Approaches to the Biological Control of Insect Pests
Approaches to the Biological Control of Insect Pests
Department of Entomology
Connecticut Agricultural Experiment Station
123 Huntington Street
P.O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8458 Fax: (203) 974-8502
Biological control is the use of living organisms to suppress pest populations, making them less damaging than they would otherwise be. Biological control can be used against all types of pests, including vertebrates, plant pathogens, and weeds as well as insects, but the methods and agents used are different each type of pest. This publication will focus on the biological control of insects and related organisms.
Recognizing the role of natural enemies of pest insects
Pests are those species that attack some resource we human beings want to protect, and do it successfully enough to become either economically important or just a major annoyance. They are only a tiny fraction of the insect species around us. Even many of the species we would recognize as important pests only occasionally do significant damage to us or our resources.
Natural enemies play an important role in limiting the densities of potential pests. This has been demonstrated repeatedly when pesticides have devastated the natural enemies of potential pests. Insects which were previously of little economic importance often become damaging pests when released from the control of their natural enemies. Conversely, when a non-toxic method is found to control a key pest, the reduced use of pesticides and increased survival of natural enemies frequently reduces the numbers and damage of formerly important secondary pest species.
The three categories of natural enemies of insect pests are: predators, parasitoids, and pathogens.
Predators: Many different kinds of predators feed on insects. Insects are an important part of the diet of many vertebrates, including birds, amphibians, reptiles, fish, and mammals. These insectivorous vertebrates usually feed on many insect species, and rarely focus on pests unless they are very abundant. Insect and other arthropod predators are more often used in biological control because they feed on a smaller range of prey species, and because arthropod predators, with their shorter life cycles, may fluctuate in population density in response to changes in the density of their prey. Important insect predators include lady beetles, ground beetles, rove beetles, flower bugs and other predatory true bugs, lacewings, and hover flies. Spiders and some families of mites are also predators of insects, pest species of mites, and other arthropods.
Parasitoids: Parasitoids are insects with an immature stage that develops on or in a single insect host, and ultimately kills the host. The adults are typically free-living, and may be predators. They may also feed on other resources, such as honeydew, plant nectar or pollen. Because parasitoids must be adapted to the life cycle, physiology and defenses of their hosts, they are limited in their host range, and many are highly specialized. Thus, accurate identification of the host and parasitoid species is critically important in using parasitoids for biological control.
Pathogens: Insects, like other animals and plants, are infected by bacteria, fungi, protozoans and viruses that cause disease. These diseases may reduce the rate of feeding and growth of insect pests, slow or prevent their reproduction, or kill them. In addition, insects are also attacked by some species of nematodes that, with their bacterial symbionts, cause disease or death. Under certain environmental conditions, diseases can multiply and spread naturally through an insect population, particularly when the density of the insects is high.
An example of an established population of an insect pathogen which has been successfully controlling its host is the fungus Entomophaga maimaiga, a pathogen of the gypsy moth. This fungus is believed to have been introduced about 1911, but was not discovered in forests until 1989, when it was widespread and abundant in New England. It has continued to control gypsy moth populations here for several years. It overwinters in leaf litter as resting spores, which germinate when gypsy moth larvae are present. First-instar caterpillars are dispersed by wind, and those that fall to the forest floor are probably infected while crawling to a tree. While these larvae are feeding in the tree canopy, if there is adequate rainfall, the fungus in their bodies produces spores that spread to other caterpillars. If conditions are suitable, this infection cycle will occur again during the larval stage. Large caterpillars rest during the day in forest litter, where they are also susceptible to infection by germinating resting spores. In late June, as infected caterpillars die in large numbers, new resting spores are produced to survive the next winter. This biological control agent is dependent on rain at appropriate times during the season to be successful.
Using biological control in the field
There are three primary methods of using biological control in the field: 1) conservation of existing natural enemies, 2) introducing new natural enemies and establishing a permanent population (called "classical biological control"), and 3) mass rearing and periodic release, either on a seasonal basis or inundatively.
1. Conservation of existing natural enemies
Reducing pesticide use: Most natural enemies are highly susceptible to pesticides, and pesticide use is a major limitation to their effectiveness in the field. The original idea that inspired integrated pest management (IPM) was to combine biological and chemical control by reducing pesticide use to the minimum required for economic production, and applying the required pesticides in a manner that is least disruptive to biological control agents. The need for pesticides can be reduced by use of resistant varieties, cultural methods that reduce pest abundance or damage, methods of manipulating pest mating or host-finding behavior, and, in some cases, physical methods of control. Many IPM programs, however, have not been able to move beyond the first stage of developing sampling methods and economic thresholds for pesticide application.
