Increasing trophic complexity influences aphid attendance by ants (Hymenoptera: Formicidae) and predation

Authors: Lundgren, Jonathan; LAYMAN, MARISSA - South Dakota State University

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2015
Publication Date: 4/15/2016
Citation: Lundgren, J.G., Layman, M. 2016. Increasing trophic complexity influences aphid attendance by ants (Hymenoptera: Formicidae) and predation. Journal of Entomological Science. 51(2): 151-164.

Interpretive Summary: Food webs are complex, and when scientists simplify these networks of species it is easy to overlook key interactions that can affect processes like pest management. Ants often tend aphids, which are pests, and thus ants may indirectly harm crop production (like in canola). But in tending these aphids, ants also eat other herbivores in a system (like imported cabbageworm eggs), thus potentially having a positive effect on a crop plant, depending on the relative importance of caterpillars and aphids as pests. Other generalist predators (like lady beetles) are adversely affected by ants as well. We studied these complex interactions on herbivore survival in a series of controlled experiments. We found that ants and lady beetles were important predators of imported cabbageworm eggs (fewer than 10% survived in any trial when predators were included). Both ants and lady beetles reduced aphids to a minimum threshold of approximately 30 aphids per plant. Ants invariably killed the lady beetles, and when lady beetles were present, ants did not consume any of their aphid attendees. This suggests that ants can be an important source of control for non-aphid herbivores in the system, and that the complex interactions among predators and herbivores can be difficult to predict.

Technical Abstract: 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 ants 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. Ants visited the aphids less frequently when a lady beetle was present in the system; this attendance was consistent over time in all treatments except that aphid attendance increased significantly over time when eggs and aphids were offered together. All of this suggests that ants can be an important source of pest management of other herbivores (and even of aphids that they are attending), but that these interactions are mediated by whether an additional predator within the system.