Collect large caterpillars from unsprayed cabbage, broccoli, or cauliflower plants. Place them in plastic containers (clean cottage cheese cartons, deli cups, margarine tubs, etc. are fine), along with some leaves from unsprayed plants. Replace the leaves every day or two as the caterpillars eat them, and clean up the mess they leave behind. Check the condition of the caterpillars daily. Have some started to pupate (forming a green chrysalis)? Others may never pupate, but wasp larvae will emerge instead from the mature larva. Cotesia glomerata is a very common parasite of imported cabbageworm caterpillars. Several wasp larvae develop inside each caterpillar, and they emerge together from the caterpillar's body to spin their yellow to orange cocoons in a group. What percent of the caterpillars you collected were parasitized? What types of parasites did you find? Were any caterpillars diseased?
You can do the same thing with the pupal stage. Imported cabbageworm larvae usually pupate on the underside of lower leaves. The 3/4-inch long green to gray chrysalis may be difficult to see because it blends in well with the leaf. Hold the collected pupae in containers until butterflies or parasites emerge. Pteromalus puparum is a common parasite of the pupae. Once again, numerous wasp larvae develop inside each pupa. They cut one or two small exit holes in the side of the pupa through which the adult wasps emerge. Parasitized pupae are easily identified: they turn dull brown compared with unparasitized pupae that change to yellowish white as the pupa matures.
- Susan Mahr, University of Wisconsin - Madison
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