Note. This is the second of 3 parts of an article that contains important information on advances in the uses of green lacewings in biological control. This is my condensed version of a more detailed review paper, entitled "Commercialization of predators: recent lessons from green lacewings (Neuroptera: Chrysopidae: Chrysoperla)". printed in American Entomologist, 2000, vol. 46 (1): 26-38. For additional information, the reader is urged to go to the original paper, which cites over 120 references to other review papers and the original scientific literature. For general information on lacewing biology and their use in biological control, see green lacewings. Many other articles containing information about green lacewings are in our archives; see our green lacewing index. Our sincere thanks go to the authors for allowing us to condense their original paper to produce this version; special thanks to the Taubers for reviewing and correcting the condensed manuscript. Dan Mahr, BCN Project Director
Go to Part I = Mass Production
Augmentation with Mass-reared Chrysoperla
Given the amenability of Chrysoperla spp. and biotypes for mass rearing, it is clear that species or biotypes can be chosen strictly on the basis of how well they are matched with the pest management situation. Below, we discuss aspects of the habitat, crop, and target pest that can influence the success of augmentative biological control with Chrysoperla. We also consider new information on release methods and rates, and we offer suggestions for research in the future.
Chrysoperla spp. and biotypes exhibit considerable variation in their responses to physical and biotic factors in the habitat, habitat preference, adult cryptic coloration, seasonal cycles, and, perhaps, prey preferences. Similarly, release sites for lacewings vary greatly (they range from cotton fields in Texas and apple orchards in Washington to greenhouses in a variety of locations). Unfortunately, the innate variation among Chrysoperla taxa and the differences in geographic areas, agroecosystems, or environmental conditions rarely have been considered in developing release tactics; consequently, the effectiveness of releases varies greatly. However, a few studies that incorporated these issues led to recommendations for matching species and biotypes with certain pest management situations, and some of these recommendations have resulted in improved augmentative biological control. Nevertheless, evaluation of the recommendations under field conditions is necessary. Two examples provide useful lessons.
First, comparative studies of the developmental and reproductive responses of C. carnea and C. rufilabris to relative humidity led to different recommendations for using each species. C. rufilabris does not perform well in dry areas but generally is the best choice for use in greenhouses or sites with moist conditions. In contrast, C. carnea does well under low humidity and should be used in dry regions. Some U. S. insectaries successfully adopted these recommendations in their sales promotions, and the species are marketed appropriately.
In the second example, studies on the variation in seasonal responses and habitat preferences among the various C. carnea biotypes (species or populations) led to tentative recommendations for matching biotypes with specific pest management situations (cropping systems). For example, the dark green C. downesi is recommended for use in evergreen trees, whereas the light green C. carnea from eastern U.S.A. is recommended for annuals or deciduous perennials (e.g., field crops or vineyards). However, it should be noted that these recommendations were based on limited information and comprehensive recommendations require additional data (e.g., on the responses of the biotypes to plant characteristics and to prey). Most importantly, the recommendations should be evaluated under commercial conditions.
Even seemingly small differences in plant structure and chemistry may influence lacewing effectiveness. For example, the smooth and hirsute leaf surfaces of certain cotton cultivars affect C. rufilabris larval mobility and prey consumption differentially. The effectiveness of C. carnea also varies in response to the surface and structure of cabbage and wheat plants. These examples illustrate the necessity of matching the predators biological characteristics not only with the physical conditions of the environment, but to the crop as well. They also illustrate the necessity for comparative studies so that species-specific recommendations for using lacewings can be developed.
Similar types of comparative studies should examine Chrysoperla responses to prey. Currently, all Chrysoperla spp. are considered generalist predators of soft-bodied insects and mites, a trait that underlies their great commercial demand. However, their prey preferences appear to vary significantly. These preferences should be defined better and the differences among species and biotypes should be clarified in comparative quantitative studies. With data from such investigations, reliable recommendations could be made for the improved use of Chrysoperla species (and biotypes) against specific types of pests.
