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Biological Control

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Definition

Biological control is the use of natural enemies to suppress pests. Biological control tactics include the importation, conservation, and augmentation of natural enemies. Biological control is an environmentally safe method and is the basis for some integrated pest management programs.

Types of Biological Control

Types of biological control include importation (use of exotic natural enemies for pest control) and conservation (use of selected control tactics that spare natural enemies and cultural practices to modify the environment to favor natural enemies).

The release of natural enemies (predators, parasites and pathogens) to control pests is a type of biological control called augmentation. This approach uses commercially available species that are applied in a timely manner to prevent population increases, or to suppress a pest population.

Importation

Many pests are exotic and have no natural enemies in Texas. Reuniting pests with their natural enemies often provides the most dramatic and sustainable method of suppressing them. The importation of such natural enemies is classical biological control. The parasite Neodusmetia successfully suppressed Rhodesgrass mealybug in Texas after being widely distributed by airplane. The search for exotic beneficial organisms which can control major plant pests in Texas is a major mission of the biological control scientists within the Department of Entomology at Texas A&M University.

Conservation

Pesticides kill beneficial predators, parasites and pathogens as well as pests, and can cause outbreaks of secondary pests or rapid resurgence of pests that were initially suppressed. Using nonchemical control methods, or pesticides which kill only the target pest, protects natural enemies. Some easily seen predators are spiders, lacewings, lady beetles, ground beetles, rove beetles, syrphid flies, flower flies, hover flies, true bugs (including minute pirate bugs, big-eyed bugs and damsel bugs), predatory mites and even fire ants. However, many important natural enemies are rarely seen, such as parasitic wasps and flies (more than 8,500 species), nematodes and pathogenic bacteria and fungi.

Augmentation

Natural enemies can be released all at once or over time to suppress pests or keep their numbers low. Also, the environment can be enhanced to favor natural enemies. Although research has shown that releases of natural enemies can be very effective in greenhouses and interiorscapes, outdoor releases are affected by unpredictable environmental conditions. Furthermore, if a second pest is unaffected by the released organism, pesticides used to control the second pest often eliminate the natural enemy of the first pest. Specific recommendations for Texas are still being developed.

The application of microorganisms in a manner similar to conventional pesticides is a type of augmentation. These products are referred to as "microbial insecticides." Several products available contain varieties of the bacterium, Bacillus thuringiensis, which controls certain caterpillars, beetles and flies but does not affect other arthropods. Microbial insecticides are relatively slow acting and are most effective if applied when pest numbers are low and pests are in early stages of development.

Examples of augmentive biological control products

Commercial products available for use in augmentive biological control include microbial insecticides containing living pathogens (bacteria, fungi and viruses) and multicellular animals (predators, parasites and nematodes). Other products occasionally used with biological control agents include synthetic honeydew, flowers to attract and conserve beneficial insects in and around pest-prone or pest-infested sites, and traps using colors or scents as attractants.

Best use for augmentive products

Purchasing and releasing natural enemies for pest suppression is a rapidly developing technology but there is still much to be learned to assure effective use of these products. Results are often difficult to evaluate and can be inconsistent because of differing conditions (e.g., environmental, meteorological, etc.). Natural enemies are living and their behavior under different environmental conditions can influence the degree of pest control. Cost-effective use of augmentive releases requires an understanding of the pest(s), natural enemies, economic goals and the environment. Commercial uses often demand intensive monitoring or scouting of the cropping system.

Augmentive releases are meant to reduce populations at points in time. Releases at low pest densities are more effective than attempts to reduce high pest densities. Action levels or economic thresholds for release of natural enemies and effective release rate(s) have often not been established through scientific research.

Timing of the release of natural enemies is critical since most require some time to affect the pest population. In addition, many natural enemies attack only certain life stages (e.g., egg or larval stage) of the pest.  Multiple releases may also be necessary to maintain pest suppression.

Biological control using parasites is generally pest-specific. When multiple pests occur (e.g. aphids, thrips, plus beetles), natural enemies are needed for each pest. In cases where natural enemies are unavailable for augmentation, use of a selected pesticide that spares other natural enemies may be necessary.

Environmental conditions change dramatically and outdoor releases of natural enemies can be negatively affected by high winds, rain, hot or cold weather and other insects in the ecosystem (e.g., red imported fire ants). These factors are often unpredictable and result in erratic results from releases. Release of appropriate natural enemies in greenhouses and interiorscapes often provide more consistent results.

Insecticide residues on the crop or site, or insecticide drift from adjacent areas, can remain toxic to natural enemies long after the pesticide was applied. Residues should be mitigated prior to releases.

