Effects of Simulated Bear Attacks on Thatch Ant (Formica obscuripes) Defense of Honeydew-Producing Aphids

Abstract

Defensive mutualisms occur in many ecosystems, but the impacts of environmental stressors on defensive services are unclear. Predation stress can create trophic cascades that influence relationships in lower trophic levels in food webs, including defensive mutualisms. Eusocial species often engage in defensive mutualisms, and stressors experienced by individuals within a colony may affect the behavior of the entire colony, such as cooperative and aggressive behaviors in defensive services. We tested whether experiencing a simulated bear attack (i.e., predation stress) affects mutualist defense behavior of a eusocial species, western thatch ants (Formica obscuripes), which defend aphids against predators in exchange for honeydew food. We exposed randomly selected thatch ant nests to a simulated bear attack to mimic natural bear predation (2022: n = 6 simulated bear attack, n = 22 not attacked; 2023: n = 8 simulated bear attack, n = 29 not attacked). We then exposed aphid-tending thatch ants to an aphid predator stimulus (e.g., lady beetle) approximately 5 weeks and 13 weeks after the simulated bear attack. Ants attacked the lady beetle more frequently during the second trial compared to the first trial, but this was more pronounced for ants whose colony did not experience the simulated bear attack. Year also interacted with trial time point to affect ants' behavior on the plants near aphid aggregations. Our results suggest that predation stress caused by the simulated bear attack may cause changes in ant defensive services that can last for months and which may be complicated by yearly differences in abiotic and biotic factors.