Group Defense against Predators and Parasitoids

Dilution Effects - Aggregation allows caterpillars to collectively defend themselves against the attacks of predators and parasitoids .  Such defense may be passive or active.  Passive modes of collective defense involve dilution effects since the mathematical probability that any one individual will be randomly singled out by a predator decreases with group size.  Individuals also gain protection from predators by surrounding themselves with others.

Communal Shelters - Shelters collectively-built by caterpillars play an important role in antipredator defense.  The tough silk shell of the nest formed by a colony of E. socialis caterpillars is virtually impregnable to both birds and invertebrate predators.  Like many other shelter builders, the resident caterpillars venture from the nest only under the cover of darkness when birds and predatory wasps are inactive.  Bound-leaf shelters, and the more loosely spun shelters of the tent caterpillars and the fall webworm, Hyphantria cunea, cannot exclude predators completely but all are likely to be  deterred to some degree, providing the residents with greater protection than they would enjoy resting in the open.

Aposematic displays - It is generally thought that aposematic coloration is most effective in deterring predators when insects group together.  Indeed, the most common mode of active defense among social caterpillars is aposematic display, often combined with synchronous body rearing, flicking, and “en masse” regurgitation of toxic or unpalatable chemicals.   Studies indicate that the spread of alarm through colonies of social caterpillars is mediated largely by tactile and, possibly, visual cues.  Caterpillars can detect the airborne sounds generated by the beating wings of flying wasps and flies and respond with rapidly jerking movements.   Vibrational signals set up by the agitated caterpillars and propagated by the communal web would appear the most likely means of alerting the colony to danger.