A CHEMICAL BASIS FOR DIFFERENTIAL ACCEPTANCE OF ERYSIMUM-CHEIRANTHOIDES BY 2 PIERIS SPECIES

Wormseed mustard, Erysimum cheiranthoides, is unacceptable as a host for the cabbage butterfly, Pieris rapae. However, it is preferred for oviposition by Pieris napi oleracea in the greenhouse. Isolation and identification of the oviposition stimulants to P. napi oleracea were accomplished by C18 open-column chromatography, TLC, ion-exchange chromatography, HPLC, UV, and NMR spectroscopy. Glucoiberin and glucocheirolin were identified as the most active stimulants. The extracted glucoiberin was as stimulatory as glucocheirolin, although its concentration in the Erysimum plants was about 10 times lower than that of glucocheirolin. These glucosinolates were only weak stimulants to P. rapae. Furthermore, P. rapae was strongly deterred by the cardenolides, erysimoside and erychroside, from E. cheiranthoides, and P. napi oleracea was less sensitive to these compounds. No other deterrent to P. napi oleracea was detected in this plant species. The results explain the differential acceptance of E. cheiranthoides by these two Pieris species.