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Beneficial Bugs Control Pests in Garden

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BURLINGTON – Insect pests get a lot of attention because we see the damage they cause. Less obvious are beneficial insects that provide “biocontrol” of insect pests. These include predators that kill or disable their prey quickly and parasitoids that kill pests more slowly.

photo by Vern Grubinger Parasitized tomato hornworms are often seen covered with multiple white, cottony cocoons of the parasitoid wasp Cotesia congregate.

Predators include lady beetles, ground beetles, lacewings and mites. Predators are larger than their prey and eat a lot of them. Most are generalists that attack a variety of prey.

Parasitoids include wasps and tachinid flies. They specialize in one insect species, which is usually bigger than they are. Females search for a host, then deposit eggs or larvae in, on or nearby it.

The immature parasitoid grows, slowly consuming the pest. Unlike parasites, such as fleas and ticks, parasitoids typically kill their host eventually.

Lady beetles are familiar predators. Both adults and larvae prey on soft-bodied insects like aphids. The larvae don’t look like their parents. They have an elongated body like an alligator that’s dark with yellow or orange flecks. There are many species of lady beetles, native and introduced.

The pink spotted ladybeetle is pink to red, oval, with six spots on each forewing. It feeds on lots of pests like European corn borer, corn earworm, imported cabbageworm, fall webworm and Colorado potato beetle. It also needs to eat pollen. Flowering plants, including dandelions, help support adults in the spring.

photo by Vern Grubinger A lady beetle prowls for aphids on an eggplant leaf.

The convergent lady beetle is slightly elongated, with white lines converging behind its head, and a few to 13 black spots on red forewings. It feeds on aphids.

Other beneficial lady beetle species include the two-spotted lady beetle, nine-spotted lady beetle and transverse lady beetle. The multicolored Asian lady beetle was introduced to feed on aphids, scales and psyllids. Its annoying habit is congregating in homes in the fall. Another species, the Mexican bean beetle, is a well-known plant pest.  

Ground beetles are dark and shiny, often found under stones and debris. Those active at night are black, and those active in the day may be a metallic color. Pests consumed include eggs and larvae of the Colorado potato beetle, root maggots and “cabbageworms.”

Rove beetles, soldier beetles and flower beetles are also insect pest predators.

Lacewings have net-like, delicate wings, long antennae and prominent eyes. They feed on aphids, leafhoppers, scales, mites and eggs of butterflies and moths.

Syrphid flies, or hover flies, are brightly colored, sometimes resembling bees. Adults can be seen feeding on flowers. The larvae, a tapered maggot, eats dozens of aphids a day.

Hunting wasps take their prey, whole or in pieces, back to their mud, soil or paper nests to feed their young. The common Polistes paper wasps can help control caterpillar pests.  

Predatory mites eat plant-feeding spider mites. They’re a little larger and move faster than their prey.

Tachinid flies are parasitoids that look like common flies. Females lay eggs near the heads of caterpillars, beetles and bugs. The eggs hatch fast, and the larvae tunnel into the host, feeding for a week or more before killing it.

Braconid and Ichneumonid wasps include small species that attack small insects such as aphids and larger wasps that attack caterpillars or wood-boring beetles. Diamondback moth larvae parasitized by the wasp Diadegma insularis appear as white fuzzy cocoons underneath cabbage leaves. Many gardeners have seen white cocoons of the parasitoid wasp Cotesia congregata attached to the outside of tomato hornworms.

One way to support beneficial insects is by providing pollen sources, which can be in weedy areas where dandelions, wild carrot and goldenrod are left to flower. In addition, try not to kill good bugs. And avoid the use of broad-spectrum insecticides.

For details about good bugs in the garden, visit

Dr. Vern Grubinger is the Extension Vegetable and Berry Specialist at the University of Vermont.

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