Effects of individual variation of host plants on the pattern of host selection in a phytophagous ladybird beetle species.

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Effects of individual variation and interspecific difference of plants on host selection by polyphagous phytophagous insects were studied using a ladybird beetle Epilachna pustulosa and its host plants thistle (Cirsium kamtschaticum) and blue cohosh (Caulophyllum robustum) in Sapporo, central Hokkaido. Through adult food choice tests and rearing of larvae using leaves from a number of individual plants, it was shown that thistle and blue cohosh constitute a continuous series of food resource for E.pustulosa and that the beetles select, irrespective of the plant species, the plants on which their larvae will perform well. However, the results obtained by laboratory experiments were contradictory to the patterns of host selction realized in the field ; the plants preferred in the laboratory were not always utilized in the field, and vice versa. Host selection by E.pustulosa is thus governed not only by the factors clarified in the laboratory but also by certain other factors whose effects are not yet clarified. Spatial distribution patterns and biomasses of host plants, habitat preference of beetles, and phenological correspondence between beetles and plants are possible candidates of such unknown factors. We also conducted preliminary host choice tests by E.pustulosa using thistles and blue cohosh collected in southern Hokkaido where E.niponica and E.yasutomii, closely related species to E.pustulosa but specialized to thistle and blue cohosh, respectively. In this case, too, E.pustulosa chose particular stocks irrespective of the plant species.