found the permethrin and pyrethrin reduced the number of adult Bracon hebetor Say produced when parental females were exposed to hosts and insecticides simultaneously. Topical application of carbaryl on adult female Bracon hebetor results in reduced numbers of eggs that develop from vitellogenic oocytes, and resorption of mature ova (Grosch 1975). Residues of pyrethrin significantly reduce parasitization rates of T. pretiosum (Riley) on H. zea eggs (Jacobs et al. 1984). Formamidines are especially recognized for their ability to disrupt pest mating, reproductive and feeding behavior (Knowles 1982, O'Brien et al. 1985). Whether or not these compounds have such effects upon entomophages is not well known. Parasitized hosts have been found to be more susceptible to insecticides than nonparasitized hosts, thus preventing normal development of immature parasitoids. Lymantria dispar (L.) larvae parasitized by Apanteles melanoscelus (Ratzburg) are significantly more susceptible to carbaryl than nonparasitized larvae, and more time is required for surviving parasitoids to develop (Ahmad & Forgash 1976). Fix & Platt (1983) found that H. virescens larvae parasitized by C. nigriceps are 1.42X more susceptible to methyl parathion, and 2.5X more susceptible to permethrin than are treated unparasitized larvae. These cases show how insecticides can have an additional, indirect action on entomophages.
Entomophage abundance may also be reduced when their hosts have been decimated by insecticides. This was the case of the predator Orius insidiosus (Say) feeding on the cotton leaf perforator Bacculatrix thruberiella Bush in cotton treated with chlordimeform. Numbers of O. insidiosus steadily declined when populations of the leaf perforator was reduced by chlordimeform sprays (Lingren & Wolfenbarger 1976). Direct mortality is the most severe way that insecticides can impact, chlordimeform may become important in certain pest management strategies, due to the property of controlling pests behaviorally and physiologically at low sublethal doses. Lo doses of chlordimeform significantly decrease fecundity and egg viability of adult female tobacco bollworms, and prevent moths from separating after mating (Phillips 1971). Although chlordimeform decreased the number of eggs laid by Lepidoptera, it is not clear whether reduced fecundity was caused by interference with ovarian development or with oviposition behavior (Hollingworth & Lund 1982). Chlordimeform also reduces fecundity in the cotton aphid, Aphis gossypii Glover (Ikeyama & Maekawa 1973) and in the cattle tick, Boophilus microplus (Masingh & Rawlins 1979). Feeding behavior is upset by chlordimeform in larval tobacco cutworms, Spodoptera litura F. (Antoniosus & Saito 1981), armyworm, Leucania separata Walker (Watanabe & Fukami 1977) and in cockroaches, Periplaneta americana L. (Matsumura & Beeman 1982). But the effects of chlordimeform vary with species and there is much selectivity between species and stages for actual acute toxicity; some insects are very sensitive and others are immune, as in the case of adult boll weevils (Wolfenbarger et al. 1973). Additional evidence for the suitability of chlordimeform for some pest management strategies is given by Platt & Vinson (1977), who found that chlordimeform is ca. 100X less toxic to the parasitoid C. sonorensis than organophosphates similar to azinphosmethyl. By controlling pests at sublethal doses, the problem of pest resistance may be lessened. Dittrich (1966) found that chlordimeform is effective against some pests that have already become resistant to organophosphate and carbamate insecticides.
The work with Trichogramma augmentation may provide clues for other species of entomophages. From 1981-83 King et al. (1985) collected Heliothis larvae from insecticide treated and untreated cotton fields and found 1/3rd of the larvae were parasitized, particularly by the braconid M. croceipes. These levels of parasitism of M. croceopes were greater than any reported in cotton since the advent of organochlorine insecticides in the 1940's (King 1986). As M. croceipes has been commonly found in cotton in the SE United States (King et al. 1985) and due to the apparent tolerance of this parasitoid to some commonly used insecticides (King et al. 1985, Powell et al. 1986, Bull et al. 1987), augmentation releases of the parasitoid are anticipated in the future (King 1984). This parasitoid was recently exposed to insecticides commonly used in cotton using a spray tower (Elzen et al. 1987b). Direct treatment with the pyrethroid fenvalerate, a mixture of the formamidine chlordimeform plus fenvalerate, and the carbamate thiodicarb resulted in nearly 100% survival of both sexes at both the lowest and highest field rates recommended for these insecticides. The organophosphate acephate and the carbamate methomyl were extremely toxic to adult M. croceipes, causing 100% mortality at the lowest recommended field rates.
Marking studies have shown that lady beetles, lacewings, syrphid flies and parasitic wasps fed on nectar or pollen provided by borders of flowering plants around farms. Many insects were shown to have moved 250 ft. into adjacent field crops. The use of elemental marker rubium also showed that syrphid flies, parasitic wasps and lacewings fed on flowering cover crops in orchards and that some moved 6 ft. high in the tree canopy and 100 fleet away from the treated area. The use of nectar or pollen by beneficial insects helps them to survive and reproduce. Thus, planting flowering plants and perennial grasses around farms may lead to better biological control of pests in nearby crops (Long et al. 1998).
