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Intraguild Predation Among Aphidophagous Lady Beetles
Department of Entomology
This project will increase our understanding of the chemical ecology of intraguild predation among invasive and native species of Coccinellidae in North America. Previous laboratory studies have documented intraguild predation among Coccinellidae, but the underlying role of alkaloid defenses has not been examined for most species. An additional aspect of this study will address the effect of the invasive predatory species (Harmonia axyridis) as an intraguild predator of several native species of Coccinellidae. This work will provide a better understanding of the role of multiple species of predatory Coccinellidae in the biological control of selected insect pests of agricultural crops.
2011 Project Description
We conducted several laboratory experiments. First, to understand effects of heterospecific alkaloids in three congener Coccinellids, Coccinella septempunctata, C. transversoguttata, and C. novemnotata, we conducted feeding experiments using their eggs and first instars. We found that these three congeners have similar alkaloids, coccinelline and precoccinelline. Our previous data show a link between heterospecific alkaloids and their toxicity and/or costs in predatory ladybeetles, Harmonia axyridis and Coccinella septempunctata (Kajita et al. 2010), but this finding suggests that heterospecific alkaloids may not be toxic for three congener ladybeetles because these species have similar alkaloids.
Second, we conducted feeding experiment and quantification of alkaloids in predatory ladybeetles, Harmonia axyridis, Hippodamia convergens, and Adalia bipunctata Ladybird prey vary in their suitability for the intraguild predator H. axyridis. Much of this variation is attributable to differences in prey chemical defenses, particularly in the types of alkaloids that different prey ladybird species possess.
We examined another factor that plays a role in egg suitability and nutrient content. Ladybird eggs vary inter-specifically 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.
We found that three Coccinella species possess the same alkaloids, coccinelline and precoccinelline. Our previous data show a link between heterospecific alkaloids and their toxicity and/or costs (Kajita et al. 2010), but this finding suggests that heterospecific alkaloids may not be toxic for these ladybeetles if the heterospecific alkaloids are similar.
Our feeding experiments with three congeners, Coccinella septempunctata, C. transversoguttata, and C. novemnotata, showed that there are no significant effects of consumption of heterospecific alkaloids (eggs) on development and survivorship of the first instar of C. septempunctata, C. transversoguttata, and C. novemnotata.
This suggests that heterospecific alkaloids are not always toxic for predators, if predators possess the similar defensive chemicals. We will use the methods from Kajita et al. (2010) to quantify alkaloids in 1st instars of these three ladybeetle species. These methods will provide an estimate of the accumulation or reduction of consumed alkaloids by comparing the levels in eggs and larvae from the GC-MS analyses.
Feeding experiments with H. axyridis, H. convergens, and A. bipunctata showed 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 older eggs of the same prey species than freshly laid ones, and larvae developed more rapidly on younger eggs.
Eggs of the suitable prey H. 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 older 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.
Kajita Y, Obrycki JJ, Sloggett JJ, Haynes KF (2011). Intraspecific alkaloid variation in ladybird eggs and its effects on con- and heterospecific intraguild predators. Oecologia (163): 313-322, DOI10.1007/s00442-009-1551-2