OVIPOSITION BY FIELD POPULATIONS OF PIERIS-RAPAE (LEPIDOPTERA, PIERIDAE) DETERRED BY AN EXTRACT OF A WILD CRUCIFER

Cabbage plants were sprayed in the field with a butanol extract of Erysimum cheiranthoides, a wild crucifer normally rejected by ovipositing Pieris rapae (L.) because of the presence of deterrent cardenolides in the foliage. Extract-treated plants received significantly fewer eggs than did solvent-treated control plants, both in field plots sprayed weekly and exposed to a natural butterfly population, and in 24-h experiments exposing plants to artificially concentrated wild butterfly populations in a large outdoor screen cage. The level of deterrence varied over time, perhaps because of susceptibility of the water-soluble deterrents to wash-off by rain water. Nevertheless, wild butterflies responded to the Erysimum deterrents in a fashion similar to that observed in earlier greenhouse experiments conducted with laboratory-reared butterflies. These results provide additional support for the hypothesis that host plant selection depends on a balance of both positive and negative stimuli. The results also suggest that protection of plants by deterrents may be possible in the field over short periods of time.