Consequences of Mutualisms Between Aphids and an Invasive Ant to Arthropod Communities and Their Host Plants

Abstract

Mutualisms between ants and honeydew-producing Hemipteran insects (e.g., aphids) are abundant and widespread in arthropod food webs, yet their ecological consequences are very poorly known. Previous work in several agricultural systems in Alabama show that the community-level effects of red imported fire ants (Solenopsis invicta) vary among crops and that this variation may be correlated with the presence and absence of aphids. Fire ants have stronger and more pervasive effects in cotton, for example, where they are attracted onto plants by cotton aphids (Aphis gossypii) than they do in soybean, which has not historically hosted aphids. Alternatively, fire ants may be hindered from foraging on soybean plants by dense trichomes on soybean stems and leaves. In chapter one, I present field and greenhouse experiments in which I manipulated fire ant density in plots of three soybean isolines varying in trichome density to test this hypothesis. Trichomes did not inhibit fire ants from foraging on plants in the field or in the greenhouse, and fire ant predation of herbivores in the field was actually greater on pubescent plants relative to glabrous plants. In chapter two, I review several studies that investigate explicitly the consequences of ant-Hemipteran mutualisms to plants. In most cases, plants benefit from these mutualisms as a consequence of increased predation or harassment of more damaging herbivores by Hemipteran-tending ants, resulting in decreased plant damage and increased plant reproduction. In chapter three, I describe field and greenhouse experiments in which I manipulated the presence and absence of cotton aphids on cotton plants to test the hypothesis that a mutualism between cotton aphids and red imported fire ants benefits cotton plants by increasing fire ant suppression of beet armyworm caterpillars (Spodoptera exigua). Greater numbers of fire ants foraged on plants with cotton aphids than on plants without cotton aphids, resulting in a significant reduction in caterpillar survival and caterpillar herbivory of leaves, squares, and bolls on plants with aphids. In chapter four, I describe field and greenhouse experiments to test the hypothesis that the range expansion of the soybean aphid (Aphis glycines), will cause a dramatic increase in the effects of fire ants as predators in soybean. In field experiments in which I misted soybean plants with artificial honeydew as a surrogate for soybean aphid honeydew, fire ants were significantly more abundant on honeydew-misted plants, resulting in decreased beet armyworm caterpillar abundance and caterpillar damage to plants. Results from greenhouse experiments with soybean aphids were consistent with the field experiments.