The effectiveness of resident insect predators as biological control agents of peach twig borer was tested in a series of field experiments. It was shown that the native gray ant, Formica aerata was the most common and effective generalist predator. Treatments with native gray ant present had significantly lower peach twig borer abundance and peach shoot damage. Ant population densities were studied in seven commercial orchards. However, results showed that although this ant is found in most peach and nectarine orchards, its abundance was not clearly associated with any single cultural practice and may be difficult to manipulate (Daane & Dlott 1998).
Spray tower treatment of Microplitis croceipes with insecticides applied directly to the insects was followed in another study by exposure of parasitoids to plants which were sprayed in the spray tower. Parasitoids were then caged on these plants and mortality observed after 24 hrs. The fenvalerate/chlordimeform mixture caused 10-23% mortality, with thiodicarb causing a similar percent mortality, whereas methomyl caused significantly high mortality, ranging from 23-70%. It is probable that the use of thiodicarb as an ovicide and larvicide for Heliothis control will increase in the future because of resistance to pyrethroids, and fortunately M. croceipes seems relatively tolerant to this insecticide.
Useful information for models to predict the impact of entomophages on reducing herbivore induced damage or plant stress is obtained by monitoring and sampling entomophages that are indigenous or released by augmentation. Modelling of population interactions requires accurate tools to determine absolute densities of entomophages and pests. Monitoring entomophage populations, particularly parasitoids, may be complicated by factors such as lack of a stable sex ratio, movement (females must forage for often patchily distributed hosts), weather, and lack of synchrony with host populations. However, monitoring methods to evaluate parasitoid populations have been suggested. The most reasonable approach to this problem would involve estimating population numbers from captures of males or females in traps baited with an appropriate attractant, such as sex pheromone. Powell (1986) suggested that monitoring systems be explored using some volatile host or host habitat attractant to trap female arthropods, thereby capturing the agent responsible for doing the parasitizing, and perhaps obviating any problems which may arise from an unstable sex ratio.
There has been no system developed whereby a parasitoid can be monitored with sex pheromone for decision making in an agricultural crop. Although much effort has been expended in the field of insect sex pheromones, few studies have resulted in identification of parasitoid sex pheromones. Robacker & Hendry (1977) identified neral and geranial from female Itoplectis conquisitor (Say), and demonstrated that these chemicals were attractive to males in the laboratory. Eller et al. (1984) identified and demonstrated the field effectiveness of ethyl palmitoleate, a female sex pheromone of Syndipnus rubiginosus Walley, a parasitoid of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer).
Powell & King (1984) showed that males of M. croceipes were attracted to virgin females in the field, and diurnal activity of males and females was found to differ. From these observations it was believed that knowledge of parasitoid activity periods would be important in developing techniques for sampling parasitoid populations in the field. Subsequently it was determined that SentryR wing traps (Albany International) were more effecting in capturing M. croceipes males than were Pherocon II traps (Zoecon). Studies in unsprayed cotton in 1984 revealed that wing traps baited with living virgin females could be used to estimate parasitoid populations, and recently M. croceipes mating behavior and sex pheromone response were reported by Elzen & Powell (1988), and a tentative identification of the female-produced sex pheromone has been made.
Control guidelines often recognize the impact of entomophage populations on pest populations (Rude 1984). But, explicit instructions for using entomophages in decision making are lacking, and where present are used with reservation. Two exceptions are Michelbacher & Smith (1943) who recommended that insecticide control decisions in alfalfa for Colias eurytheme Boisduval be made only after determining that the number of Apanteles medicaginis Muesebeck present was capable of maintaining the pest under control and Croft (1975) reported on a decision making index for predicting the probability of adequate control of a phytophagous mite that would occur depending ont the predator/prey ratio per apple leaf.
Pheromone traps baited with living virgin females attracted large numbers of male cereal aphid parasitoids when placed in cereal fields (Powell & Zhang 1983). Aphidius rhapalosiphi DeStef and Praon volure Haliday males were caught in separate traps baited with females. Monitoring may be useful in this situation to achieve maximum impact when the parasitoid/aphid ratio is particularly high, especially early in the season (Powell 1986). Methods for isolating sex pheromones (Golub & Weatherston 1984), as well as bioassay directed fractionation, identification (Heath 7 Tumlinson 1984), and synthesis (Sonnet 1984) are detailed. These methods are adaptable for identification of parasitoid pheromones (Elzen & Powell 1988).
