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Quantitative food webs were constructed from the data collected, using visual observation technique, from May to July in 2005 and 2006 to describe separately the trophic relationships between the community of aphids and their natural enemies of predators and parasitoids in agricultural and semi-natural habitats in Gembloux, Belgium. In the web, a total of six aphid attacked by six parasitoid and 21 predator species were recorded in this study; 50 and 33 links between aphid and natural enemy species were respectively observed in 2005 and 2006. Aphid density varied seasonally and among years, and three species of aphid were particularly common in different habitats. The ratios of the number of aphid species to the number of species of either predators or parasitoids changed also seasonally. Most communities of insect herbivores are likely to be structured by apparent competition mediated by shared natural enemies more than to be structured by resource competition. The potential of two guilds of natural enemies to contribute in structuring aphid community through indirect interactions was assessed. The potential strengths of apparent competition between hosts mediated by parasitoids and predators were assessed using quantitative parasitoid/predator overlap diagrams. Symmetrical links were uncommon, and rare species were severely influenced by the presence of common aphids with which they shared parasitoids or predators or both. The study’s results suggest that (i) stinging nettle aphids, acting as potential sources of apparent competition mediated by natural enemies, may be important in these highly connected communities and can control whole-community dynamics, and (ii) stinging nettle habitats, in providing an important alternative prey for natural enemies, are likely to play a key role in conservation biological control. |
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