What support can I expect from the companies selling these products?

Companies selling products and promoting their use should provide the consumer with directions on how to use their products, and support their claims of product performance. Insectaries and brokers, the companies producing and marketing parasites and predators, assure the delivery of viable natural enemies of the stated species or strain. They usually do not  guarantee results from releases of these biological control agents even when used as directed. Although researchers and Extension faculty at The Texas A&M System are involved in evaluating some of these products, suggestions for their most effective use are still being developed.

Are these products regulated by any laws?

Microbial insecticides (bacteria, fungi, viruses) are regulated like pesticides by the Environmental Protection Agency (EPA) under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). Multicellular animals (arthropod predators, parasites, nematodes, etc.) are NOT registered or regulated by the EPA under FIFRA. Complaints regarding product performance can be reported to the Federal Trade Commission (FTC).

The user of purchase-and-release natural enemies must be aware of legal and biological limitations of augmentive biological control methods. Just restricting frequent use of broad spectrum insecticides often will allow a diverse group of naturally occurring beneficial organisms to survive, sometimes profoundly impacting pest population densities. As the cost of natural enemy products continues to decrease and delivery systems and methods are improved, the economic feasibility of using these methods in commercial pest control will undoubtedly improve.

Considerations for Purchasing and Releasing Biological Control Products

Bastiaan M. Drees, Professor and Extension Entomologist and Allen Knutson, Professor and Extension Entomologist

Purchasing and releasing natural enemies for control of insect and mite pests is an attractive alternative to the potential hazards associated with chemical insecticides (i.e., toxic effects on non-target organisms, development of pesticide resistance and persistence in the environment).  Furthermore, releasing natural enemies, such as lady beetles, is educational and fun for children and adults alike. However, consumers are sometimes disappointed with the level of pest control achieved by releasing natural enemies. Successful use of natural enemies requires the use of appropriate species under proper conditions. A better understanding of this method of biological control can help improve your chances of success.

Microbial Insecticides

(Modified from R. Weinzieri and T. Henn, 1989)

Single cell organisms, such as bacteria, fungi and protazoa, and viruses, have been mass produced and formulated for use in a manner similar to insecticides. Products containing these organisms are regulated by the Environmental Protection Agency and use is governed by the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).

Bacillus thuringiensis

The bacterium, Bacillus thuringiensis (B.t.), reproduces by spores. The spores are produced in the bacterium cell along with a crystalline protein called an endotoxin. The endotoxin, with or without the spores must be ingested by the target insect in order to be effective. Once ingested, the endotoxin is activated by the alkaline conditions in the insect's stomach. The toxin attaches to specific receptors on the gut wall, causing the gut lining to break down. This method normally allows the spores to enter the hosts blood (hemolymph) where the bacterium can proliferate.

Different species and strains and Bacillus bacteria are known to affect different groups of insect pests, primarily due to differences in endotoxin receptor sites on the gut wall:

  • Bacillus thuringiensis var. kurstaki (Dipel®, Javelin® and others) - caterpillars of moths and butterflies
  • Bacillus thuringiensis var. israelensis (Vectobac®, Gnatrol®) - larvae of flies such as fungus gnats
  • Bacillus thuringiensis var. san diego (M-One®) - larvae of beetles such as elm leaf beetles and Colorado potato beetles
  • Another species, Bacillus popollia, or milky spore disease has been marketed for controlling Japanese beetle larvae. Occasionally, this product is marketed in Texas for white grub control. Research conducted in Texas does not indicate that this species affects the species of white grubs found in the state.

For most effective use, B.t. products must be applied when insects are in their early larval stages (first or second instars) and are actively feeding. Several days may be required for larvae to die, although feeding usually stops soon after ingestion. For foliar applications, additives such as feeding stimulants and stickers are often added to the spray mixture to ensure that target pests rapidly ingest the treated leaves and that rain does not wash treated surfaces.  On foliage, B.t. treatments degrade rapidly. Applications are most effective when made in the evening or on cloudy days.

B.t. is formulated in liquid concentrates, wettable powders, dusts and granules. One product, MPV®, has been developed by inserting the genes that code for the B.t. endotoxin into another hard-bodied microorganism. The micro-organism is then killed and used as a capsule in which the endotoxin is protected. These endotoxin genes have also been genetically engineered into several plants, including tobacco, tomatoes and cotton. These plants have been shown to be resistant to caterpillars. Unfortunately, there have been several documented cases of insect pests becoming resistant to B.t. endotoxins. 

