Functional response of all instars, adult males and females of the ladybird Coccinella septempunctata L. was studied against two species of aphids, namely Lipaphis erysimi (Kalt.) and Brevicoryne brassicae L., infesting mustard and cabbage crops, respectively. The predatory stages of C. septempunctata exhibited a type II response on both the prey species. The attack rate was highest (0.0192) with the lowest handling time (7.9 min) when the adult female consumed L. erysimi. There was a significant difference in the predicted values of consumption of L. erysimi by fourth instars, adult males and females compared with those of B. brassicae, indicating that the former prey is preferred over the latter one. The adult females prey upon significantly more numbers of aphids than the adult males.
We investigated the predatory potential and food preference of different life stages of Coccinella septempunctata L. for a nutritious aphid (mustard aphid, Lipaphis erysimi) and toxic aphid (cabbage aphid, Brevicoryne brassicae). We provided all the life stages of C. septempunctata with either L. erysimi or B. brassicae and found that the second, third and fourth instar larvae and adult females of this predator consumed daily greater numbers of L. erysimi. However, the fi rst instar larvae and adult males consumed similar numbers of both of these aphids. In choice condition, each larva, adult males and females were each provided separately with a mixed aphid diet in three proportions (i.e. low: high, equal: equal and high: low densities of L. erysimi: B. brassicae). We hypothesized that life stages of C. septempunctata will prefer L. erysimi regardless of its proportions. Laboratory experiments supported this hypothesis only at the adult level in terms of high values of β and C preference indices. However, it rejects this hypothesis at the larval level, as larvae preferred B. brassicae when provided with certain combinations and showed no preference in a few combinations. We infer that mixtures of nutritious and toxic aphids may enable this ladybird to overcome any probable nutritional defi ciency and/or reduce the toxicity of a toxic diet, especially for the larvae. Results of the treatment in which a high proportion of B. brassicae were consumed along with fewer L. erysimi indicates that a mixed diet could be better for the development of immature stages of C. septempunctata.
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