An important consideration in examining predator-prey interactions is that in open-field releases, introduced predators may themselves become prey. Indeed, the absence of resident predators in enclosed systems, and thus the avoidance of predator-predator interactions, may explain the high success rates of Chrysoperla spp. in glasshouse and cage studies. In some circumstances, ants, assassin bugs, earwigs and other predaceous arthropods can attack lacewing eggs and sometimes larvae, thereby disrupting the effectiveness of releases. For example, the Argentine ant, Linepithema humile, removed 98% of the C. carnea eggs that were dispensed on tulip trees to control the aphid Illinoia liriodendri. To reduce disruption, pre-fed larvae, rather than eggs, can be used; however, at present, the high cost of larvae makes this method prohibitive in most agroecosystems. But, the lesson is clear: intraguild predation is a factor that should receive more attention than it has in the past.
Less well-documented, but of equal importance, is the potential disruptive effect of parasitoids in augmentative release programs. Several species of parasitoids attack Chrysoperla eggs and larvae. Rates of parasitism can be high, especially at the end of the season. In pecan orchards with season-long releases of C. carnea eggs, parasitization increased such that overall lacewing densities (introduced and resident populations) were lower in experimental than control ("non-release") fields. Similarly, in one of two trials, the scelionid Telenomus tridentatus parasitized considerably more eggs in experimental plots (~30%) than in control plots (~2%). In some cases, release of mature (rather than newly laid or young) eggs can reduce parasitism greatly.
The impact of pathogens on augmentative releases of predators is poorly understood. However, recent studies report that C. carnea larvae are susceptible to Bacillus thuringiensis toxins that are being incorporated into corn, potato, and other crop plants. Thus, the large-scale use of transgenic plants should be tested for significant negative effects on this important predaceous insect.
Pesticides constitute another common disruptive component in many agroecosystems. Here, C. carnea may have an advantage over other introduced or resident natural enemies because it has a relatively broad tolerance to many insecticides, particularly during the larval and cocoon stages. However, tolerance can vary; for example, C. carnea associated with heavy pesticide usage often are less vulnerable than those from areas with low insecticide usage. In contrast, C. rufilabris displays generally higher vulnerability to insecticides than does C. carnea. Insectary managers should consider these issues when they choose or market lacewings. Also, generalized statements regarding lacewing susceptibility to insecticide residues may not be appropriate.
Development of efficient methods for commercial releases is a crucial factor in the success of augmentative biological control. Nevertheless, until recently there was little field evaluation of lacewing release tactics since the 1970s when they were originally developed and tested. When considered together, recent studies provide a crucial lesson: there is a great need for quantitative evaluations under commercial conditions. Below are three examples.
Delivery Systems. Historically, chrysopid eggs were dispensed manually, typically mixed with a solid medium such as rice hulls or vermiculite; this practice fostered uniform field distribution. New delivery systems are being developed to improve lacewing delivery to the crop.
Recently, agricultural engineers tested new, much improved mechanized systems. In one test, C. rufilabris eggs and larvae were mixed with vermiculite mechanically and distributed evenly over the plants without significant mortality.
One disadvantage of solid carriers is poor retention of eggs on the plants eggs fall off the leaves whereas liquid carriers help attach eggs to the targeted plants. Recently there have been notable advances in the development of liquid carriers and commercial sprayers. For example, distributing C. carnea eggs in an agar solution rather than sucrose-based carriers has the advantage of lowered attractiveness to ants and other predators. In "prototype" applicators, C. rufilabris eggs were immersed in a commercial liquid carrier (BioCarrier TM, Smuckers Mfg., Harrisburg, OR), pneumatically agitated to create uniform egg suspension, and discharged into the targeted crop without damage to the eggs and with good retention on the leaves. A commercial sprayer for delivering insect eggs to the field efficiently is also under development (BioSprayer, TM, Beneficial Insectary, Oak Run, CA).