Toxicology of Bacillus thuringiensis. The varieties of Bacillus thuringiensis used commercially survive when injected into mice, and at least one of the purified insecticidal toxins is toxic to mice. Infections of humans have been extremely rare (two recognized cases) and no occurrences of human toxicosis have been reported. From studies involving deliberate ingestion by human subjects, it appears possible, but not likely, that the organism can cause gastroenteritis. B.t. products are exempt from tolerance on raw agricultural commodities in the United States. Neither irritative nor sensitizing effects have been reported in workers preparing and applying commercial products. A single case of corneal ulcer caused by a splash of B.t. into the eye was successfully treated (Morgan, D.P. 1989. Recognition and management of pesticide poisonings. EPA-540/9-88-001. U.S. Environmental Protection Agency, Washington, D.C. 205 pp.).

Example of registered product: Green Light® Plus Bio-Worm Killer® Concentrate - For tomato hormworms on tomato, cabbage looper on broccoli, cauliflower, collards, kale, mustard greens, turnip greens, cabbage, celery, lettuce, melons and tomatoes. Imported cabbage worm on broccoli, cabbage, cauliflower, collards, kale, mustard greens and turnip greens.  On shade trees and ornamentals for leaf-feeding worms: spring cankerworm, fall cankerworm and tent caterpillar. Apply at first sign of infestation and repeat at weekly intervals when needed to maintain control. Apply thoroughly; cover all foliage surfaces.

Fungi

Several fungi have been studied as potential microbial insecticides. Beauveria bassianacan affect a wide variety of arthropods. However, no products containing fungi are currently registered for use in Texas. Environmental conditions, particularly temperature and humidity are important factors effecting the success of fungal treatments, particularly when using preparations of fungal spores.

Protozoa

The protozoan Nosema locustae is available in a few products such as Grasshopper Attach®, and Hopper Stopper. These products are sold for the control of grasshoppers. Effectiveness of these products for small-scale use, such as gardens and yards has not been demonstrated. The disease must be ingested to be effective and it is very slow acting. Grasshoppers are strong fliers and can easily move long distances, making the effectiveness of these treatments on a small scale questionable.

Commercially Available Natural Enemies (Multicellular Animals)

(Extracted from T. Henn and R. Weinzieri, 1990)

Predators

Convergent lady beetle, Hippodamia convergens. The adult beetle is orange with six small, black spots on each side and a black area with white markings behind the head. The larva is soft-bodied, alligator-shaped, grey and orange in color with rows of raised black spots. Larvae and adults feed on aphids and other small, soft-bodied insects and mites.  Adults also feed on nectar, pollen and honeydew.  Development (egg to adult) takes 2 to 3 weeks, and adults live up to several months.

This species occurs throughout North America.  In California, adult beetles overwinter in huge aggregations on mountainsides.  These are harvested, stored at cool temperatures and shipped to customers in the spring and summer for release in gardens or crops. Unfortunately these releases have limited usefulness because the beetles fly away soon after release.  They provide long- term, adequate aphid control only if they reproduce.  Female beetles cannot produce eggs until they have fed on prey, and they lay eggs only where prey are abundant.  Larvae provide better aphid control.

Suppliers recommend release rates ranging from 1 pint to 1 quart of beetles per home garden, and from 1 gallon of beetles per acre to 1 gallon per 15 acres for larger areas. 

Mealybug destroyer, Cryptolaemus montrouzieri.  The adult of this Australian lady beetle is a small (3/16 inch), black beetle with orange on the wing tips and behind the head.  The larva is covered with white, waxy material  and resembles a mealybug. Adult and larval stages feed on above ground mealybug species, but will also consume aphids and immature scale insects.  This natural enemy won't reproduce without large numbers of mealybugs and optimum environmental conditions (72 to 77 degrees F and 70 to 80 percent relative humidity). The mealybug destroyer may not control mealybug infestations during winter months. It is most effective when used for quick reductions of heavy mealybug infestations.

Suppliers recommend releasing one beetle per 2 square feet of planted area or two to five beetles per infested plant.  Supplies are often limited because colonies are difficult to maintain.

The lady beetle, Delphastus pusillus, has been shown to suppress sweet potato whitefly infestations. Commercial availability is very limited.

