Phytophagous specialists among insects occasionally accept plants they do not or only rarely utilize under natural conditions. This non-host acceptance could represent an initial stage in host-range evolution. Here, we examined adult acceptance of and larval survivorship on a non-host plant - wild thistle, Cirsium kamtschaticum Ledeb. (Asteraceae) - for two geographically separate populations of the phytophagous ladybird beetle Henosepilachna yasutomii Katakura (Coleoptera: Coccinellidae), which normally utilizes blue cohosh, Caulophyllum robustum Maxim. (Berberidaceae), in the wild. The two beetle populations showed lower acceptance of and survivorship on the non-host plant (thistle) than on the normal host plant (blue cohosh). Furthermore, they differed significantly in their responses to thistle but not to blue cohosh. Even for the beetle population that more readily accepted thistle as adults, acceptance was moderate and the eclosion rate was low, indicating reduced performance on the non-host plant. Although studies have infrequently focused on non-host acceptance, this is likely common in phytophagous specialists and thus is a potentially important evolutionary factor, as it may determine the future direction of food-range evolution.
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