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Parasitic Wasps: Beneficial Insects

Parasitic wasp eggs on hornworm1
Ichneumon Wasp
Ichneumon Wasp

Parasitic wasps are small, even minute mini-wasps that control pests including many garden pests.

Parasitic wasps kill their hosts by feeding on them. Pests controlled by parasitic wasps include aphids, beetles, caterpillars, flies, sawflies, and scale.

Parasitic wasps range in size from 1/100 to ¾ inch long. They vary in shape and coloration but have a notable narrow “waist” between the thorax and abdomen and long thread-like antennae. The larval stages usually do the most damage to pest insects developing inside or outside the hosts then chewing the host for food. This is called “host feeding” and reduces the population of host pest insects.

Parasitic wasp eggs on hornworm
White parasitic wasp cacoons on hornworm

Parasitic wasps are noticeably seen victimizing tomato hornworms; their egg-like white cocoons are seen traveling on the backs of hornworms—the host has already been parasitized by the wasp larvae and is near death or will not be able to pupate.

Parasitic wasps are attracted to the garden by an ample supply of water—a shallow birdbath or garden pond will do, and a good supply of nectar—plant alyssum, dill, cilantro, parsley, and yarrow–and host insects to feed on.

There are three notable families of parasitic wasps (all belong to the order Hymenoptera):

Braconid wasps: feed on aphids, garden webworms, tomato hornworms, armyworms, strawberry leaf rollers, and tent caterpillars; they are just 1/16 to 5/16 inch long and brown or red bodied.

Chalcid wasps including the trichogramma wasp: feed on moth and butterfly larvae, including cabbage worms, tomato hornworms, corn earworms, cutworms, armyworms, webworms, cabbage loopers, and corn borers and also aphids and mealybugs; they are 1/64 to 5/16 inch long and metallic black or golden bodied.

Ichneumon wasps: feed on cutworms, corn earworms, white grubs, caterpillars; they are ⅛ to 1½ inch long.

Braconid wasp
Braconid wasp illustrated 36X Actual Size

Overview of Braconid beneficial wasps:

Here is an overview of the Braconid wasp, one of the most important and common parasitic wasp:

Scientific name: Family Braconidae, between 50,000 and 150,000 species

Range: Worldwide including North America

Life cycle: Usually several generations per year. Most species hibernate as larvae or pupae in their hosts. Females inject eggs into host insects including grubs and caterpillars. Larvae hatch and grow inside the host, weakening and often killing it, or the adult wasp paralyzes the host insect before laying the eggs. The larvae pupate on the back of the host.

Adults: Black, yellowish, or red; 1/10 to ¼ inch long; narrow, threadlike waists.

Larva: White to cream-colored, wormlike; develop and feed inside the bodies of host insects.

Eggs: Laid in bodies of host insects.

Feeding habits: Wasps feed on the pollen and nectar of small flowers including dill, parsley, and yarrow. Larvae feed inside caterpillars, beetle grubs, and aphids. Adults feed on aphids, codling moths, cucumber beetles, cutworms, gypsy moths, imported cabbage worms, tent caterpillars, and tomato hornworms.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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