Protect Your Garden With Beneficial Insects

Plant the right flowers to attract beneficial insects to your garden.


| August/September 2004



Parasitic Wasps

Large paper wasps are predators, but many parasitic species such as braconids look like tiny flying ants.


Photo courtesy Fotolia/kelly marken

Potatoes were planted for the first time last summer at Clemson University’s Calhoun Field Laboratory Research Farm in South Carolina. Soon after the plants emerged, potato beetles showed up and began eating the plants. Then came a rowdy band of soldier bugs, sometimes called predatory stink bugs. “It was neat and exciting to see them,” says Dr. Geoff Zehnder, a professor of entomology at Clemson. “The stink bugs really did a job on the potato beetles. We still had to spray once with Bt (Bacillus thuringiensis), but the rest of the time, the stink bugs kept the potato beetle population down.”

Zehnder, who also is coordinator of Clemson’s IPM (integrated pest management) and sustainable agriculture programs, says he doesn’t know exactly where the helpful stink bugs came from but he thinks they were in residence as a consequence of plant diversity on the farm, which is home to plenty of flowering plants intended to attract beneficial insects. “It’s not a silver bullet solution to pest problems, but it does have an effect,” Zehnder says. “If you are going to farm or garden organically, you need to build in attractants for beneficials.”

Ten years ago, Dr. Whitney Cranshaw of Colorado State University headed a team that surveyed more than 150 plant species to document those most effective at attracting beneficials. The study was conducted at the Denver Botanic Garden and the Cheyenne Botanic Garden; the target beneficials were five leading “good bug” categories — hover flies, braconid wasps, tachinid flies, lacewings and lady beetles. “The beneficials liked plants with tiny flowers, which had easily accessible nectar chambers,” Cranshaw says, noting the favorites were members of the mint, carrot and aster families, and sedums and alyssums. (The flowers they surveyed and the beneficials attracted are listed below.) Including some of these ornamentals in your garden should help attract and sustain good bugs to help keep pests in check. (Crenshaw has just published a superb new book, Garden Insects of North America: The Ultimate Guide to Backyard Bugs.)

Plants Versus Pests

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.

warren_7
7/14/2007 8:00:54 PM

exactly the information needed






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