Geographic variation of color polymorphism in two sibling ladybird species, Harmonia yedoensis and H.axyridis (Coleoptera: Coccinellidae)

Geographical variation of elytra color pattern in two sibling ladybird species, Harmonia yedoensis and H.axyridis (Coleoptera: Coccinellidae), was examined. The two species are distributed sympatrically in central Japan; however, only H.yedoensis and H.axyridis occur in the Ryukyu Islands (southern Japan) and Hokkaido island (northern Japan), respectively. The frequency of elytra color patterns was significantly different between the two species in all sympatric locations and our results were inconsistent with the classical theory on Mullerian mimicry. The most dominant pattern of H.axyridis was the least dominant of H.yedoensis in all sympatric populations. Furthermore, the frequency of the non-melanic form (red ground color with or without black spots) increased towards the south in H.yedoensis. This tendency was in contrast to the known geographical cline in H.axyridis in which the melanic form (black ground color with red spots) was gradually displaced with the non-melanic form northwards in the Japanese archipelago. We discuss possible selective factors including predator avoidance, thermal adaptation and reproductive character displacement, all of which might contribute to the maintenance of the color polymorphism in the two Harmonia species.