TRAIL MARKING BY THE LARVA OF THE MADRONE BUTTERFLY EUCHEIRA-SOCIALISAND THE ROLE OF THE TRAIL PHEROMONE IN COMMUNAL FORAGING BEHAVIOR

The larva of the Madrone butterfly Eucheira socialis (Lepidoptera: Pie ridae) secretes a trail pheromone from the ventral surface of the post erior rip of its abdomen. Caterpillars mark trails by bringing the sec retory site into brief contact with the substrate during a locomotive cycle. Foragers mark most heavily when they move onto new branches and little, if at all, when they move over established trails or when the y return to the communal shelter after feeding. The caterpillars make careful comparisons of alternative pathways at choice points and selec t newer and stronger trails over older and weaker trails. Differential marking of new and established trails during nightly forays, coupled with sensory discrimination of trails by strength and age, leads colon ies to abandon old trails in favor of new trails. When applied at a ra te as low as 2.5 x 10(-10) g/mm, caterpillars followed synthetic trail s prepared from 5 beta-cholestane-3-one, a trail pheromone previously reported from the tent caterpillars (Malacosoma spp.). Although both E ucheira and Malacosoma mark with the tip of the abdomen and have near- identical sensitivites To 5 beta-cholestalze-3-one, our study shows th at Eucheira employs a relatively unsophisticated system of trail-based communication and does not recruit to food. The trail-based communica tion system of Eucheira appears to represent an early stage in the evo lution of cooperative foraging that is derived from, and motivationall y linked to, conflict behavior.