Several USDA and EPA surveys of pesticide use in major crops indicate that the quantity of pesticides used in the U.S. has been stable or increasing since the late 1980’s. Although there are variations by crop and class of pesticide, the overall trend is that previous reductions, due to the substitution of economic thresholds for calendar spraying and the use of pesticides effective at lower dosages, are being reversed by increases in acreage treated and number of treatments per season. This stagnation of pest management has resulted in calls for IPM to be re-focused toward preventing pest problems by greater understanding of pest ecology, enhancing the ability of plants and animals to defend themselves against pests, and building populations of beneficial organisms. This strategy is sometimes called "biointensive IPM."
Selecting and using pesticides to minimize the effect on natural enemies
The effect of a pesticide on natural enemy populations depends on the physiological effect of the chemical and on how the pesticide is used -- how and when it is applied, for example. While insecticides and acaricides are most likely to be toxic to insect and mite natural enemies, herbicides and fungicides are sometimes toxic as well. A database has been compiled on the effects of pesticides on beneficial insects, spiders and mites (summarized in Croft 1990 and Benbrook 1996). This database compares the toxicity of different pesticides and the "selectivity ratio" -- the dose required to kill 50% of the target pest divided by the dose that kills 50% of the affected natural enemy species. Among the insecticides, synthetic pyrethroids are among the most toxic to beneficials, while Bacillus thuringiensis and insect growth regulators were among the least toxic. In general, systemic insecticides, which require consuming plant material for exposure, and insecticides that must be ingested for toxicity affect natural enemies much less than pests.
Pesticides may also have more subtle effects on the physiology of natural enemies than direct toxicity. Several fungicides, such as benomyl, thiophanate-methyl, and carbendazim, inhibit oviposition by predacious phytoseiid mites. Certain herbicides (diquat and paraquat) make the treated soil in vineyards repellent to predacious mites.
The impact of pesticides on natural enemies can be reduced by careful timing and placement of applications to minimize contact between the beneficial organism and the pesticide. Less persistent pesticides reduce contact, especially if used with knowledge of the biology of the natural enemy to avoid susceptible life stages. Spot applications in the areas of high pest density or treatment of alternating strips within a field may leave natural enemies in adjacent areas unaffected. The effectivenss of limiting the areas treated may depend on the mobility of the natural enemy and the pest.
Providing habitat and resources for natural enemies
Natural enemies are generally not active during the winter in the Northeast, and thus, unless they are re-released each year, must have a suitable environment for overwintering. Some parasitoids and pathogens overwinter in the bodies of their hosts (which may then have overwintering requirements of their own), but others may pass the winter in crop residues, other vegetation, or in soil. A classic example is the overwintering of predacious mites in fruit orchards. Ground cover in these orchards provides shelter over the winter, refuge from pesticides used on the fruit trees, and a source of pollen and alternate prey.
The adults of many predators and parasitoids may require or benefit from pollen, nectar or honeydew (produced by aphids) during the summer. Many crop plants flower uniformly for only a short time, so flowering plants along the edges of the field or within the field may be needed as supplemental sources of pollen and nectar. However, diversification of plants within the field can also interfere with the efficiency of host-finding, particularly for specialist parasitoids. Populations of generalist predators may be stabilized by the availability of pollen and alternative prey, but the effectiveness of the predators still depends on whether they respond quickly enough, either by aggregation or multiplication, to outbreaks of the target pest. Thus, diversification of plants or other methods of supplementing the nutrition of natural enemies must be done with knowledge of the behavior and biology of the natural enemy and pest.
For example, the native lady beetle Coleomegilla maculata is a potentially important predator of the eggs and early instar larvae of Colorado potato beetle. The population feeding on the potato beetle depends on the availability of aphid prey in surrounding fields, including crops of alfalfa, brassicas, cucurbits, and corn, and on the availability of pollen from corn and several weeds, such as dandelion and yellow rocket. Although this predator does not currently control Colorado potato beetle on its own, more knowledge about managing C. maculata populations in the agricultural landscape could make it more effective.
