Good housekeeping: why do shelter-dwelling caterpillars fling their frass?

Caterpillars of the silver-spotted skipper butterfly ballistically eject their individual fecal pellets (frass) as far as 40 body lengths away from their resting places in leaf shelters. Why do these and many other shelter-dwelling caterpillars go to such great lengths to distance themselves from their waste?

In a paper to appear in the April issue of Ecology Letters, M. Weiss shows that predatory wasps are attracted to skipper frass placed in leaf shelters, and that caterpillars in proximity to frass are particularly vulnerable to wasp predation. Caterpillars reared in close contact with accumulations of their own waste do not suffer ill effects; nor is repeated shelter-building, another possible consequence of frass accumulation, energetically expensive. Thus selection by natural enemies is likely to have been a driving force behind the evolution of frass ejection behavior in skipper caterpillars.

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