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Pteromalus puparum, Parasite of Imported Cabbageworm

Pteromalus puparum is a tiny pteromalid wasp that develops inside pupae of the imported cabbageworm and other caterpillar pests. Adult wasps are 3-4 mm (1/8 inch) long and feed on flower nectar. Females are shiny black, while males are a metallic greenish bronze and are normally smaller than the females. The wasps fly short distances of less than an inch, which gives them the appearance of hopping. Females lay eggs in either the prepupae or newly formed pupae of the host, depositing up to 700 eggs during her lifespan. More than 200 offspring can be produced within each cabbageworm pupa. Parasitized pupae turn from green or gray to dull brown as the wasp larvae develop over about three weeks. Unparasitized pupae change from green or gray to yellowish white as the butterfly inside reaches maturity. One or more small round exit holes in the pupae indicate the wasp parasites have emerged. The wasps overwinter as mature larvae within the host pupae.

The wasps will parasitize other caterpillar species when the imported cabbageworm is scarce. Because the wasps attack only the pupae, caterpillar feeding is not reduced and significant damage can occur, especially if cabbageworm population levels are high. The benefit of this parasite is in reducing the number of adults, and thereby reducing the size of the subsequent generation.

This parasite was accidentally introduced into the United States from Europe in the late 1800s and is now found throughout the country, although its abundance varies in different geographical areas. Naturally occurring parasites usually reduce cabbageworm populations, but not enough to prevent damage until late in the season. The rate of parasitism varies considerably throughout the year, from field to field, and from season to season. Average parasitism in Wisconsin over four years was 13%, but ranged from 0% to 38%. In Minnesota parasitism was 90% among late August pupae. In commercial cabbage fields in southwestern Virginia, 50% of the pupae in late July and 67% of overwintering pupae were parasitized. In many areas, 60 to 70% of the overwintering imported cabbageworm pupae may be parasitized. Augmentative releases would probably help, but this wasp is not available commercially.

- Susan Mahr, University of Wisconsin - Madison

Adult photo by Joe Ogrodnick. Larval photo by Susan Mahr.

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