2. Introducing new natural enemies and establishing a permanent population
This is a process which requires extensive research into the biology of the pest, potential natural enemies and their biology, and the possibility of unintended consequences (e.g. negative effects on native species which are not pests or on other natural enemies of the pest). After suitable natural enemies are found, studied, and collected, they must undergo quarantine to eliminate any pathogens or parasites on the natural enemy population. Then, the natural enemies are carefully released, with attention to proper timing in the enemy and pest life cycles, in a site where the target pest is abundant, and where disturbance of the newly released enemies is minimized. Although this process is long and complex, when it is successful, the results can be impressive and permanent, as long as care is taken in production practices to minimize negative effects on the natural enemy.
One of many examples of a pest controlled by successful introduction of new natural enemies is the alfalfa weevil. The alfalfa weevil is native to Europe, and was first reported in the US in 1904. It appeared in the eastern US about 1951, and by the 1970’s was a major pest across the country. Larval densities were high enough to require most growers to spray one or more times per year. Several parasitoids were introduced from Europe against this pest. The most successful introductions include two species of parasitoids attacking the larvae, one attacking the adult, and a parasitoid and predator attacking the eggs. A program to collect the most effective natural enemies, rear them in large numbers, and release the progeny assisted in the spread of some of these species. These natural enemies, plus a fungal disease that infects larvae and pupae, have kept the densities of alfalfa weevil far below the economic injury level in most years in the Northeast. The success of this biological control has been enhanced by cultural methods, such as timing cuttings to reduce weevil populations and avoid disruption of natural enemies. The introduction of additional natural enemies against other alfalfa pests and the use of pest-resistant alfalfa varieties have mimimized the insecticide use against alfalfa blotch leafminer and aphids, thus avoiding disruption of the natural enemies of alfalfa weevil.
3. Mass culture and periodic release of natural enemies
a. Seasonal inoculative release
In some cases, a natural enemy is not able to overwinter successfully here in the Northeast, due to the weather or the lack of suitable hosts or prey. In other cases, such as in greenhouses, all possible habitat for the natural enemy is removed at the end of the season or production cycle. Thus, particularly in annual crops, or in other highly disturbed systems, the natural enemy may need to be reintroduced regularly in order to maintain control of the pest.
Seasonal inoculative release of insect parasitoids and predators has been a highly successful strategy for biological control in greenhouses in Europe. This strategy was adopted by growers because of the prevalence of resistance to insecticides in many greenhouse pests, and the rising costs of chemical control. The program was originally built around use of the parasitoid Encarsia formosa against the greenhouse whitefly and the predacious mite Phytoseiulus persimilis against the two-spotted spider mite. Over the years, additional natural enemies have been added to control other pests, such as thrips, leafminers, aphids, caterpillars, and additional species of whiteflies, as needed. The costs of using biological control are now much lower in Europe than using chemical control for insect pests. Growers are informed about the details of implementation of the program, new developments, and new natural enemies through a network of extension advisers, specialized journals and grower study groups.
Two examples of seasonal inoculative release in the field are the use of the parasitic wasp, Pediobius foveolatus, against Mexican bean beetles, and the parasitic wasp, Edovum puttleri, against the Colorado potato beetle. Neither of these parasitoids can survive the winter in the Northeastern U.S. However, methods have been developed for rearing them in the laboratory and releasing them annually, and they multiply in the field, killing their hosts through the season. P. foveolatus is commercially available, and E. puttleri is being reared and released by the New Jersey Department of Agriculture for IPM of eggplant.
b. Biological insecticides or inundative release
These two approaches are fundamentally different from all the other approaches to biological control because they do not aim to establish a population of natural enemies that multiplies to a level where it reaches a long-term balance with the population of its hosts or prey. Instead, the idea is to use biological agents like an pesticide -- to release them in quantities that will knock down the pest population. Most commercially available formulations of insect pathogens are used inundatively.
Products based on the bacteria Bacillus thuringiensis are the best known example of a biological insecticide. A Bt spray is essentially an insecticide which works by paralyzing the gut of the insect (depending on the strain used, either caterpillars, Colorado or elm leaf beetle larvae, or mosquito or fungus gnat larvae). A protein produced by the bacterium is the active ingredient which paralyzes the gut, and in many products, there are no viable bacterial spores present, just a formulation of the active protein. Thus, the disease does not continue to spread in the insect population.
