Host plant suitability of a recently naturalized thistle Cirsium vulgare (Asteraceae) for a phytophagous ladybird beetle, Epilachna pustulosa (Coleoptera: Coccinellidae)

The phytophagous ladybird beetle, Epilachna pustulosa Kôno occurs mainly on the thistle Cirsium kamtschaticum Ledeb. In recent years, we have confirmed feeding and oviposition by E .  pustulosa , and some degree of larval development of this beetle, on a naturalized congeneric thistle, Cirsium vulgare (Savi) Tenore. In this study, we assessed the host suitability of C. vulgare for E. pustulosa , using larval rearing and simple food choice tests, by comparing the performance on, and preference for, their legitimate host C. kamtschaticum . All experiments were initiated in July, when the utilization of C. vulgare by E. pustulosa became obvious. For larval performance, E. pustulosa showed a lower eclosion rate, shorter developmental duration and smaller body size when reared on C. vulgare than on C. kamtschaticum . Cirsium vulgare appeared to be a less adequate larval food than C. kamtschaticum . Adult feeding preference was altered seasonally; females preferred C. kamtschaticum to C. vulgare in a food choice test conducted in early July, whereas they showed the opposite preference in tests conducted in mid and late July. The utilization of C. vulgare by E. pustulosa appeared to be induced by qualitative deterioration of C. kamtschaticum leaves in July, which resulted in increased dispersion of adult beetles.