Protect Your Garden with Beneficial Bugs
(Page 2 of 6)
August/September 2004
By Barbara Pleasant
Plants Versus Pests
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Researchers, farmers and gardeners the world over have experimented with ways to use plants to attract beneficial insects. People have long believed that plants respond to insect attacks defensively, but it’s only recently that this process has been better understood. Let’s take tomatoes, for example. When tomato hornworms begin feeding on tomato leaves, the tomatoes do two things — they change their leaf chemistry so they become a less nutritious food, and they release volatile chemicals that attract natural enemies of hornworms. In tomato and many other plants, these volatile chemicals include jasmonic acid, a natural plant hormone that works like a dinner bell to beneficials such as braconid wasps, which are major parasites of tomato hornworms. At Michigan State University’s Plant Research Laboratory, Dr. Gregg Howe has found that the same “signaling pathway” decreases feeding by spider mites on tomatoes; other researchers have found that jasmonic acid even attracts carnivorous mites, which then feed on the pest spider mites.
In the world of plant-pest communication, jasmonic acid may be a generalized “scream for help,” but plants can emit much more specific signals to attract very specific beneficial insects. For example, when a beet army worm feeds on a plant, molecules in its saliva help the plant fine-tune its scream to call parasitic wasps, specialists in killing beet army worms. Scientists do not know much about these specific communications yet, but that is changing. “This is a very new area, and we expect that more plant-predator systems will be discovered in the future,” says Dr. Gary Felton, head of the entomology department at Pennsylvania State University.
Hungry beneficials are on the lookout for big, fat caterpillars, teeming colonies of aphids or runaway populations of potato beetles; they seek pests the plants have not been able to bring under control. Some feeding on plants by pest insects must occur in order to attract the beneficials’ attention, according to one study of cabbage aphids on broccoli conducted in Corvallis, Ore. In this research, alyssum, buckwheat and other beneficial host flowers were planted to help lure beneficial insects, and though plenty of beneficial hover flies appeared, they did not start laying eggs on broccoli plants until aphid populations grew to 50 aphids per plant. Meanwhile, the hover flies enjoyed taking nectar and pollen from the flowers, which probably increased the number of eggs they were able to lay.
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