Lacewing. Chrysoperla carnea, the common green lacewing, is the most widely available lacewing species. Chrysoperla rufilabris is an eastern lacewing species that is better adapted for use in tree crops.  Green lacewings occur naturally throughout North America.  The adult has a delicate, light green body with large, clear, veined wings. Larvae are small, elongated and greyish brown with sickle-shaped mandibles.  Eggs are deposited singly on silken stalks. Although C. rufilabris and most other lacewings are predaceous as adults, the adult C. carnea feeds only on nectar, pollen and aphid honeydew, and females cannot produce eggs if these foods are unavailable.  Adults fly at night and disperse soon after emerging whether or not ample food is present. Artificial foods (Bug Chow®, BugPro® or Wheast®) may be a useful supplement to natural foods (nectar and honeydew) to attract and concentrate adult lacewings. 
Lacewings can be purchased as eggs shipped in a mixture of rice hulls and frozen caterpillar eggs or larvae.  Suppliers recommend releasing from one to five lacewing eggs per square foot for gardens, and from 50,000 to 200,000 lacewing eggs per acre in field crops and orchards.  Releases are made singly or sequentially at 2-week intervals, depending on the pest to be controlled.  The costs of purchasing and releasing such high numbers of lacewing eggs may be prohibitive.

Preying mantid. Several species occur naturally in most of the U.S.  In the fall, females produce egg cases that may contain up to 200 eggs.  Eggs hatch in the spring. Nymphs and adults are territorial and general predators, feeding on a wide variety of insects, including other mantids.  They are not effective in controlling aphids, mites or most caterpillars.  Mantids are nearly useless for pest control in gardens because of their feeding habits and high mortality rate.  Egg cases of the Chinese praying mantid, Tenodera aridifolia sinensis, are most commonly available for purchase. 

Predatory mites. Spider mite predators in the genera Phytoseiulus and Amblyseius are quick breeding, fast moving, pear shaped predators with short life cycles (from 7 to 17 days, depending on temperature and humidity). They are pale reddish and distinguished from twospotted spider mites by their lack of spots, their long legs and rapid movement. Predatory mite eggs are elliptical and larger than the spherical eggs of spider mites.  Adults feed on all stages of spider mites, whereas the nymphs feed on eggs, larvae and nymphs.

Phytoseiulus persimilis does best in a temperature range of 70 to 80 degrees F and a relative humidity of 60 to 90 percent.  The suppliers suggest releasing from two to 30 predatory mites per plant, depending on the stage and susceptibility of the crop.  Some experimentation may be necessary to determine the best release rate and method for specific situations. U.S. insectaries often recommend releasing P. persimilis when one or fewer spider mites per leaf occur throughout the greenhouse.  Where spider mite populations are larger, it is a good idea to apply an insecticidal soap or other nonresidual insecticide to reduce the infestation before predatory mites are  released. Spot releases and uniform, area-wide releases both are occasionally advocated, depending on the distribution of the spider mite.  In Europe, spider mites are sometimes released into the greenhouse at a low rate soon after planting, followed later by the release of predators. This practice allows the predatory mites to become established. In other cases, spider mites and predators are released together early in the season.

Phytoseiulus longipes tolerates temperatures up to 100 degrees F when humidity is high, and tolerates lower relative humidities (down to 40 percent) at lower temperatures (70 degrees F).  Amblyseius californicus also tolerates high temperatures (up to 90 degrees F), but consumes mites at a slower rate than Phytoseiulus species and survives better when spider mite numbers are low.  Mixed releases of the two predators have been made in greenhouses where conditions and spider mite numbers are variable.

Thrips predators. Amblyseius cucumeris and A. mckenziei (or A.barkeri) are mites that feed on the western flower thrips and onion thrips.  They can also subsist for short periods on pollen, fungi or spider mite eggs. These mites require high humidity and are sensitive to insecticides.  They do not produce eggs during the winter, which makes thrips control difficult at that time.

Suppliers recommend releasing large numbers of these predators to control thrips. Rates vary from 10 to 50 predatory mites per week per plant, plus an extra 25 to 100 mites per infested leaf in the greenhouse, until there is one predatory mite for every two thrips.  Very little research has been published about these types of releases, but  literature from Europe indicates that lower release rates may be feasible. This is not an effective method of vector control for dealing with tomato spotted wilt virus.

Parasites

Greenhouse whitefly parasite, Encarsia formosa. This is a tiny parasitic wasp. Adults lay eggs during the third and fourth nymphal whitefly stages.  Parasitized whitefly nymphs blacken within 2 to 3 weeks and die as the wasp larvae develop inside.  Adults also feed on and kill early and late nymphal stages. Encarsia develops best in bright light, 70 to 80 degrees F temperatures and 50 to 70 percent  relative humidity. Under these conditions it reproduces faster than whiteflies.  Commercially available Encarsia are not effective parasites of the sweet potato whitefly, although adult wasps will feed on and kill the immature stages.