Developmental Stage for Release. Although lacewings commonly are sold and dispensed as eggs, larval releases may sometimes be more effective. Releases of C. carnea larvae were superior to releases of eggs for control of the Colorado potato beetle. Although larval releases remain expensive, new advances in insectary production and dispensing systems (discussed above) may improve the economics of commercial releases of larvae. Meanwhile it is crucial to evaluate the biological and economic advantages of releasing one or the other developmental stage and to begin devising efficient methods for introducing the larval stage.
Release Rates. Few
studies have assessed release rates in relation to pest reduction
and costs of application. In most early studies, large numbers of
lacewings were dispensed to insure a reduction in pest densities;
the release rates generally were too high to be commercially
practical at prevailing insectary costs. Recently, a few field
studies addressed this problem by testing lacewing release rates
that approximate commercially feasible rates. However, these
tests yielded conflicting results. For example, C. rufilabris
were dispensed on grapevines at rates varying from 6,175 to 1,235,000 larvae per
hectare; in one test, there was a positive correlation between
release rate and pest density, but in another, no significant
correlation occurred. Clearly, more field tests using
commercially feasible release rates are necessary.
Habitat Manipulation: Food Sprays
Chrysoperla adults are not predaceous; rather they feed on honeydew and pollen. Consequently, the behavioral responses of Chrysoperla adults to flowering plants and to chemical and other stimuli associated with their habitats and food can be used to augment populations in targeted areas. For example, populations of C. rufilabris were greater in the pecan canopy in orchards with a leguminous ground cover than in those with a grass cover. Similarly, food sprays that imitate honeydew can attract or retain adults and stimulate egg laying. However, the effectiveness of food sprays in manipulating field populations varies, and recent studies indicate that further basic and applied research in this area is needed.
The best commercially available food sprays contain both enzymatic protein hydrolysates and sugar or honey. In most trials, protein-sprays without sugar fail to increase the number of lacewings, and sugar-sprays without protein attract lacewing adults but do not stimulate egg laying.
Early work showed that by using food sprays to attract and induce chrysopids to lay eggs before natural honeydew becomes abundant, it is possible to suppress honeydew-producing or other pests before their numbers become large. However, in some cases the application of food sprays increased the densities of lacewing adults, but not the eggs or larvae. Given the above, we recommend two areas of research that could be of great value: (a) the seasonal variation in the reproductive responses of lacewings to food and (b) the impact of food sprays on nontarget organisms that could reduce lacewing effectiveness.
A few studies have combined augmentative releases with the application of food sprays to induce both released and naturally occurring lacewings to remain within the crop. Augmentation of C. externa in soybeans and corn did not affect the resulting number of lacewing larvae or the three targeted noctuid pest species, but applying "Wheast" and sugar at the time of the release gave a 2- to 6-fold increase in the densities of adults and eggs of C. externa in corn fields. These results indicate that with some well-focused research, novel uses of food sprays have considerable potential for application in commercial agriculture.
If the goal is to attract and retain Chrysoperla
adults in the field, then there is a particular need to focus new
research on their seasonal patterns of movement. In Chrysoperla,
the adult is both the dispersing and reproductive stage and the
stage that undergoes hibernal and aestival dormancy. Effective
and reliable manipulation of Chrysoperla, therefore,
requires knowledge of the seasonal timing of (a) adult movement
(dispersal, migration) and reproductive development, and (b)
responsiveness to the chemical, visual, and other cues that
attract adults, arrest their movement, and promote egg laying.
In the next issue, Part
III: Evaluation of Augmentative Releases
Maurice J. Tauber, Catherine A. Tauber, (both Department of Entomology, Cornell University, Ithaca, NY), Kent M. Daane, and Kenneth S. Hagen (both Center for Biological Control, University of California, Berkeley
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