Abstract

Group living has both negative and positive effects, and various ecological factors can mediate decisions between remaining cohesive and splitting up. We investigated how food quality mediates the trade-off in Malacasoma disstria, the forest tent caterpillar, between (i) remaining in a group on a known food source, thus benefitting from group basking and group predator defenses, but potentially incurring opportunity costs, and (ii) increasing movement in search of a potentially better quality food source and/or splitting into more numerous groups, risking predation. During laboratory experiments in spring 2013, and field experiments in 2014, groups of caterpillars were reared on the foliage of two different tree species; one of high quality: trembling aspen (Populus tremuloides), and the other of lower quality: sugar maple (Acer saccharum). Results indicate that forest tent caterpillar colonies feeding from maple increased locomotion, traveled farther, took more numerous, but smaller meals, and split into smaller, more numerous groups when compared to colonies feeding from aspen. Though other ecological factors were found to mediate group decisions, namely developmental stage, colony size, and wind speed, the presented experiments suggest that a decision between remaining cohesive or otherwise in the presence of different quality food sources exists. The findings suggest that the potential increase of encountering a predator when increasing movement and decreasing cohesion can increase early larval mortality and may be a contributing factor to the rise and decline of an outbreaking population.