Asymmetric local adaptation to host plants (Cirsium spp.) by the phytophagous ladybird beetle Henosepilachna niponica and its implications

Abstract

Divergent host specialization by phytophagous insects is often detected as local adaptation and is thought to have played an important role in their diversification even within an ecological specialist. The phytophagous ladybird beetle Henosepilachna niponica Lewis (Coleoptera: Coccinellidae) predominantly depends on thistles (Cirsium spp., Asteraceae). The distribution of H. niponica occupies multiple areas dominated by different thistle species. This implies the possibility of the occurrence of host‐associated divergent specialization of H. niponica. In this study, we investigated the pattern of host‐use ability of three allopatric H. niponica populations (Aomori, Iwate, and Yamagata) on three thistle species – Cirsium alpicola Nakai, Cirsium nipponicum (Maxim.) Makino, and Cirsium tonense Nakai – under laboratory conditions. The results displayed asymmetric local adaptation by the beetles. The adults and larvae of the Aomori population showed sufficient acceptance and performance on C. nipponicum and C. tonense, the hosts of the Iwate and Yamagata populations, respectively. On the other hand, the Iwate and Yamagata populations fed small amounts of and performed poorly on C. alpicola, the host of the Aomori population. In contrast, the adults from all the populations clearly preferred feeding on C. nipponicum or C. tonense to C. alpicola. We concluded that the small but significant population differentiation and asymmetric local adaptation by beetles to congeneric host plant species could be a sign of the earliest stage of population divergence by divergent natural selection, given that these divergences will act as ‘immigrant inviability’ and ‘habitat isolation’ at least asymmetrically when these populations come into contact.