Entomophage culture is treated extensively in a different section , as previously discussed. It is obvious that efficient and cost effective methods of rearing entomophages must be developed if augmentative releases are to be feasible. Large numbers of beneficial insects may be employed in greenhouses, field cages and laboratory studies. Thousands of entomophages available at unpredictable times, may be required for commercial augmentation. Considerable attention has been devoted to development of techniques to produce quality entomophages in large numbers (King & Leppla 1984). The genetic implications of long term laboratory rearing of insects are vast (Bouletreau 1986, Mackauer 1976). Powell & Hartley (1987) described techniques for producing large numbers of parasitoids efficiently. These researchers adapted a multicellular host rearing tray technique (Hartley et ala. 1982) to rear M. croceipes and some other parasitoids. Techniques reduced parasitoid harvest time by 1/2 and simultaneous release of nearly 17,000 wasps was possible using low temperature storage. Powell & Hartley (1987) also noted several factors that were important for maintaining this large scale rearing program, and which may be applicable to other programs. Included were (1) a continuous host supply, (2) use of environmental chambers to alter developmental rates of hosts and parasitoids, (3) constant appropriate environmental conditions, (4) sanitary rearing conditions with flash sterilization of diet, (5) use of laminar flow hoods, (6) autoclaving reusable supplies, (7) disinfecting work areas, (8) acid or antibiotics in water or food, (9) adequate technical support, space and equipment. Elzen & King (1999) show a list of beneficial insects that have been reared for augmentative purposes by the U. S. Department of Agriculture.
Costs.--King et al. (1985) cited costs for release of Trichogramma pretiosum to control Heliothis spp. at US$7.68/ha. per application. This cost compared well with the cost of a commonly used pyrethroid, fenvalerate, at US$16.18/ha when applied at 0.11 kg/ha. The pyrethroid had to be applied only once for every two to three parasitoid applications. However, the development of resistance to pyrethroids by Heliothis (Luttrell et al. 1987) and to dimethoate by Lygus lineolaris Paisot de Beauvois (Snodgrass & Scott 1988), and in general, possible development of resistance, makes augmentation attractive.
In Vitro Rearing.--Augmentation may be commercialized only for selected organisms for which suitable diets and storage methods are developed. Artificial rearing would offer possibilities, but as is discussed in the section on Entomophage Nutrition, this technology is poorly developed. Various groups have made progress in in vitro rearing of parasitoids nevertheless. Over 22 entomophage species have been reared in vitro. Several Hymenoptera (1 ectoparasitoid, 4 pupal parasitoids and 4 species of Trichogramma) and 3 species of Diptera have been cultured with varying success (King et al. 1984). Predators have been reared on artificial diets, notably Chrysopa carnea Stephens (Vanderzant 1973, Martin et al. 1978). While there have been numerous successes in oviposition stimulant identification or partial rearing (Nettles & Burks 1975, Nettles 1982), definite development of a feasible in vitro rearing system for entomophages has yet to be developed. Presently most parasitoids are expensive to rear, and the costs involved would preclude mass rearing in vivo preparatory to thrifty augmentation efforts. Although considerable advances have been made in in vivo rearing, the advances have not been achieved with in vitro rearing to such an extent. The work of Wu et al. (1982) illustrates an instance in which a completely synthetic artificial host egg was produced which contains no insect derivatives, and supports Trichogramma oviposition and development. Greany et al. (1984) suggested that mass rearing of Trichogramma using completely artificial hosts would soon become economical, however.
Hymenopterous larval endoparasitoids have not been successfully reared to the adult stage on artificial diet. However, Cotesia marginiventris and M. croceipes have been reared on artificial media through the first instar. Larval endoparasitoids have evolved complex mechanisms that interact with the host's internal dynamics and organs without damaging this environment or causing untimely death of the host. The function of these interacting factors must be understood for in vitro rearing of larval endoparasitoids to become a reality. Developments in artificial rearing of entomophages on artificial diet may allow production of sufficient numbers of individuals to practically implement the further evaluation of entomophages in biological control.
Parasitoids which have been reared to the adult stage on artificial media include the larval ectoparasitoid Exeristes roborator (F.) (Thompson 1982), the endoparasitoids of eggs: T. pretiosum Riley (Hoffman et al. 1975) and T. dendrolimi (Wu et al. 1982); of larvae: Lixophaga diatreae (Towns) (Grenier et al. 1978) and Eucelatoria bryani Sabrosky (Nettles et al. 1980), and of pupae: Brachymeria lasus (Walker) (Thompson 1983), Pachycrepoideus vindemiae Rondani (Thompson et al. 1983), Itoplectis conquisitor (Say) (House 1978), and Pteromalus puparum L. (Hoffman & Ignoffo 1974).
Exercise 48.1-- When should manipulation be attempted to enhance the activity of natural
Exercise 48.2-- What methods can be considered in manipulation?
Exercise 48.3-- Give some examples of successful manipulation.
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