Ladybird egg cannibalism and intraguild predation by Harmonia axyridis: the effects of egg age
Abstract: Ladybird prey vary in their suitability for the intraguild predator Harmonia axyridis. Much of this variation is attributable to differences in prey chemical defences, particularly in the types of alkaloids that different prey ladybird species possess. We here discuss another factor that plays a role in egg suitability: nutrient content. Ladybird eggs vary interspecifically in concentrations of important nutrients, notably lipids and protein. Nutrients also decline as an egg develops, leading to a predictable decline in nutritional content with egg age. Experimental results are given here showing that starved H. axyridis neonates fed younger eggs survive for longer than those fed on older, more developed eggs. By contrast, alkaloid content remains stable throughout egg development. In feeding experiments, first instar H. axyridis larvae ate more old eggs of the same prey species than freshly laid ones, and larvae developed more rapidly on the younger eggs. Eggs of the suitable prey Hippodamia convergens remained suitable irrespective of age, when compared to similarly aged conspecific eggs. However old eggs of the more toxic prey, Adalia bipunctata disproportionately slowed the development of H. axyridis larvae, by comparison with similarly aged conspecific eggs. This interactive effect of chemical defence and nutrient content, mediated by egg age, appears to arise because H. axyridis cannot metabolise the unsuitable A. bipunctata alkaloids. Because larvae must eat more old eggs, a higher level of toxic alkaloid will accumulate in the body of the larva. The results suggest that in studies of egg cannibalism and intraguild predation, absolute quantitative measures of performance on or consumption of egg prey need to be treated cautiously if egg age or stage of development is unknown or not recorded.