Non-target effects of herbicides on the Zerene silverspot butterfly, a surrogate subspecies for the threatened Oregon silverspot butterfly.
Herbicides are used as management tools to improve habitat for native plants and animals, but their application may also have harmful effects on the native community. The federally threatened Oregon silverspot butterfly (Speyeria=Argynnis zerene hippolyta) resides in remnant native grasslands along the Pacific Northwest coast. However, like many grasslands, many of these areas have high incidences of invasive plants, such as false dandelion (Hypochaeris radicata) and velvet grass (Holcus lanatus). These and other invasive plants severely limit the abundance of the Oregon silverspot's larval host plant, the early blue violet (Viola adunca). Selective herbicides, such as clopyralid and fluazifop-P-butyl, can reduce invasive plant abundance. However, non-target effects of these herbicides, and of adjuvants applied with these herbicides, on Oregon silverspots are unknown. In our study, we applied herbicides and adjuvants to host plants and Zerene silverspot (S. z. zerene) larvae, a subspecies closely related to Oregon silverspots. Responses in silverspot larvae measured in two experiments included survival, sex ratio, development time, mass, morphology, fecundity, and behavior. Our results suggest that negative effects of herbicides, clopyralid and fluazifop-P-butyl, and adjuvants, Agri-Dex® and Nu-Film®-IR, are limited. However, we detected weak effects from clopyralid and fluazifop-P-butyl with and without Agri-Dex® on larval and pupal development time and pupal mass.