Roles for structural and temporal shelter-changing by fern-feeding lepidopteran larvae

Larvae of the pyralid moth, Herpetogramma aeglealis, construct feeding shelters upon the Christmas fern (Polystichum acrostichoides). Field and laboratory study involving 532 shelters showed that as the larvae mature, they sequentially inhabit approximately 5 shelters of 3 distinct types, constructed at night on different fronds of the same plant. The bundle shelter, simple and ephemeral, is first to be inhabited and constructed. The fiddlehead shelter which houses slightly older larvae strongly resembles contemporaneously emerging frond fiddleheads. The final shelter form, the globe, is a silk-bound ball of leaflets at the frond tip. An individual larva usually constructs 3 globe shelters on different fronds of the same plant before completing its development. As shelter sites, sterile Polystichum fronds are chosen preferentially over fertile fronds. The bundle and fiddlehead shelter forms, less abundant, appear cryptic to humans and perhaps to other vertebrates. The final globe shelter form is larger and quite conspicuous. However, the persistence of empty globe shelters left on the plant as the larva moves to a new one may serve to make searching for larvae less profitable for potential predators and parasites. We suggest that the energetic costs of constructing and occupying multiple shelters may be offset by circumvention of reduced frond palatability and reduced exposure to predators and parasites.