Understanding caterpillar movement to guide habitat restoration for the Oregon Silverspot Butterfly, Speyeria zerene hippolyta

Background/Question/Methods

Integrating concepts from landscape ecology and animal behavior can produce useful guidance for management of sensitive species. We illustrate this notion with a study of the Oregon Silverspot (Speyeria zerene hippolyta), a federally threatened butterfly subspecies restricted to 4-5 populations along the coast of Oregon and northern California. 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. Despite considerable management efforts, including captive rearing and controlled burning of its grassland habitat, Silverspot recovery remains elusive. So far, managers have not had a specific target for appropriate host plant densities. We used a combination of a greenhouse experiment, field observations of larval foraging behavior, and a simulation model to estimate what host plant density is required to support a Silverspot population.

Results/Conclusions

The greenhouse experiment determined that larval survival increases with host plant density, and that at low host plant densities larvae are limited by their ability to find hosts, rather than by actual host plant availability. In field conditions, 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 larvae to have a reasonable chance of survival to pupation. This study was conducted at The Nature Conservancy’s Cascade Head Preserve, where current densities rarely exceed 1 plant/m². TNC staff are using these findings to guide a violet restoration program for this sensitive species.