Quantitative evaluation of suitability of the greenbug, Schizaphis graminum, and the bird cherry-oat aphid, Rhopalosiphum padi, as prey for Hippodamia convergens (Coleoptera: Coccinellidae)

The nutritive value of two cereal aphid species, Schizaphis graminum and Rhopalosiphum padi, for Hippodamia convergens development was compared as pure- and mixed-species diets and as suboptimum and ad libitum quantities. Comparisons were based on the following daily aphid quantities and combinations: 4 mg R. padi, 4 mg S. graminum, 3:1 mg mixture of R. padi and S. graminum, 2:2 mg mixture of R. padi and S. graminum, 1:3 mg mixture of R. padi and S. graminum, ad libitum R. padi, and ad libitum S. graminum. Preimaginal survival levels of H. convergens were high across all treatments and were neither influenced by aphid species, daily amounts of aphids, nor the mixtures supplied to the larvae. There was no diet effect on the sex ratio of individuals that developed to adulthood. Only the development time of fourth instars was significantly influenced by the larval prey regimes. Fourth instars fed ad libitum aphids had shorter development times that did not vary between aphid species. However, among the suboptimal aphid levels, fourth instars that fed on higher proportions of R. padi had longer development times. Both diet and gender of developing larvae had individual and interactive effects on the body size of adult H. convergens. Under ad libitum aphids feeding on R. padi resulted in smaller females whereas males showed no difference. Our results suggest that both R. padi and S. graminum are essential prey for H. convergens development with S. graminum being relatively more suitable than R. padi.