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6. The Predator Patrol: Beneficial insects in your garden can help keep the bad guys under control
by Joanna Poncavage
If you've ever seen a tomato hornworm sicken and die, its back thick with little white knobs that look like grains of rice, you've seen natural pest control at its most graphic. Those little white grains are pupae (cocoons) that have developed from eggs laid on the hornworm by a braconid wasp. As the wasps develop, they kill their host. Then they go looking for more hornworms.
We call these tiny wasps "beneficial" because what they do naturally has horticultural benefits. (An added benefit is that these wasps don't sting humans.) To gardeners, beneficial insects are the "good" insects. They help us grow fruits, vegetables, trees and flowers by eating or parasitizing pest insects. Almost every insect has a specific set of natural enemies -- other insect species that will eat it, or use its body or eggs as a place to lay their own eggs.
The chinch bug is a common turf pest across North America. Because they prefer hot, dry conditions, you can discourage them by irrigating well.
Thanks to a growing awareness of the dangers of chemical insecticides, there's been increased interest in beneficial insects and other beneficial organisms, such as mites and nematodes. Many scientists are studying how best to put insects to work in our gardens, fields and orchards. Now you can buy lures that entice beneficial insects to your garden with sexual smells (called pheromones) or food aromas. You can also buy insects such as lady beetles and lacewings, two of the most common (and voracious) beneficials.
Using beneficials doesn't get the immediate results that spraying an insecticide does. It also requires more understanding and observation of what's going on, bug-wise, in your garden. The goal of using beneficial insects is not total eradication -- rather, it is to provide a natural balance between pests and their predators so that the populations of bad guys don't get out of control.
So how do you go about using beneficials in your garden? First, identify the insects that are causing problems. (A magnifying glass and a good insect identification field guide are helpful.) Not every beneficial eats or parasitizes every pest, so you need to match the correct beneficial or beneficials to your particular pests. Timing is important, too. If you want to release an egg parasite, you must do so when the pest to be controlled is in its egg stage. And if you buy hungry beneficials, they've got to have something to eat when you set them free.
The following are some of the most commonly available beneficial insects. They're also some of the least fussy eaters, preying on a variety of pests.
Beneficial Insects for the Garden
Lady Beetles
Better known as ladybugs, these are the most widely recognized beneficials. The species that is usually sold is the convergent lady beetle -- the classic red beetle with black spots. Its favorite food is aphids, but it will help control other soft-bodied pests, too. When you buy lady beetles, you'll receive the adult stage. Open the container and sprinkle them with a little water to give them a drink, then close them back up and keep them in your refrigerator until conditions are right for their release. Don't set them loose in the middle of a hot day; wait until evening, preferably after a rain. (Water your garden first if it hasn't rained.) And make sure there is something in your garden for them to eat, preferably aphids, but pollen and nectar of blooming herbs and flowers will do. You want the adults to stay around long enough to lay eggs. These will hatch into voracious larvae that will eat up more aphids by the hundreds.
Lacewings
The lacewing is found naturally throughout North America. The larvae are big aphid eaters, too, but they'll also feed on thrips, mealybugs, scale, moth eggs, small caterpillars and mites. If you buy lacewings, you'll receive the egg stage. Scatter about 1,000 lacewing eggs around each 500 square feet of garden.
Spined Soldier Bugs
These beneficial bugs are also common throughout North America, but you can buy them, too. They are shield-shaped, yellow or brown bugs about one-half inch long. After the nymphs hatch from their eggs, they become voracious predators. They will eat many kinds of caterpillars, including gypsy moth caterpillars, plus Colorado potato beetle larvae and Mexican bean beetle larvae. You'll need about 100 spined soldier bug nymphs per 20 feet of bean row, or two to five nymphs for each square yard.
Trichogramma Wasps
These are tiny insects that will lay their eggs in the freshly laid eggs of many notorious garden pests, including corn earworms, cutworms, cabbage loopers, codling moths and tomato hornworms. The wasp larvae develop inside the host eggs, then eat the contents of the eggs, which turn black as the wasps mature.
Trichogramma wasps do not sting humans. They are native to North America but can be purchased from farm supply companies.
Nematodes
These are extremely tiny worms that live in the soil. Some nematodes are parasitic on insects that live at least part of their lives underground, including cutworms, root weevils, wireworms and white grubs. When you buy beneficial nematodes, you'll receive a small container that holds millions of them. First, rake back any mulch or thatch from the area you intend to apply them to. Then water thoroughly until the soil is very moist -- the wetter the better. Mix the nematodes with water according to package instructions. Use a watering can to sprinkle them where you want them, again noting package instructions for the rate of application. Then water the area again to soak the nematodes into the soil. This whole process is best done in the evening, because exposure to sunlight and dry air can kill nematodes.
The praying mantis is also widely available for sale as a beneficial insect. However, it has fallen into disfavor as entomologists have told us how truly omnivorous this insect is. It will eat anything -- including other beneficials and its own young. In other words, it does as much harm as good.
Beneficial Insects for the Home and Greenhouse
If you have a greenhouse or lots of indoor plants, you may have problems with aphids, whiteflies, spider mites, mealybugs or scale. Greenhouses are ideal battlegrounds for beneficials -- closed environments from which they can't escape. Lady beetles are very effective in greenhouses, preying on most of the above greenhouse pests.
Encarsia formosa
Tiny wasps with the scientific name of Encarsia formosa parasitize the pesky whitefly. The wasps lay their eggs in immature whitefly or sweet potato whitefly larvae. As the wasp larvae develop inside the whitefly, the whiteflies die. These little wasps will not survive cold winters, but thrive in greenhouses and mild-winter areas, or can be released outdoors in colder locales when the weather is warm. About 1,000 wasps should be enough for most home gardens or greenhouses.
Predatory Mites
These are beneficial cousins of pests such as spider mites, two-spotted mites, red mites and Pacific mites. At the first sign of pest mites, release up to 10 predatory mites per plant. Misting plants will encourage predators and hinder spider mite development.
Mealybug Destroyer
Another greenhouse beneficial is best known as the mealybug destroyer, after its favorite food. It's a non-native lady beetle that won't survive cold winters outdoors. For a single infested indoor plant, release about five beetles. Trap the mealybug destroyers around the infested houseplant by draping sheer fabric over the plant and tying it shut around the pot.
While it may seem odd at first to think of bringing insects into your home and letting them run wild, they'll mostly be polite guests and go about their business of eating what's bugging your plants.
Joanna Poncavage is a freelance writer and former senior editor of Organic Gardening magazine.