Geographical proliferation of ongoing novel host use by the herbivorous ladybird beetle Henosepilachna vigintioctopunctata (Coleoptera: Coccinellidae)

The acquisition of novel host plants can be regarded as an incipient phase in the speciation continuum of herbivorous ladybird beetles. Therefore, describing the geographical pattern of the establishment of associations with novel hosts is indispensable for elucidating the whole picture of their speciation continuum. The herbivorous ladybird beetle Henosepilachna vigintioctopunctata (Fabricius) (Coccinellidae: Epilachnini) depends basically on solanaceous plants, but also occurs on the introduced legume Centrosema molle Mart. ex Benth. (Fabaceae) in Southeast Asia; yet the geographical extent of this novel host‐use has not been fully elucidated. In this study, we documented the geographical extent of the occurrence of H. vigintioctopunctata on C. molle through intensive field surveys. The use of C. molle by the beetles was newly detected in various regions. We also investigated the ability of larvae of H. vigintioctopunctata from several geographical populations to develop on C. molle in the laboratory. Geographic variation in the degree of adaptation to C. molle by H. vigintioctopunctata populations was also detected. Our results suggest that the degree of adaptation to C. molle by the respective beetle populations does not correspond with the frequency of occurrence of the beetles on C. molle in the wild at the regional level. It was interpreted that the degree of adaptation to C. molle by respective H. vigintioctopunctata populations is determined not only by the time since the novel host‐use has started but also by ecological situations such as the relative abundance and availability of various host plants in respective local habitats.