Parasitoid (parasitic) wasps are tiny wasps whose larvae develop inside or on another insect and eventually kill it. In gardens they quietly suppress aphids, caterpillars and other pests. The clues you will actually see are swollen, brown or black “aphid mummies” with a round exit hole, and white cocoons on a hornworm. Leave both alone, and keep flowers and some pests around so the wasps persist.
What are parasitoid wasps?
Colorado State University describes them as insects in the order Hymenoptera, from families such as Braconidae, Ichneumonidae, Eulophidae, Aphelinidae and Trichogrammatidae. Most are about 1 to 10 mm long with brown or black bodies and long, slender antennae, though some ichneumonid wasps exceed 10 cm and Trichogramma wasps are only 0.25 to 1 mm. The Colorado State page does not say whether they sting people, and the sources reviewed here do not address it, so this guide makes no claim either way. Most species are specialists that attack a single kind of host (Colorado State).
How do they kill pests?
- Females paralyze or lay eggs in an adult insect, a larva or an egg, using an ovipositor.
- The wasp larva feeds inside (endoparasitoid) or outside (ectoparasitoid) the host and eventually kills it (Colorado State, Missouri).
- The adult wasps feed on nectar, pollen and honeydew (Colorado State).
- Hosts include aphids, scales, psyllids, stink bugs, beetles and caterpillars (Colorado State). Trichogramma wasps attack eggs of pests such as corn borers, corn earworms, cabbage loopers and hornworms (Missouri).
How do you recognize aphid mummies and hornworm cocoons?
| Sign | What you see | What it means |
|---|---|---|
| Aphid mummy | A swollen, hardened aphid that is brown, black or discolored | A wasp larva is developing inside; the adult chews a round exit hole in the rear when it leaves (Colorado State, University of Delaware) |
| Early parasitism | The aphid looks normal for a few days after the egg is laid | Parasitism may already be underway (Delaware) |
| Cocoons on a hornworm | Small cocoons attached to the caterpillar | Parasitoid pupae; the hornworm is already doomed (Colorado State, Missouri) |
University of Delaware says the adult wasps are under 1/8 inch long and hard to spot, but the mummies are easier to find. Development takes about 2 weeks at summer temperatures, with typically several generations per year.
How effective are they?
Delaware says each female lays about one egg in every 100 aphids on average, which suppresses aphid populations meaningfully over time, and that parasitoids work best preventatively, before damaging outbreaks. Colorado State gives examples from orchards and crops, such as about 80% of leafroller larvae parasitized in unsprayed Washington apple orchards in 1992 and up to 90% of brown marmorated stink bug eggs parasitized by the samurai wasp. These figures come from specific crops and are not a promise for a home garden. Overwintering happens as larvae or pupae in hosts or soil, and adults emerge in spring or summer (Missouri).
How do you attract and protect parasitoid wasps?
- Grow shallow flowers for nectar and pollen. Colorado State recommends diverse flowers with shallow blossoms. Missouri recommends large, concentrated stands in sun with little wind, mixed varieties, heights and bloom times, and water if possible.
- Leave some pests and parasitized hosts so the wasp population persists (Colorado State).
- Keep shelter. Leave leaf litter, ground covers and crop residues (Colorado State, Missouri).
- Use as little insecticide as possible. Missouri says many insecticides are broad-spectrum and harm beneficials, so choose microbial products or treat selectively, and spray when winds are lowest to reduce drift.
Use the companion plant finder to plan shallow-flowered herbs and flowers, and check the pest identifier before treating. See also lady beetles, green lacewings and hover flies.
Can you buy parasitoid wasps?
Delaware notes that commercially available species can be released in field or greenhouse crops, and Colorado State says Aphidius colemani is used commercially against aphids in greenhouses and outdoor systems. These are mainly commercial-crop tools, and most parasitoids are specialists, so a release must match the pest you have. For a home garden, habitat and low-pesticide management are the practical steps.
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Frequently asked questions
What are the swollen brown aphids on my plants?
They are likely aphid mummies, parasitized by a tiny wasp. A round hole in the back means the adult has already left.
Should I remove a hornworm with white cocoons?
The cocoons are parasitoid pupae, so the caterpillar is already being parasitized. Leaving it lets the next generation of wasps emerge.
Do parasitoid wasps need flowers?
Yes. Adults feed on nectar, pollen and honeydew, so shallow flowers help them persist.
Reviewed November 2026. Sources: Colorado State University: Parasitoid wasps; University of Delaware Extension: Meet the wasps helping to suppress early season aphids; University of Missouri Extension: Increasing beneficial insects in row crops and gardens
