Principles of Insect Parasitism Analyzed from New Perspectives Practical Implications for Regulating Insect Populations by Biological Means
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Principles of Insect Parasitism Analyzed From New Perspectives Practical Implications for Regulating Insect Populations by Biological Means W ■* * K -m^ ' -m. - ■* "■, • ■^ ^¡^ United States Agricultural Agriculture È) Department of Research Handbook Agriculture Service Number 693 Principles of Insect Parasitism Analyzed From New Perspectives Practical Implications for Regulating Insect Populations by Biological Means E.F. Knipling United States Agricultural Agriculture Department of Research Handbook Agriculture Service Number 693 Abstract E.F. Knipling. 1992. Principles of Insect Parasitism Analyzed From New Perspectives: Practical Implications for Regulating Insect Populations by Biological Means. U.S. Department of Agriculture. Agriculture Handbook No. 693, 349 pp. This publication presents the results of a theoretical study on the roles that parasitic insects can and cannot play against populations of their insect-pest hosts under natural conditions More impor- tantly, the feasibility of managing major insect-pest problems through highly augmentative releases of parasites in the host ecosystems is critically examined by theoretical deductive proce- dures. Ke3rwords: augmentation release, beneficial insect, biological agent, biological control, entomophagous insect, host insect, insect, insect management, insect pest, insect suppression, integrated pest management, parasitic insect, pest control, pest management, predator insect, prey insect. While supplies last, single copies of this publication may be ob- tained free of charge from USDA-ARS, Information Staff, 6303 Ivy Lane, Room 400, Greenbelt, MD 20770. The United States Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for commu- nication of program information (Braille, large print, audiotape, etc.) should contact the USDA Office of Communications at (202) 720- 5881 (voice) or (202) 720-7808 (TDD). To file a complaint, write the Secretaiy of Agriculture, U.S. Depart- ment of Agriculture, Washington, DC 20250, or call (202) 720-7327 (voice) or (202) 720-1127 (TDD). USDA is an equal employment opportunity employer. June 1995 This publication slightly revises Principles of Insect Parasitism Analyzed From New Perspectives: Practical Implications for Regulating Insect Populations by Biological Means, issued April 1992. Foreword The improved use of parasitoids as biological control agents is at the forefront of our quest for more effective, lasting, and environ- mentally safe technology for pest management. Various avenues are available for utilizing parasitoids, including the importation of new species, mass-propagation and release of parasitoids, and habitat management techniques to increase the abundance and performance of feral and released individuals. Our limited under- standing of parasitoid/host interactions and other factors affecting their efficacy is a central barrier to consistently effective use of parasitoids with all of these approaches. The author herein pre- sents an analysis that is of major value in our endeavor to under- stand host/parasitoid interactions. Dr. Knipling has for many years advocated the need to seek envi- ronmentally compatible and lasting solutions to our major pest problems versus the more conventional, reactive measures and has continually stressed the importance of approaching these pest problems from a total-population perspective. He further has envisioned parasitoids as potentially invaluable weapons in our pest management arsenal and sought ways to develop that poten- tial. This treatise is the fruit from his years of diligent pursuit toward a fundamental understanding of parasitoid/host interac- tions at the total-population level, and it presents the results that could be derived by the mass-release of parasitoids over a major area of the pest ecosystem. He has assembled the key information on the variety of factors that are known to affect parasitoid-host coexistence patterns and used it in a thorough analysis of the population dynamics of several parasitoid-host examples, each species having certain unique characteristics. The results derived from these analyses and especially the conclusions as to certain common principles govern- ing parasitoid populations and their efficacy offer a truly new and important perspective. The author's highly significant central conclusion is that the rate of parasitism (at the total-population level) is primarily a function of the ratio of parasitoid numbers to host numbers—^with host density, of itself, being of little conse- quence. A major supporting premise for this conclusion is reported research which demonstrates that parasitoids have highly sophisti- cated host-detection mechanisms and, thereby, waste very little time searching in spaces other than the immediate sites of host infestation. It is important to emphasize, as does the author, that because of limited information many of the parameters used in the analyses were derived by deductive reasoning and may be of limited preci- sion. However, the author has taken great care to reduce errors in his assumptions by placing them through rigorous validation trials with numerous models. As with any such conceptual work, various aspects of the conclu- sions are certain to be amended and modified with new data and further analysis. In fact, some important sources of variation which have an important bearing on the subject, such as recent findings on learning by parasitoids, are not analysed. These factors, which argue for more dynamics in the models, should be kept in mind as we continue to explore the subject. These consid- erations do not, however, negate the primary value of the work, which is in the central conclusions; the new perspective for ap- praising parasitoid/host interactions; and the practical implications for augmentative use of parasitoids. The findings show that augmentation on a total-population scale can be an extremely powerful tool for pest suppression. In fact, when releases raise the overall population parasitization much over 50 percent, a very strong "self-increasing" effect can be set in motion. Some colleagues may feel that the appraisals and conclusions are oversimplified and too generalized. Such impressions should not, in my opinion, be drawn because of the simplicity of the models. The relatively few parameters used in developing the models are based on a judicious selection of the factors pertinent to the questions addressed. My experience in following Dr. Knipling's various works is that he has a tremendous gift for sorting through complex matters and bringing the essence to focus. I believe he has again achieved that end with this major contribution to the science of biological control. W. Joe Lewis Research Entomologist USDA-ARS Insect Biology & Population Management Research Laboratory Tifton, Georgia 31793 Preface For many years I have encouraged research and development on techniques and strategies for insect-pest control that in theory offer possibilities of being more effective, more economical, and less environmentally hazardous than those in current use. The investigation herein described was focused on appraising the role that certain parasites (parasitoids) play as natural control agents, and the role they might play for regulating populations of their pest hosts if the number of parasites in the host ecosystem were sufficiently increased by artificicil means. The appraisals were made in the light of (1) important new basic information that insect behaviorists have obtained during recent years on the highly efficient mechanisms that biological agents possess for finding their hosts or prey; (2) reports, by a number of investigators, that for many parasite species there is no evidence of a positive correlation between host density and the proportion of the host population parasitized by the coexisting parasite population; and (3) new, more general information on the biology and behavior of numerous parasite species. The appraisals were made by deductive procedures, and the results are presented and discussed in detail in this publication. While theoretical, the results are considered to be entirely consistent with the principles of insect parasitism, the factors that influence the dynamics of insect populations, and the basic principles of insect population suppression. Many complex biological actions and interactions are involved in natural parasitism processes. However, pest hosts and their associated parasites have evolved biological and behavioral charac- teristics which virtually ensure that the relative numbers of hosts and parasites in the ecosystem they inhabit and the rates of parasitism that occur will remain within rather narrow limits. This is to be expected even though the coexisting populations may fluctuate by up to 100-fold during a single season or over a period of several years for cyclic species. The rather close numerical relationships assumed are entirely consistent with nature's balanc- ing mechanisms, which permit closely associated organisms to coexist in reasonable harmony. Thus in natural populations, the ratios of parasites to their coexist- ing hosts are not high enough to result in dependable control of many pest hosts. Without question, however, technology can be developed to efficiently mass-rear parasites