Restoring habitat for a threatened butterfly: The role of larval behavior

The Oregon silverspot butterfly, Speyeria zerene hippolyta, is a threatened subspecies with 4 extant populations on the Pacific Northwest coast. One of these populations is found on the Nature Conservancy’s Cascade Head preserve. At this site, numbers of the butterfly’s larval food plant, Viola adunca, have been declining, apparently because of competition with introduced perennial grasses. Although prescribed burning is necessary for restoring butterfly habitat, as it stimulates V. adunca seed germination six-fold, it appears not to be sufficient to reverse V. adunca declines, because of low seed bank densities. Transplanting adult V. adunca may be an important complement to prescribed burning. Studies of the foraging behavior of larval S. z. hippolyta can provide information crucial for setting target densities for transplanting V. adunca. In preliminary field studies of larvae in a range of V. adunca densities, larvae appear to move randomly; there are no differences in movement speed or turning angle among larvae placed different distances from food plants. These results have implications for the design of a program of habitat restoration at Cascade Head and elsewhere.