Parasitism in the Garden - March 16, 2005
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

For many people, parasites carry a negative connotation. When you think of parasites, you might imagine ticks or tapeworms. Gardeners might think of mistletoe and dodder: parasitic plants that suck energy from their hosts. Root knot nematodes also feed on plant roots, weakening the host plant. However, in your garden, parasites can also be a very positive influence, especially in controlling plant-feeding insects.

A parasite is an organism that lives and feeds in or on a larger host. Unlike predators, parasites have a prolonged and specialized relationship with their hosts, usually parasitizing one individual host in their lifetimes. Parasites often weaken their hosts but usually do not usually kill them. In some cases, they have little negative impact on the host. However, insect parasites that significantly weaken or kill plant-feeding insect hosts are a gardenerís best friends.

A special category of parasites are called parasitoids. Parasitoids keep the host alive just long enough for the immature young to feed. Once they reach maturity, the host dies. Many wasps and flies are parasitoids. Parasitoid wasps are not familiar to most people because they are very small and lack flashy yellow stripes found on larger wasp species. Yet, scientists estimate that there are hundreds of thousands of parasitoid wasp species, more than all the species of ants, bees, and non-parasitoid wasp species combined.

Parasitoid wasps feed on many garden pests such as aphids, beetle grubs, caterpillars, leaf miners, grasshoppers, crickets, leafhoppers, mealybugs, psyllids, scale, whiteflies, and eggs of other insect species. Broad spectrum insecticides, such as malathion, often provide good control of plant feeding pests, but also decimate populations of beneficial insects (predators and parasites as well as pollinators). This is ample reason to exercise discretion when applying pesticides.

Gardeners that have a better understanding of beneficial insects will tolerate higher levels of pest species early in the growing season. They know that as pest insect populations increase, predators and parasites populations follow. When considering insect control strategies, gardeners should use methods that target the pest species as closely as possible and have the least affect on beneficial insect populations. For example, learn to tolerate early season aphid damage, spray them with soapy water, or blast them with a high pressure hose. Some insecticides, such as the various strains of Bacillus thuringiensis (Bt), target only the larval stages of flies, beetles, moths, and butterflies.

Many predators and parasites are available for purchase and can be shipped by mail. Ladybug and lacewing larvae and preying mantid egg cases are often sold to gardeners. These insects will usually show up in your garden of their own accord when their preferred food sources are present. In addition, these particular species can easily migrate away from your yard. In the case of preying mantid egg cases, the first to hatch often waits around to gobble up the siblings as they emerge.

The purchase of insects for population control is most effective in closed systems or where the predator or parasite has limited mobility. In greenhouses and other enclosed settings, parasitoid wasps are released to control a wide range of plant feeding pests. Parasitic nematodes are used successfully to control beetle grubs because they move slowly in soil and organic material. Predatory mites move slowly and are used to control plant-feeding mites, fungus gnats, and thrips.

These are only a few examples of how parasitic (and predatory) insects and their relatives are used to aid in control of plant-feeding pests. They will never provide 100% control, but they are extremely important piece of your pest management puzzle.

The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 8, 2005
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