Diaeretiella rapae, a small aphidiid wasp, is the most important parasite of the cabbage aphid. The dark brown adult female is 3 mm long and, over a lifetime, deposits an average of 85 eggs internally in separate aphids. The wasp larvae consume the body contents of the aphids. By the time the wasp larvae are fully grown, the empty bodies of the host aphids turn into hardened, light brown shells called mummies. The adult wasp emerges through a circular hole cut in the back of the mummy. Although this wasp parasitizes large numbers of cabbage aphids, it is often ineffective in controlling the cabbage aphid by itself because by the time the parasite population builds up to a significant level, the aphid numbers have already exceeded threshold levels. The effectiveness of the parasite is also reduced by several species of wasps that develop on and kill D. rapae.

Lady beetles, syrphid fly larvae, lacewing larvae, and other predators are often effective in destroying cabbage aphids, especially small colonies. These predators seem to be favored by wet weather, which is unfavorable to the aphid. Entomogenous fungal epidemics cause cabbage aphid populations to decline, on occasion killing entire colonies. This occurs only when the weather is wet and/or humid, which is necessary for the development and spread of the fungi.

Source:

Mahr, S., D. Mahr and J. Wyman. 1993. Biological control of insect pests of cabbage and other crucifers. N. Cent. Reg. Pub. 471; 54 pp.