Insect predators (such as ladybugs, praying mantids, and lacewings) and parasites (such as certain species of tiny wasps) are the garden dwellers typically referred to as beneficial insects, since they take aim at garden pests.
Predators consume other insects, whereas parasites lay eggs on or inside other insects. When the larvae emerge, they feed in or on their hosts, destroying them in the process.
Since insects are the most successful living organisms on Earth, it makes good sense to take advantage of these relationships by pitting one against another, maintaining a reasonable balance. A healthy garden will host a variety of insects that fall into both the pest and beneficial categories. Some will even turn up in both, depending on their life cycle stage and available food.
These insect allies interact in different ways within your garden. A predator might go straight to a pest, take care of it, and then leave or die, for instance. At other times, a desirable insect may have a limited effect on a large population of target pests, but will settle in with a small population that reduces the pest levels over time. Some "good" guys, such as praying mantids, might eat just a few pests, and then fly away, or eat a wide variety of pests, but fail to control any outbreaks of a single pest type. Realize, though, that predators and parasites certainly don't make our distinctions between "bad" and "welcome" garden visitors, so they may also dine on your preferred visitors, such as butterfly larvae.
The following information features the lowdown on a few of the most common beneficial insects that target pests. You can order such organisms from a science or beneficial insect supplier if you want to control an outbreak or set up investigations. Better yet, try to create a garden or habitat that will attract helpful creatures. In addition to those listed, there are scads of other beneficial insects that probably already inhabit your garden. These include such seamy-sounding characters as assassin bugs, soldier beetles, big-eyed bugs, spined soldier bugs, and hover flies. Challenge your students to be keen garden sleuths who observe and identify these characters that call your school garden home.
Green lacewings target leafhoppers, mites, aphids, thrips, mealybugs, and whiteflies. The larvae (sometimes called aphid lions) are voracious aphid predators. The adults feed on flower nectar and honeydew produced by aphids and other sucking insects. You can buy eggs or larvae from suppliers and can attract adults in most areas of the U.S. by planting flowers with abundant nectar.
Lady beetles target aphids, leafhoppers, scales, mites, mealybugs. These familiar creatures, in both larval and adult stages, feed on soft-bodied insects, especially aphids. You can attract them by planting nectar plants (nectar is an alternate food source) and those that attract aphids. These include alyssum, legumes, and flowers in the Umbelliferae family (dill, wild carrot, fennel, yarrow, and so on).
Parasitic or predatory wasps target caterpillars, aphids, mealybugs, leafhoppers, greenhouse whiteflies. Encarsia formosa are small wasps that parasitize greenhouse whiteflies. Trichogramma wasps parasitize wasps parasitize eggs of leaf-eating caterpillars such as cabbage loopers.
Praying mantids target most pest insects and eggs. They can be wonderful allies for gardeners (and great fun to watch), but they eat such a variety of insects that you wouldn't want to use them for an outbreak of any one pest.
Insect Appeal: Inviting in the Good Guys
Create a garden that offers food, shelter, and nesting sites that beneficial insects need, and they'll beat a path to your oasis. Here are some tips on making your garden alluring.
Ages and Stages
All insects go through different life cycle stages. Can students find evidence of these stages in their outdoor laboratory? The changes that occur as insects change form and mature are classified as either complete or incomplete metamorphosis. In the four stages of complete metamorphosis, an adult lays eggs in a place where offspring will be able to find food (Consider Monarchs and milkweed plants, for instance.) When the larvae hatch, they eat voraciously, repeatedly shedding skin (molting) as they grow. At a certain point, they stop eating and enter the pupa stage, in which they might make cocoons or chrysalids. While inside, the body changes form and an adult emerges that looks very different from the creature of earlier stages. The insects that go through this cycle include butterflies, beetles, and wasps.
Incomplete or gradual metamorphosis involves three stages. The insect also begins life as an egg, which hatches into a nymph (a small-sized version of the adult.) As they feed and grow, nymphs repeatedly shed their skin (exoskeleton). This typically happens several times until the insect reaches its adult size. Examples of insects that go through these stages are true bugs, grasshoppers, and praying mantids.