Several species of wasps are parasites of garden pests; most common are ichneumon wasps, braconid wasps, and chalcid wasps. The work of these parasitic wasps is usually more visible than the insects themselves. Examples include a tomato hornworm with a packet of white rice-like pupae on its back, or a mummified aphid with a tiny hole in it where a parasitic wasp emerged.
Parasitic wasps pose no danger to humans; few species are able to sting and they do so only when mishandled. They are found throughout North America.
The ichneumon wasp parasitizes garden pests such as cutworms, corn earworm, white grubs and various caterpillars. It is the largest type of parasitic wasp and its slender body may measure 1/8–1 1/2″ long. Females have tail-like egg-laying tubes. Colors range from black to yellowish.
Braconid wasps are 1/16–5/16″ long, with stout bodies. They attack various species of aphids, garden webworm, tomato hornworm, armyworms, strawberry leaf roller and tent caterpillar, among others. Most braconid wasps are dark with some colored markings, but the markings can be seen only under magnification.
Chalcid wasps range from 1/64–5/16″ in length. This group includes the trichogramma wasp, which is available commercially for pest control purposes. The trichogramma wasp is an effective parasite of many moth and butterfly larvae, including cabbage worms, tomato hornworms, corn earworms, cutworms, armyworms, webworms, cabbage loopers and corn borers. There are other chalcid species that parasitize aphids and strawberry leaf rollers.
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