Putting more eggs in the best basket:: clutch-size regulation in the comma butterfly

1. Many Studies have identified different factors influencing clutch-size regulation, primarily within various groups of insects. One prediction is that ovipositing females should increase clutch size with host quality. However, in many studies it is not clear whether ovipositing females are responding to host quality or quantity. 2. Females of the polyphagous comma butterfly, Polygonia c-albian (L.), were allowed to oviposit on two hosts differing greatly in quality: the preferred host, stinging nettle (Urtica dioica L.), and the low-ranked host, birch (Betula pubescens Ehrh). Ovipositing females were observed visually and clutch sizes were recorded. The experiment was repeated in three different years; in total, 938 observations of oviposition events were made. 3. In all three years, females ovipositing on U. dioica laid larger clutches (median 1.6-1.85) compared with females ovipositing on B. pubescens (median 1.0-1.3). The difference was significant in two out of three years and when all three years were pooled. 4. Thus, P. c-album females exhibit clutch-size regulation, with larger clutches on better hosts. It is suggested that the proximate mechanism is likely to be a response to the same stimuli used for female ranking of host plants in the preference hierarchy.