Diet-mediated effects of specialized tansy aphids on survival and development of their predators: Is there any benefit of dietary mixing?

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Some predators have a broad prey range, but not all prey are equally suitable for a predator. We tested the suitability of three specialized tansy (Tanacetum vulgare) aphids, Macrosiphoniella tanacetaria, Metopeurum fuscoviride and Uroleucon tanaceti, on biological parameters of their major predators, Coccinella septempunctata and Chrysoperla carnea, when aphid species were offered either alone, or as a mixed diet of all the three aphid species. For the lacewing C. carnea, a diet consisting only of the generally ant-tended M. fuscoviride resulted in the shortest larval and total developmental time, in highest larval survival and pupation success. For the ladybird C. septempunctata, a pure diet of the non-tended M. tanacetaria was most suitable, resulting in the shortest total developmental time, heaviest fresh and dry weight. Larvae of both predator species had low survival and none of them developed to pupal stage when offered only U. tanaceti that has a bright red colour, is not ant-tended and feeds on the underside of lower leaves. The mixed diet of all three aphid species was worse than the best single-aphid diet for both predators, for almost all fitness parameters. Thus, while diet mixing may allow predators to avoid the negative impact of unsuitable prey, diet mixing does not necessarily result in the highest fitness of predators.