Caterpillar talk: acoustically mediated territoriality in larval Lepidoptera

We provide evidence for conspecific acoustic communication in caterpillars. Larvae of the common hook-tip moth, Drepana arcuata (Drepanoidea), defend silk nest sites from conspecifics by using ritualized acoustic displays. Sounds are produced by drumming the mandibles and scraping the mandibles and specialized anal “oars” against the leaf surface. Staged interactions between a resident and intruder resulted in escalated acoustic “duels” that were typically resolved within minutes, but sometimes extended for several hours. Resident caterpillars generally won territorial disputes, regardless of whether they had built the nest, but relatively large intruders occasionally displaced residents from their nests. All evidence is consistent with acoustic signaling serving a territorial function. As with many vertebrates, ritualized signaling appears to allow contestants to resolve contests without physical harm. Comparative evidence indicates that larval acoustic signaling may be widespread throughout the Lepidoptera, meriting consideration as a principal mode of communication for this important group of insects.

Caterpillars are key constituents of most terrestrial ecosystems and include some of the most serious pests of crops and forests. Their ecological and economic importance has promoted extensive research on how they interact with heterospecifics, including host-plants, predators, and parasitoids (1, 2). Conspecific communication among caterpillars has received little attention, however, despite many species living gregariously, where interactions involving group defense, aggregation, competition, and foraging may be crucial for survival (3, 4). The extent to which caterpillars communicate with one another, the modalities used, and the function of any communication all remain largely unexplored (3). Pheromones and tactile cues have been described for a few species (3, 5), and vision is unlikely to be an important modality for communication, because of the simplistic nature of the larval optical system (6). Acoustic signaling has not been investigated rigorously, despite its obvious potential for substrate-bound organisms like caterpillars.

Drepana arcuata Walker is locally common throughout deciduous woodlands of northeastern North America, feeding primarily on birch (Betula) and alder (Alnus) (7). Despite the common occurrence of this species, little is known of its larval behavior and life history. Sounds produced by modified anal structures had been noted long ago for Drepana (8, 9), but the functional significance of these sounds had not been investigated. On discovering that D. arcuata larvae produced a combination of three distinct sounds, we investigated the nature of these signals and the context in which they occurred, and tested the hypotheses that D. arcuata are territorial and that the acoustic signals serve in territoriality. If Drepana larvae are territorial, then: (i) residents should maintain exclusive use of nests, (ii) residents should defend nests against conspecifics, (iii) intruders should only rarely displace residents, and (iv) intruders experimentally given vacant nests should defend those nests against original owners (10, 11). If acoustic displays signal territory ownership, then displays will be: (i) restricted to the territory and (ii) given only in response to intruding conspecifics.