Biological control is the beneficial action of parasites (technically parasitoids), pathogens, and predators in managing pests and preventing or reducing their damage. Biocontrol provided by these living organisms called "natural enemies" is especially important for reducing the abundance of pest insects and mites and their damage to crops.
Conservation
Preserve resident natural enemies by choosing cultural, mechanical, and selective chemical controls that do not kill these species or disrupt their beneficial activities. Many insect and mite pests in flower and nursery crops have natural enemies that can keep their populations below economically damaging levels. Conservation is the primary way to successfully use biological control. For example, using selective pesticides that do not kill natural enemies or disrupt their beneficial activities is a key component of integrated pest management.
To enhance the effectiveness of biological control
- Consult the Natural Enemies Gallery to become familiar with common and important parasites and predators of pests.
- Control ants, which protect some pests from parasites and predators.
- Exclude pests (e.g., bring only pest-free plants into growing areas, screen greenhouses) and use excellent sanitation (e.g., promptly remove infested crop debris and eliminate weeds and other nearby noncrop hosts of pests). These methods provide direct control and reduce or prevent the in-migration of abundant pests that can overwhelm the potential effectiveness of natural enemies that are resident or released.
- Grow flowering insectary plants in or near outdoor nurseries to provide nectar and pollen to feed adult parasites and predators. Grow a series of plants that flower sequentially so blossoms are available throughout the growing season. Ideal insectary plants are those that can also be marketed, such as for culinary herbs and cut flowers. Avoid insectary-plant species that host arthropod pests or plant pathogens that can move to damage nearby crops. Consult Flowers; Fruit Trees, Nuts, Berries, and Grapevines; Trees and Shrubs; and Vegetables and Melons for lists of the pests reported on these plants.
- Keep growing areas and plants clean and free of dust, which can impede parasites' and predators' ability to locate and attack prey.
- Minimize or avoid the application of broad-spectrum, persistent (long-residual) acaricides and insecticides.
- Rely on pesticides that do not kill natural enemies or disrupt their beneficial activities where this is feasible. Consult the table RELATIVE TOXICITIES OF PESTICIDES USED IN FLORICULTURE AND NURSERIES TO NATURAL ENEMIES AND HONEY BEES to learn which insecticides and miticides can be applied with minimal impact on natural enemies.
- When pesticides are used, apply them in a selective manner. For example, spray only the more heavily infested portions of the crop (hot spots) and where needed to prevent crop damage. Conserving resident natural enemies elsewhere in the unsprayed growing area allows parasites and predators to reproduce and the adults to disperse and attack pests elsewhere in growing areas where the plant-feeding species are not currently abundant or causing noticeable damage.
Augmentation
When the desired natural enemies are not present, or they are not abundant enough to sufficiently reduce pest numbers, biological control can sometimes be augmented with the purchase and release of commercially reared natural enemies. Augmentative releases can be inoculative or inundative. As discussed below and in more detail in Natural Enemy Releases for Biological Control of Crop Pests, there are various release strategies (e.g., banker plants discussed below). Be aware that in most situations, employing practices that conserve resident natural enemies is more effective and less expensive and time consuming than purchasing and releasing them.
Inoculative release
When pest populations are low, relatively few natural enemies may be needed for effective release. The introduced parasites or predators are expected to reproduce, and it is their progeny that are expected to provide biological control. Releasing the mealybug destroyer lady beetle (Cryptolaemus montrouzieri) in late winter or spring to control foliar-feeding mealybugs is an example of inoculative release. The lady beetle is a highly effective predator of mealybug species that lay eggs in a cottony mass (ovisac). But this predator is native to the semi-tropics and does not survive cold weather, so to be effective it must be purchased and released early each growing season where mealybugs have been a problem. A parasitic wasp, Leptomastix dactylopii, can be released to kill nymphs of citrus mealybug. The mealybug destroyer and Leptomastix dactylopii parasite can be released in combination where citrus mealybug is a pest.
Inundative release
When releasing large numbers of natural enemies, often several times over a growing season, the individuals released are expected to provide biological control. Periodically releasing Trichogramma spp. egg parasitic wasps to destroy moth eggs is an example of inundative biological control. Trichogramma releases to kill moth eggs are compatible with applying Bacillus thuringiensis (Bt) to control the caterpillar stages and drenching soil with entomopathogenic nematodes (Heterorhabditis and Steinernema spp.) to kill the soil-dwelling, mature larvae and pupae of armyworms, cutworms, and European pepper moth (Dufo moth).
Periodically releasing large numbers of convergent lady beetles (Hippodamia convergens) to temporarily control aphids is also inundative release. Each convergent lady beetle can consume several dozen aphids per day. Because the beetles will virtually all disperse within 1 or 2 days of being released, and generally will lay few eggs and not reproduce in the crop, releases must be repeated about weekly to provide control where aphids are an ongoing problem. Because the purchased beetles are field collected from mountainous foothills during their overwintering (diapause) phase, when collected, transported, and released, the convergent lady beetles are physiologically obliged to fly and disperse before settling to lay eggs among aphids. For more information, see Lady Beetle Releases for Aphid Control: How to Help Them Work (PDF ) from the newsletter UC IPM Retail Nursery and Garden Center IPM News.
Banker plants
One strategy for releases is to maintain natural enemies by rearing them on alternative insects that are not pests of the desirable crops. For example, barley, oats, or wheat infested with bird cherry-oat aphid might be used to maintain Aphidius colemani parasitic wasps in the greenhouse. This aphid is not a pest of many greenhouse bedding plants, but the wasps will feed on many other kinds of aphids (like green peach aphid) that are pests. These banker plants (e.g., planted in containers) with nonpest aphids and parasites or predators can be scattered throughout a greenhouse or nursery to provide a continual source of natural enemies that disperse to consume pest species on crops, such as for aphid control in greenhouses (PDF).
Effectiveness of releases
Augmentation is likely to be effective only in situations where researchers or other pest managers have previously demonstrated success. Guidelines for releasing natural enemies are provided in this publication in the sections on aphids, foliar-feeding mealybugs, fungus gnats, twospotted spider mite, and whiteflies.
Because natural enemies are living organisms, they require food (adults of many species require nectar and pollen), shelter from harsh conditions, suitable environmental conditions, and water. Natural enemies may be adversely affected by low humidity or extreme conditions such as hot temperatures. Residues of certain pesticides can persist for weeks or months, harming natural enemies long after losing their effectiveness against pest species. Many beneficial species stop reproducing under short day length or prolonged cool conditions; supplemental light may be necessary for them to reproduce and be effective year-round. Environmental conditions required by natural enemies (such as long days) may not be compatible with production needs of certain crops.
Desperate situations where pests are already abundant or damaging are not good opportunities for augmentation. Because pest presence is necessary to sustain natural enemies, choose crops where some levels of the target pests and their feeding can be tolerated (crops with moderate to high pest thresholds). Begin making releases early in the production cycle even before infestations are observed. Consider what other pests may occur in the crop and how they can be managed in ways that are compatible with conserving natural enemies. For more information, see Natural Enemies Handbook: The Illustrated Guide to Biological Pest Control, UC ANR 3386 (print edition), 9038 (ePub), Natural Enemy Releases for Biological Control of Crop Pests, and RELATIVE TOXICITIES OF PESTICIDES USED IN FLORICULTURE AND NURSERIES TO NATURAL ENEMIES AND HONEY BEES.