Social Behavior of Larvae of the Neotropical Processionary Weevil Phelypera distigma (Boheman) (Coleoptera: Curculionidae: Hyperinae)

Socially gregarious behavior among free-living leaf-eating insect larvae occurs mostly among Lepidoptera, Symphyta, and a few Chyrsomelidae (Coleoptera). However, the Neotropical hyperine curculionid Phelypera distigma has also evolved this lifestyle, exhibiting a suite of social behaviors unique among beetles. The larvae are nomadic processionary foragers that punctuate foraging bouts with rosette-shaped resting formations (cycloalexy). Larvae also vibrate or bob their heads rapidly when moving, especially when in contact with conspecifics, and this suggests acoustic or vibrational communication. In this study we used observational and experimental approaches to investigate the basis of processionary, cycloalexic,and head-vibration behavior of this species. Larvae used both trail pheromones and thigmotactic signals to organize themselves into head-to-tail processionary columns. The trail pheromone, produced from the center of the abdomen, remains active for up to 4 h. Processions are not consistently led by particular individuals, but dynamically change over time and often temporarily break into two or more subprocessions. Subprocessions reunite through use of the trail pheromone. We found no evidence that head-bobbing generates attraction through substrate-borne or acoustic signals, but this behavior functions in direct contact to excite group activity. Time-lapse videography used to analyze cycloalexic group formation showed that larvae transition from feeding in a line along the leaf margin to cycloalexic formations on the upper leaf surface via a coordinated back-up movement that brings the posterior tip of their abdomens into contact. We identify three phases of cycloalexic formation: line-up, back-up, and an adjustment phase. Complete assembly can be achieved in as little as 5 s, but often the two phases establishing the basic rosette lasts 5–10 min, while the adjustment phase slowly tightens the group over a period of up to an hour. Collectively these studies present the first documented case of chemical trail marking in a beetle, and provide insight into a remarkable social-behavioral repertoire convergent in key respects with the better-studied social caterpillars and sawflies.