Beneficial insects Gardening

Body-Snatching Wasps Help Control Pests

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Parasitoid wasp preparing to attack a small caterpillar. Source: Ian Alexander, Wikimedia Commons

There’s a whole range of tiny insects called parasitoid wasps (some 700,000 species worldwide) that are helping to control insect pests in your garden without you even knowing it. They lay their eggs on or in host insects (aphids, beetles, caterpillars, flies, true bugs, whiteflies, etc.) and their larvae literally eat the host alive. Sometimes the larvae even take control of the host, telling it what to do, or the female injects a poison or fatal virus that paralyzes the host insect. Gruesome indeed, but what a boon for gardeners!

A Very Visual Example

Wasp parasitism is usually done sight unseen, though, so it’s hard for gardeners to appreciate what is being done. There is at least one exception: the hornworm brachnid wasp (Cotesia congregata).

This parasitoid wasp is specific to hornworms, huge caterpillars up to 4 inches (10 cm) long, by far the largest caterpillars found in most gardens. Two species are very common in North American gardens: the tomato hornworm (Manduca quinquemaculata) and its look-alike cousin, the tobacco hornworm (Manduca sexta). Both devour tomato plants and other plants in the Solanaceae family (eggplants, peppers, potatoes, tobacco, etc.). A single hornworm can entirely defoliate a tomato plant!

Wasps to the Rescue!

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Hornworm covered with wasp cocoons. At this stage, just let nature take its course! Source: 8thstar, Wikimedia Commons

The female hornworm brachnid wasp injects her eggs into the caterpillar where they develop into larvae that feed on the caterpillar, digesting its internal organs. The weakened caterpillar stops feeding and the larvae soon emerge and form white cocoons on the outside of the caterpillar’s body. This is the only stage where they are noticeable and, to be honest, you really can’t miss them. The caterpillar looks like it’s slipping on a thick, lumpy, white winter coat.

Normally, I would have suggested handpicking any hornworm you see in your garden and disposing of it by dropping in a cup of soapy water or squishing it underfoot, but if wasp cocoons are visible, that’s exactly what you shouldn’t do. Instead, just let the wasps continue their life cycle. Within 4 days, they’ll be gone and the caterpillar will finally die … but next season, you and your neighbors will have more beneficial wasps to help control future generations of hornworms.

How to Encourage Beneficial Wasps

Again, there are many more beneficial wasps at work than you’d ever suspect, controlling a whole horde of insect pests: without them, our gardens would be overrun by harmful insects. But they’re tiny, rarely seen … and even more rarely recognized. Any tiny brown, black, red or yellow wasp, recognized as such by its narrow “wasp waist,” is likely to be an ally. And no, they don’t sting people.

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Brachonid wasp visiting a flower. Source: Gilles Gonthier, Flickr

Even if you can’t readily see parasitoid wasps, you can help increase their population. Here’s how:

  1. Avoid using pesticides, even organic ones.
  2. Offer plenty of tiny flowers. Adult wasps feed on nectar and pollen and need plants with small, shallow blooms, like those in the carrot family (coriander, dill, etc.), cabbage family (sweet alyssum, mustard, etc.) and daisy family (feverfew, goldenrod, yarrow, etc.).
  3. Leave out a source of very shallow water (too shallow for mosquito larvae). Perhaps a plate with a thin coating of water and a few stones on which the wasps can land.
  4. Don’t kill insects showing signs of parasitism.

Body-snatching wasps: who’d ever have thought they’d be our friends?

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

2 comments on “Body-Snatching Wasps Help Control Pests

  1. Great info….I love the look of tomato hornworms and the moths they become but I like tomatoes more!

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