Ants commonly form mutualistic associations with honeydew-producing Homopterans, in which the herbivores typically gain protection from predators and parasitoids in return for producing a significant and renewable food resource for ants. Honeydew-producing Homopterans feed on plant phloem and, in sufficient numbers, can be detrimental to plant fitness. The fact that ants are attracted to and protect Homopterans only exacerbates this problem; however, the attraction of ants onto plants might actually benefit plant fitness by ant suppression of other herbivores. In a heuristic model of the mutualism between red imported fire ants (Solenopsis invicta) and cotton aphids (Aphis gossypii) on cotton plants, we develop the hypothesis that the consequences to cotton plant fitness of the fire ant – cotton aphid mutualism depend on the intensity of non-Homopteran herbivory. We test this hypothesis using field cages in which we manipulate the abundance of cotton aphids and beet armyworms (Spodoptera exigua) in cotton fields infested with fire ants. We predict that at high aphid abundance but low beet armyworm abundance, the cost to the plant of hosting the aphids outweighs the benefit received in terms of beet armyworm suppression, and thus reduces plant fitness. Conversely, at the same level of aphid abundance but with high beet armyworm abundance, we predict that the benefit of beet armyworm suppression outweighs the cost of hosting the aphids, and thus improves plant fitness.
Species 1: Hymenoptera Formicidae Solenopsis invicta (red imported fire ant)
Species 2: Homoptera Aphididae Aphis gossypii (cotton aphid)
Keywords: mutualism, biological control
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