Biological control is the management of pests and their damage by the beneficial action of parasites (parasitoids), pathogens, and predators. These beneficial organisms, collectively, are named natural enemies.
Types of natural enemies. Parasites, pathogens, and predators are the primary groups used in biological control of insects and mites. Most parasites and pathogens, and many predators, are highly specialized and attack a limited number of closely related pest species. Some popular natural enemies are convergent lady beetles (ladybugs), syrphid flies, soldier beetles, green lacewings and predatory wasps.
Parasites. A parasite is an organism that lives and feeds in or on a host. Insect parasites can develop on the inside or outside of the host’s body. Often only the immature stage of the parasite feeds on the host. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) feed on and kill their hosts, providing an easily overlooked but important source of biological control in addition to the host mortality caused by parasitism.
Pathogens. Natural enemy pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host. Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations. In addition to a naturally occurring disease outbreak, some beneficial pathogens are commercially available as biological or microbial pesticides. For example, Bacillus thuringiensis, or Bt.
Predators. Predators kill and feed on several to many individual prey during their lifetimes. Many species of amphibians, birds, mammals, and reptiles prey extensively on insects. Predatory beetles, flies, lacewings, true bugs (Order Hemiptera), and wasps feed on various pest insects or mites. Most spiders feed entirely on insects.
How to attract natural enemies. Most likely, natural enemies are already present in your garden. Your job is to give them the food, water, and shelter they need to thrive, and they will reward you by patrolling your garden.
Here are some things you can do to attract beneficial insects into your garden and keep them there:
Avoid the use of pesticides
Pesticides tend to be indiscriminate, killing beneficial insects as well as insect pests. Eliminate or reduce the use of broad-spectrum, persistent pesticides whenever possible. When pesticides are used, apply them in a selective manner. Treat only heavily infested areas with “spot” applications instead of entire plants. Choose insecticides that are more specific in the types of invertebrates they kill, such as Bacillus thuringiensis (Bt) that kills only caterpillars that consume treated foliage.
Provide habitat. Create a garden that celebrates diversity. Include both annual and perennial ground covers, shrubs, trees, grass, and some permanent arrangements, such as stone paths and decorative rock accents, which will provide shelter for insect predators. Clumps of native grasses make good overwintering sites for a variety of predatory insects. Include varieties of flowering plants that bloom throughout the year. Make sure to include different types of flowers including natives.
A shallow dish of water filled with pebbles or gravel and placed in a sheltered area will give them a safe place to get a drink. Change the water regularly to keep mosquitos from laying eggs.
Recognize natural enemies. Proper identification of pests, and distinguishing pests from natural enemies, is essential for effective biological control. For example, some people may mistake syrphid fly larvae for caterpillars. However, syrphid fly larvae are found feeding on aphids and not chewing on the plant itself. Also, many beneficial insects that we know look nothing like the adult version when they are larvae. The larvae of the lady beetle (ladybug) looks like a little black and orange alligator and does not resemble the adult version at all.
Tolerate a small number of pests. This will provide a continuous food supply for natural enemies. Try “sacrificing” some plants in the garden as host plants for natural enemies. Many plants can tolerate a small number of pests while you wait for the natural enemies to find them.
Eliminate ants. The Argentine ant and certain other ant species are considered pests primarily because they feed on honeydew produced by insects that suck sap, such as aphids. Ants protect honeydew producers from predators and parasites that might otherwise control them.
Information for this article was obtained from the UC IPM Pest Note 74140. Natural enemy identification and information can be found on the UC IPM Natural Enemies website: ipm.ucanr.edu/natural-enemies.
If you have gardening questions, contact the Master Gardeners at (209) 953-6112 or visit our website at ucanr.edu/sjmg.