While some beetles, such as Japanese beetles and Colorado potato beetles, can be serious home garden pests, others are some of the best pest-fighters around.
Lady beetles: This family of small to medium, shiny, hard, hemispherical beetles includes more than 3,000 species that feed on small, soft pests such as aphids, mealybugs, and spider mites. (Not all species are beneficial—for example, Mexican bean beetles also are lady beetles.) Both adults and larvae eat pests. Most larvae have tapering bodies with several short, branching spines on each segment; they resemble miniature alligators. Convergent lady beetles (Hippodamia convergens) are collected from their mass overwintering sites and sold to gardeners, but they usually fly away after release unless confined in a greenhouse.
Ground beetles: These swift-footed, medium to large, blue-black beetles hide under stones or boards during the day. By night they prey on cabbage root maggots, cutworms, snail and slug eggs, and other pests; some climb trees to capture armyworms or tent caterpillars. Large ground beetle populations build up in orchards with undisturbed groundcovers and in gardens under stone pathways or in semipermanent mulched beds.
Rove beetles: These small to medium, elongated insects with short, stubby top wings look like earwigs without pincers. Many species are decomposers of manure and plant material; others are important predators of pests such as root maggots that spend part of their life cycle in the soil.
Other beetles: Other beneficial beetles include hister beetles, tiger beetles, and fireflies (really beetles). Both larvae and adults of these beetles eat insect larvae, slugs, and snails.
We usually call flies pests, but there are beneficial flies that are pollinators or insect predators or parasites.
Tachinid flies: These large, bristly, dark gray flies place their eggs or larvae on cutworms, caterpillars, corn borers, stinkbugs, and other pests. Tachinid flies are important natural suppressors of tent caterpillar or armyworm outbreaks.
Syrphid flies: These black-and-yellow or black-and-white striped flies (also called flower or hover flies) are often mistaken for bees or yellow jackets. They lay their eggs in aphid colonies; the larvae feed on the aphids. Don’t mistake the larvae—unattractive gray or transluscent sluglike maggots—for small slugs.
Aphid midges: Aphid midge larvae are tiny orange maggots that are voracious aphid predators. The aphid midge is available from commercial insectaries and can be very effective if released in a home greenhouse.