Most U.S. suppliers suggest making releases when fewer than one adult whitefly per upper leaf is found throughout the greenhouse. Releases should be made at 2-week intervals to control immature whiteflies.  Release rates range from one to five wasps per square foot or one  to eight per plant, depending on plant species and the severity of the infestation.   In European vegetable greenhouses, whiteflies are introduced at low levels before the natural enemy is released. Where this approach is not used, parasites must be released at the very first sign of whitefly infestation or on a preventive schedule. 

Caterpillar egg parasites, Trichogramma spp. These wasps are extremely small, averaging about 1/4-inch in length as adults. Females lay their eggs in the eggs of moths and butterflies.  A few species parasitize the eggs of other insects.  Trichogramma larvae develop inside host eggs, killing the embryos. Instead of a caterpillar, one or more adult wasps emerge from the parasitized egg.  There are many species and strains of Trichogramma. Some are general parasites but others parasitize only selected species. 

Trichogramma wasps are usually released as mature pupae inside host eggs.  Adults emerge within 1 to 3 days and live for about 9 days.  Releases are timed to correspond with the egg-laying period of the pest, as determined by monitoring. Single or sequential releases at rates of 50,000 to 300,000 wasps per acre per release have been made, but the results have been extremely variable. 

Two species available include:

1) T. pretiosum, which parasitizes more than 200 species of caterpillar eggs (though not equally effective against all species); and

2) T. minutum, which controls orchard and forest caterpillars. The size and host-finding ability of Trichogramma is influenced by the way the wasp is reared. 

In commercial operations they are reared in the small eggs of the Angoumois grain moth. The parasites produced this way are also small and may not do well at locating eggs of target pests in the field.  Parasites reared locally on the eggs of the intended target pest are more likely to provide successful control.

Parasitic nematodes

Insect parasitic (entomogenous) nematodes, such as Steinernema carpocapsae, have been mass reared and formulated on sponges so that they can be mixed with water and sprayed onto plants or for treating soil and growing media. Different species and strains of these nematodes have different abilities to kill specific insect pests. Since these organisms are multicellular animals, these products (BioSafe®, Exhibit®, Guardian Nematodes and others) are not regulated by the Environmental Protection Agency). These nematodes are considered to be nontoxic and nonpathogenic to plants and animals.

The juvenile (J3) stage of the nematode is used in these products. Once applied, the nematodes actively seek out arthropods. After successfully gaining entry into the arthropod body, the nematodes' symbiotic bacteria are injected into the host's blood to paralyze them. The nematodes then complete their development inside. Use of the nematodes have been more successful in the control of arthropods with restricted movement such as root-feeding weevil larvae. However, they are commonly used to control insect pests such as fungus gnat larvae and sod webworms. Foliar treatments are most effective when applied in the evening or at times when water film remains on the leaf surface for an extended period of time. However, care must be taken with these treatments to avoid promoting disease outbreaks. Effectiveness of applications is dependent upon environmental conditions. Temperature, humidity, soil microfauna and moisture all effect the free-living juvenile stages.

References:

Hunter, C. D. 1994 ed. Suppliers of Beneficial Organisms in North America. California EPA, Dept. of Pesticide Regulation, l020 N. Street, Room l6l, Sacramento, CA 95814-5604; 916/324-4100.

Knutson, A., 1998. The Trichogramma Manual. B-6071. The Texas Agricultural Extension Service, The Texas A&M System, College Station, TX.

Flint, M. L. and S. H. Dreistadt. 1999.  Natural Enemies Handbook - The Illustrated Guide to Biological Pest Control. University of California Press, 6701 San Pablo Ave., Oakland, CA 94608-1239.

Henn, T., and R. Weinzieri. 1990. Beneficial insects and mites. Circular 1290. University of Illinois, College of Agriculture, Cooperative Extension Service.

Steiner, M.Y. and D.P. Elliott. 1987. Biological pest management for interiorscape plantings. Alberta Public Affairs Bureau, Publication Services, 11510 Kingsway Ave., Edmonton, AB, Canada T5G 2Y5.

For a listing of available biological control organisms in North America, see Suppliers of Beneficial Organisms in North America from the California Department of Pesticide Regulation (CDPR).

For a report on advances in biological control technology in the greenhouse, request "Biological control of two-spotted spider mite in California greenhouses" and "Biological control of greenhouse whitefly in California greenhouses" from the Bulletin Secretary, University of California Cooperative Extension, 4205 Wilson Way, Stockton, CA 95205.

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