Seasonal variation in elytral colour polymorphism in Harmonia axyridis (the ladybird beetle): the role of non-random mating

Abstract

Seasonal variation in the frequency of elytral colour polymorphism in the ladybird beetle, Harmonia axyridis, was analysed through the combination of field observations and laboratory mate-choice experiments. Field observations involving daily mark-recapture of a local population revealed an overall preponderance of melanic morphs in the population, but the proportion of non-melanics notably increased from spring to summer due to their higher frequency of mating among different morphs of the spring generation. In the laboratory experiments, both melanic and non-melanic females preferentially chose non-melanic males in spring, but non-melanics were less successful at mating than melanics in summer. Therefore, the relative frequencies of melanics and non-melanics within a population vary throughout the seasons, with non-melanics increasing in proportion towards summer and decreasing thereafter. Laboratory experiments revealed that non-random mating among morphs is the basis of this variation, with females and, to a lesser degree, males being involved in the process of mate selection. The results of an artificial colour-change experiment indicate that male elytral colour is an important factor in female mate selection but other factors such as behaviour and chemical cues must also be involved.