Heritability and fitness consequences of cannibalism in Harmonia axyridis

We examined environmental (food levels) and genetic (heritability and evolv ability) influences on the expression of cannibalism in larvae of the ladyb ird beetle Harmonia axyridis Pallas (Coleoptera: Coccinellidae). In conjunc tion, we examined potential fitness consequences of cannibalism under diffe rent levels of food availability by measuring time of larval development an d size at adult. Using a full-sib design, we split broods into food environ ments that differed by five-fold and measured rates of cannibalism by third instar larvae on first instar conspecifics. Surprisingly, there was signif icant genetic variation in the expression of cannibalism in response to inc reased prey levels. Some families exhibited a decrease in cannibalistic beh aviour, some an increase, while some families did not alter their cannibali stic rate in response to different food levels. In the low food environment , there was a strong genetic basis for the expression of cannibalism with a heritability significantly different from zero. In the higher food environ ment, heritability was not significantly different from zero. However, evol vabilities for cannibalism were similarly high for both food level environm ents. Fitness consequences also depended on food levels. Larvae from the lo w food environments reduced their development time by approximately 1 day w hen they cannibalized an average of one first-instar conspecific larva. Alt hough in the higher food environment rates of cannibalism did not decrease significantly, development times remained unaffected. Our results suggest s ignificant genetic variation in the expression of cannibalism within a natu ral population of H. axyridis and selection favouring cannibalism under low food environments but not when prey levels are high.