The role of spatial scale and intraspecific variation in secondary chemistry in host-plant location by Ceutorhynchus assimilis (Coleoptera : Curculionidae)

To understand the ecological role of secondary plant compounds in host loca tion by phytophagous insects it is important to consider attraction at diff erent scales in natural populations. The cabbage seed weevil, Ceutorhynchus assimilis, which lays eggs in pods of crucifers where the larvae feed on s eed, is attracted to purified extracts of specific glucosinolate-derived vo latiles. We considered the possibility that C. assimilis adults are attract ed to and preferentially attack patches of plants and/or individual plants producing these volatiles. Using discrete natural populations of Brassica o leracea and Brassica nigra, we found that oviposition was highest in popula tions of B. oleracea producing high amounts of 3-butenylglucosinolate. No l inks were found between the other glucosinolates, 2-propenylglucosinolate, 2-hydroxy-3-butenylglucosinolate, 1-indolylmethylglucosinolate or 1-methoxy -3-indolylmethylglucosinolate, and oviposition in B. oleracea. B. nigra, wh ich contains only 2-propenylglucosinolate, was not attacked by C. assimilis . Within populations of B. oleracea, neither oviposition nor the number of seeds eaten was related to the glucosinolate profiles of individual plants. We suggest that C. assimilis adults use 3-butenylglucosinolate-derived vol atiles to locate host populations, whereas other cues determine oviposition on individual plants. The consequences of these results for natural select ion of glucosinolate phenotypes are discussed.