Species that are involved in multitrophic interactions are affected by the trophic levels that are above and below them in both indirect and direct ways. In this experiment, interactions among ants (Formica montana Wheeler; Hymenoptera: Formicidae), aphids (Myzus persicae [Sulzer]; Hemiptera: Aphididae), lepidopteran eggs (Pieris rapae [L.]; Lepidoptera: Pieridae), and lady beetles (Coleomegilla maculata DeGeer; Coleoptera: Coccinellidae) were examined on canola (Brassica napus L.; Brassicales: Brassicaceae) to determine whether aphids mediate interactions between ants and eggs and if aphid-tending ants will protect these species from predation by the lady beetles. The first trial examined how ant attendance affected aphid and egg survival over 72 h when these herbivores were present on their own or in combination. The second trial examined these same interactions when a lady beetle was included in the system. Ants consumed significantly more aphids when this prey was offered alone than when eggs were offered alongside. When lady beetles were present, ants did not consume aphids. Ants and lady beetles consumed nearly all of the eggs regardless of whether aphids were present or not. When a lady beetle was absent, ant attendance increased significantly, but only when eggs and aphids were present in the system together. Ants visited the aphids less frequently when a lady beetle was present in the system, possibly due to increased competition. This trend was consistent over time in all treatments. All of this suggests that ants can be an important source of pest management of other herbivores (and even of the aphids that they are attending) but that these interactions are mediated by whether an additional predator is present within the system.
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Vol. 51 • No. 2