The Oregon silverspot is a federally threatened butterfly that occupies early successional grasslands supporting its larval host plant, early blue violet (Viola adunca). Habitat loss has reduced the species to five populations, the southernmost of which occurs in Del Norte County, California, near the coastal lagoon Lake Earl and in Tolowa Dunes State Park. To aid in the recovery of the species, the Institute for Wildlife Studies is working with the U.S. Fish and Wildlife Service (USFWS) to better understand the relationship between the butterfly and its habitat in Del Norte County.
One concern for the Del Norte population is that the low-lying areas near Lake Earl have historically supported high densities of violets, but are subject to winter flooding. Thus, it is feared that female silverspots may be attracted to lay their eggs (oviposit) in areas that put their larvae at increased mortality risk. To better understand this risk, we conducted a study to determine how habitat characteristics influence where female butterflies lay their eggs. Land elevation, violet density, and other vegetation characteristics were measured on 1-meter plots centered on locations where oviposition-associated behavior was observed, and compared with data from randomly located plots. We found that high violet density was the dominant factor influencing the Oregon silverspot’s choice of oviposition site. However, the highest densities of violets occurred at elevations that exceeded the level at which the lagoon typically floods, probably because V. adunca cannot compete with the dense stands of slough sedge (Carex obnupta) that dominate the lower elevations. Thus, the risk of inundation to oviposition sites near Lake Earl is low as long as the lagoon is managed at the current level.
IWS is also developing future projects with USFWS to map butterfly habitat and to understand the phenology and quality of nectar resources for the Oregon silverspot.