Beneficial nematodes are an example of live natural enemies that are inundatively released. These nematodes travel either through the soil or on the soil surface, and actively attack their insect hosts. Once inside, they release symbiotic bacteria, which multiply and kill the host. The nematodes feed on the bacteria and insect tissue, then mate and reproduce. After one to two weeks, new young nematodes emerge from the insect cadaver to seek new hosts. Nematodes are highly susceptible to desiccation, exposure to ultraviolet light, and extremes of temperature. They are most useful against insects living on or in the soil, or in other protected environments (such as tunneling inside plants). Adequate moisture and temperatures from about 53 to 86 degrees F. are critical to success.
Inundative release of insect and mite natural enemies in the field is still rather expensive, due to the costs of mass rearing, storage, and transportation of live organisms in the numbers required. However, research into artificial diets for natural enemies and other aspects of commercial production continues to bring down the cost.
Sources of more information:
Books about basic principles and practical aspects of biological control:
- DeBach, P. 1991. Biological control by natural enemies. 2nd edition. Cambridge University Press.
- US Congress, Office of Technology Assessment. 1995. Biologically based technologies for pest control. OTA-ENV-636. US Government Printing Office.
- Van Driesche, R.G. and T.S. Bellows, Jr. 1996. Biological Control. Chapman & Hall. International Thomson Publishing Co.
For young people:
- Jeffords, M.R. and A. S. Hodgins. 1995. Pests have enemies too: Teaching young scientists about biological control. Special Publication 18. Illinois Natural History Survey.
Integrating biological control into pest management systems
- Benbrook, C. et al. 1996. Pest Management at the Crossroads. Consumers Union, Yonkers NY.
- Croft, B. 1990. Arthropod Biological Control Agents and Pesticides. John Wiley and Sons, NY.
- Pickett, C.H. and R.L. Bugg. 1995. Enhancing natural control of arthropod pests through habitat management. AgAccess Publications, Davis, California.
- Van Lenteren. J.C. 1989. Implementation and commercialization of biological control in west Europe. pp. 50-70 in International Symposium on biological control implementation. Proceedings and abstracts. North American Plant Protection Organization Bulletin No. 6.
Extension guides to general biological control
- Henn, T. and R. Wienzierl. 1990. Alternatives in insect pest management. Beneficial insects and mites. Univ. of Illinois, Circular 1298.
- Mahr, D.L. and N.M. Ridgway. 1993. Biological control of insects and mites: An introduction to beneficial natural enemies and their use in pest management. Univ. of Wisconsin.
- Weinzierl, R. and T. Henn. 1989. Alternatives in insect pest management. Microbial insecticides. Univ. of Illinois, Circular 1295.
- Van Driesche, R.G. and E. Carey. 1987. Opportunities for increased use of biological control in Massachusetts. Research Bulletin 718. Mass. Agricultural Experiment Station, Univ. of Massachusetts.
Extension guides to specific crops or pests
- Hoffman, M.P. and A. C. Frodsham. 1993. Natural enemies of vegetable insect pests. Cornell Cooperative Extension.
- Mahr, S. E., D.L. Mahr, and J.A. Wyman. 1993. Biological control of insect pests of cabbage and other crucifers. North Central Regional Publication 471.
- Malais, M. and W.J. Ravensberg. 1992. Knowing and recognizing: The biology of glasshouse pests and their natural enemies. Koppert Biological Systems, The Netherlands.
- Obrycki, J., W. Wintersteen, and D. Lewis. 1992. Reducing insecticide use in the home garden. Pm-1502. Iowa State University Extension.
- Raupp, M.J., R. G. Van Driesche, and J. A. Davidson. 1993. Biological control of insect and mite pests of woody landscape plants: Concepts, agents and methods. Univ. of Maryland.
- Steiner. M.Y. and D.P. Elliot. 1987 Biological pest management for interior plantscapes. 2nd edition. Alberta Environmental Centre, Vegreville, Alberta, Canada.
- Van Driesche, R. and D.N. Ferro. 1989. Using biological control in Massachusetts: Colorado potato beetle. Massachusetts Coop. Extension Service Pub. L596.
- Van Driesche, R. and D.N. Ferro. 1989. Using biological control in Massachusetts: Cole crop Lepidoptera. Massachusetts Coop. Extension Service Pub. L597.
