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Garden Q&A: Parasitic wasps play crucial role

| Saturday, Oct. 12, 2013, 9:00 p.m.
Jessica Walliser
The white rice-like sacs hanging off the back of this tomato hornworm is the cocoon of a parasitic wasp known as the Cotesia wasp.

Question: I've been gardening for many years and have never seen tomato hornworms with white eggs on their backs. What are they?

Answer: The white rice-like sacs hanging off the back of your tomato hornworms are not eggs. They are the cocoons of a very important parasitic wasp known as the Cotesia wasp, one of the more commonly encountered groups of parasitic wasp.

Different Cotesia species also parasitize cabbageworms, cutworms, corn earworms, gypsy moths, tobacco budworms and many other caterpillars. Female wasps lay eggs inside the hornworm, and the resulting larvae consume the caterpillar from the inside out. In 15 to 30 days, the larvae mature inside their host and then chew small holes through the skin of the caterpillar and spin external cocoons in which they pupate into adults. Once mature, the adult wasps “pop the top” off the cocoon and fly off.

They go on to parasitize and kill more hornworms and other caterpillar species.

There are literally thousands of species of parasitic wasps here in North America, and they are very important as they help control the populations of many common garden pests. Parasitic wasps are a very complex group of insects with a tremendous amount of diversity in their individual appearances and lifecycles.

Parasitic wasps can be smaller than a gnat or as long as your pinky finger (adult Cotesia wasps measure less than an eighth of an inch). Nearly all of them use other insects as hosts for their young, with many of them being host-specific and utilizing only one or two species of insects for this task.

Most often the females lay eggs singly or in groups on, or inside of, the host. The larvae then hatch and develop inside of the host, consuming the non-vital tissues first to ensure the host survives until the larvae are ready to pupate into adults. Though you will probably never encounter the larvae themselves, they are often cream-colored, legless, maggot-like creatures. Parasitic wasps attack almost every group of insects, including aphids, beetles, flies, scales, true bugs, and caterpillars of every sort.

Unlike yellow jackets, hornets, and paper wasps, nearly all species of parasitic wasps are incapable of stinging people, and what may look like a nasty stinger protruding from the abdomen of the females of some species is actually the ovipositor she uses to lay her eggs.

These valuable pest-controlling insects should be encouraged and valued. Do not destroy any hornworms you find with the cocoons already present, because at this point the caterpillars have stopped feeding and will die soon enough.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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