Modeling caterpillar movement to guide habitat enhancement for Speyeria zerene hippolyta, the Oregon silverspot butterfly

Abstract

Studies of at-risk species are likely to have greater conservation impact if they: involve managers in question formulation, set specific management or restoration goals, and are based on detailed knowledge of species’ resource needs. These ideas guided our investigation of the foraging behavior of larvae of a US federally threatened butterfly, the Oregon silverspot (Speyeria zerene hippolyta), with the goal of making management recommendations for habitat restoration. S. z. hippolyta larvae feed exclusively on Viola adunca and must consume multiple individuals to pupate successfully. Larvae forage at random through meadow vegetation to locate host plants. Observations of larvae foraging under field conditions revealed that larger, older larvae move more rapidly and turn less acutely than smaller, younger larvae; the consequence of these developmental differences is that younger larvae tend to remain in one place while older larvae tend to range more widely, presumably in search of new host plants. Results from a simulation model initialized with these data suggested that a host plant density of at least four V. adunca plants/m2 (depending on predation intensity to which larvae are exposed) is required in order for 4th instar larvae to have a 10 % chance of survival to pupation. These findings are being used to guide a violet restoration program for this sensitive species.