- Van Driesche, R., R. Prokopy, W. Coli, and T. Bellows. 1989. Using biological control in Massachusetts: Apple blotch leafminer. Massachusetts Coop. Extension Service Pub. L594.
- Van Driesche, R. et al. 1997. Biological control of forest arthropods in the Northeastern and North Central United States: A review and recommendations. U.S. Forest Service.
Lists of commercial suppliers of biological control agents:
- Hunter, C.D. 1997. Suppliers of beneficial organisms in North America. #PM 97-01. Comprehensive listing updated periodically. One free copy per request available from: California Environmental Protection Agency, Department of Pesticide Regulation, Environmental Monitoring and Pest Management, 1020 N Street, Room 161, Sacramento, CA 95814-5624. Also on Internet (see below)
- Directory of least-toxic pest control products. Produced annually by the IPM Practitioner, published by the Bio-Integral Resource Center, P.O. Box 7414, Berkeley, CA 94707.
- Midwest Biological Control News. Monthly newsletter. Annual subscription $12 per year from Susan Mahr, MBCN, Dept. of Entomology, University of Wisconsin, Madison, WI 53706. Also available on the Internet (see below).
- IPM Practitioner and Common Sense Pest Control Quarterly. Bio-Integral Resource Center, P.O. Box 7414, Berkeley, CA 94707
World Wide Web sites:
- Biocontrol Virtual Information Center: http://ipmwww.ncsu.edu/biocontrol/biocontrol.html
- Biological Control: A guide to natural enemies in North America: http://www.nysaes.cornell.edu/ent/biocontrol
- National Biological Control Institute: http://www.aphis.usda.gov/nbci.html
- Midwest Biological Control News: http://www.wisc.edu/entomology/mbcn/mbcn.html
- Suppliers of beneficial organisms in North America: http://www.cdpr.ca.gov/docs/dprdocs/goodbug/organism.htm
Research or review papers on specific examples cited here
- Coli, W.M., R. A. Ciurlino, and T. Hosmer. 1994. Effect of understory and border vegetation composition on phytophagous and predatory mites in Massachusetts commercial apple orchards. Agriculture Ecosystems & Environment 50: 49-60.
- Groden, E., F.A. Drummond, R. A. Casagrande, and D.L. Haynes. 1990. Coleomegilla maculata (Coleoptera: Coccinellidae): Its predation upon the Colorado potato beetle (Coleoptera: Chrysomelidae) and its incidence in potatoes and surrounding crops. Journal of Economic Entomology 83: 1306-1315.
- Hamilton, G.C. and J. Lashomb. 1996. Comparison of conventional and biological control intensive pest management programs on eggplant in New Jersey. Florida Entomologist 79:488-496.
- Kingsley, P.A., M.D. Bryan, W. H. Day, T. L. Burger, R.J. Dysart, and C.P. Schwalbe. 1993. Alfalfa weevil (Coleopter: Curculionidae) biological control: Spreading the benefits. Environmental Entomology 22: 1234-1250.
- Nyrop, J.P., J. C. Minns, and C.P. Herring. 1994. Influence of ground cover on dynamics of Amblyseius fallacis Garman (Acarina: Phytoseiidae) in New York apple orchards. Agriculture Ecosystems & Environment 50: 61-72.
- Russell, E.P. 1989. Enemies hypothesis: a review of the effect of vegtational diversity on predatory insects and parasitoids. Environmental Entomology 18: 590-599.
- Sheehan, W. 1986. Response by specialist and generalist natural enemies to agroecosystem diversification: A selective review. Environmental Entomology 15: 456-461.
- Stevens, L.M., A.L. Steinhauer, and J.R. Coulson. 1975. Suppression of Mexican bean beetle on soybeans with annual inoculative releases of Pediobius foveolatus. Environmental Entomology 4: 947-952.
- Weseloh, R.M. and T.M. Andreadis. 1992. Epizootiology of the fungus Entomophaga maimaiga, and its impact on gypsy moth populations. Journal of Invertebrate Pathology 59: 133-141.
Biological control is the use of living organisms to suppress pest populations, making them less damaging than they would otherwise be. Natural enemies of insects play an important role in limiting the densities of potential pests. These natural enemies include predators, parasitoids, and pathogens. Biological control of potential pest insects can be increased by: 1) conservation of existing natural enemies, 2) introducing new natural enemies and establishing a permanent population, and 3) mass rearing and periodic release of natural enemies, either on a seasonal basis or inundatively.