Experience-based food consumption by larvae of Pieris rapae: addiction to glucosinolates?

When newly hatched larvae of P. rapae were transferred to cowpea foliage, t hey readily accepted this non-host as food, whereas later instars that had fed on cabbage rejected cowpea. However, when cowpea leaf discs were treate d with aqueous extracts of cabbage foliage, they were accepted by cabbage-r eared larvae. Experiments were conducted to determine whether larvae reared on one host plant would be stimulated to feed by extracts of other hosts. Larvae reared on Brassica juncea, Cleome spinosa, Tropaeolum majus, Sinapis alba, Alliaria petiolata, Barbarea vulgaris and cabbage (Brassica oleracea ) were offered extracts of each of the other host plants on cowpea discs in choice assays. Larvae were generally stimulated to feed by extracts of all the alternate hosts, but quantitative differences in consumption occurred. In most cases, levels of discrimination between treatment and control cowp ea discs showed no significant preference for extracts of the previously ex perienced plant. Since the test plants (and their extracts) contain glucosi nolates of widely different structures, a general addiction to glucosinolat es was suggested. A single glucosinolate, sinigrin, was sufficient to elici t feeding by cabbage-reared larvae. The time required for individual neonat es to become addicted to glucosinolates as they fed on cabbage, as measured by refusal of cowpea, varied from 6 to 30 hours. Bioassays of cowpea extra cts failed to show any deterrent activity and, therefore, supported the con clusion that addiction to glucosinolates is responsible for the fixation of P. rapae larvae on their host plants.