Nesbit, Chris and Menendez Martinez, Maria Rosa and Roberts, Michael Richard and Wilby, Andrew (2016) Associational resistance or susceptibility : the indirect interaction between chemically-defended and non-defended herbivore prey via a shared predator. Oikos, 125 (12). pp. 1802-1811. ISSN 0030-1299
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Abstract
Many organisms possess chemical defences against their natural enemies, which render them unpalatable or toxic when attacked or consumed. These chemically-defended organisms commonly occur in communities with non- or less-defended prey, leading to indirect interactions between prey species, mediated by natural enemies. Although the importance of enemy-mediated indirect interactions have been well documented (e.g., apparent competition), how the presence of prey chemical defences may affect predation of non-defended prey in terrestrial communities remains unclear. Here, an experimental approach was used to study the predator-mediated indirect interaction between a chemically-defended and non-defended pest aphid species. Using laboratory-based mesocosms, aphid community composition was manipulated to include chemically-defended (CD) aphids (Brevicoryne brassicae), non-defended (ND) aphids (Myzus persicae) or a mixed assemblage of both species, on Brassica oleracea cabbage plants, in the presence or absence of a shared predator (Chrysoperla carnea larvae). Aphid population growth rates, aphid distributions on host plants and predator growth rates were measured. In single-species treatments, C. carnea reduced M. persicae population growth rate, but had no significant impact on B. brassicae population growth rate, suggesting B. brassicae chemical defences are effective against C. carnea. C. carnea had no significant impact on either aphid species population growth rate in mixed-species treatments. M. persicae (ND) therefore experienced reduced predation in the presence of B. brassicae (CD) through a predator-mediated indirect effect. Moreover, predator growth rates were significantly higher in the M. persicae-only treatments than in either the B. brassicae-only or mixed-species treatments, suggesting predation was impaired in the presence of B. brassicae (CD). A trait-mediated indirect interaction is proposed, consistent with associational resistance, in which the predator, upon incidental consumption of chemically-defended aphids is deterred from feeding, releasing non-defended aphids from predatory control.