Invitation by vibration: recruitment to feeding shelters in social caterpillars

Sociality is widespread in caterpillars, but the communication mechanisms used for group formation and cohesion are poorly understood. Here, we present the first evidence that caterpillars produce complex vibratory signals to advertise food and shelter sites to conspecifics. We first tested the hypothesis that early instars of the masked birch caterpillar (Drepana arcuata) actively form groups. Larvae placed alone on different leaves of a birch twig began assembling within minutes and forming groups of 2–6 at a median time of 2 h. In Y-choice experiments, larvae joined arms occupied by conspecifics significantly more frequently than unoccupied arms. To test the hypothesis that group formation is vibration-mediated, signals were monitored in solitary residents of silk leaf shelters before and during natural recruitment events. Four distinct signal types were recorded: anal scraping, mandible drumming, mandible scraping, and buzz scraping. Anal scraping and buzz scraping were the most common in residents prior to being approached, and these signals were strongly correlated to feeding and laying silk. Signaling occurred in 100% of residents, and higher signal rates resulted in significantly faster recruitment times. As a recruit approached a resident, complex signaling interactions occurred, which may communicate information about resource quality or location. We conclude that caterpillars, similar to other social animals, use acoustic communication to advertise resources. The vibratory signaling repertoire of these tiny caterpillars exhibits a complexity rivaling that of eusocial insects. Further investigations of vibroacoustic communication are essential to fully appreciate the intricacies of social interactions in caterpillars and other juvenile insects. SIGNIFICANCE STATEMENT: Group living provides many survival benefits to juvenile insects such as caterpillars, but little is known about the communication signals mediating social interactions such as group formation. Our study shows that caterpillars use vibration signals to “invite” conspecifics to social gatherings. Pinhead-sized early-instar caterpillars (Drepana arcuata) are capable of locating conspecifics on birch leaves to form small groups. But how do they accomplish this? We report that individual resident caterpillars established in a silk shelter produced complex vibrations by dragging their anal segments, scraping and drumming their mouthparts, and tremulating their bodies to advertise a feeding spot and shelter. These results provide the first evidence that caterpillars use vibratory signaling to form social groups, providing insight into the poorly understood role of vibratory communication in juvenile insects.