The influence of three cereal aphid species and mixed diet on larval survival, development and adult weight of Coccinella septempunctata

The developmental time, weight and fecundity of the aphid specialist predator the seven spotted ladybird (Coccinella septempunctata L.) vary considerably according to the species of aphid they are raised on (Blackman, 1967a; Ghanim et al., 1984; Malcolm, 1992). The results of these studies showed that prey of low food quality slowed larval development and produced smaller adult animals. The experiments were carried out with single-species diets. So far there seems to be no studies on the possible benefits of diet mixing in coccinellid beetles (Hodek & Hon ěk 1996). Hodek (1956) and Blackman (1967b) found evidence that C. septempunctata is unselective in its choice of prey; it cannot recognise and avoid unsuitable or toxic species of aphids. ̧ Sengonca & Liu (1994) noticed that the larvae ofC. septempunctata revealed no difference in responding to the kairomones from four different species of aphid prey. It is therefore likely that the larvae ofC. septempunctata just eat the aphids that they meet. This may be an optimal foraging strategy, when the prey is clumped and unpredictable and acceptance of toxic prey is no worse than starvation. Even if coccinellids are unselective in a choice situation, a mixture of prey species may provide a higher nutritional diversity and thus be beneficial in terms of improved performance i.e. a lower mortality, a faster development and a higher adult body weight. We used three cereal aphid species Sitobion avenae(F.), Rhopalosiphum padi (L.) and Metopolophium dirhodum(Walker) and an equal number mixture of these three species as prey and compared the performance of C. septempunctata larvae. The three cereal aphid species all have grasses as their host plants, thus the coccinellid larvae may encounter all three species on the same plant. It is therefore ecologically relevant to examine if there is an advantage of a mixed diet of these three aphid species. Our hypothesis is that an aphid specialist predator as C.septempunctata , may gain advantage from developing on a diversified diet, compared to a singlespecies aphid diet, because different species of aphids may have a different composition of nutrients, thus satisfying better a varied nutrient demand. This should be revealed by better performance of the larvae, indicating a higher food value of the mixed diet. Such relationships are known for a wide range of generalist consumers (Reichert & Harp, 1987; Waldbauer & Friedman, 1991; Wallin et al., 1992; Bernays et al., 1994).