Invasive plants severely impact habitats for more than 80% of at-risk butterflies in the United States. Controlling these invasive species is a high priority for land managers. The use of taxa-specific herbicides is a promising management technique, yet effects on non-target butterflies are virtually unknown. In Pacific Northwest Prairies, tall perennial grasses are the dominant structure-altering invasive species in many butterfly habitats. We conducted field investigations on two widely-used grass-specific herbicides. We investigated use of fluazifop-p-butyl (Fusilade) in experimental plots with Fender’s blue (Plebejus icarioides fenderi), an endangered butterfly in Willamette Valley, OR and sethoxydim (Poast) with Puget blue butterfly (Plebejus icarioides blackmorei), a Washington-state species of concern in the South Puget Sound Prairies, WA. Experiments with Fender’s blue were conducted at Baskett Slough National Wildlife Refuge, an area highly infested with tall oatgrass (Arrhenatherum elatius); experiments with Puget blue were conducted at Johnson Prairie on Joint Base Lewis-McCord in prairies with low abundance of invasive perennial grass.
Results/Conclusions
For Fender’s blue, herbicide application negatively impacted larval survivorship with egg to post-diapause larval survivorship dropping from 2.8% in control plots to 1.6% in plots treated with fluazifop-p-butyl. However, herbicides reduced tall grasses and butterflies were attracted to sprayed plots. Here fecundity increased from 25 eggs/m2 in control plots to 40 eggs/m2 in treated plots. At Johnson Prairie there were no detected effects of vegetation, but Puget blue females spent significantly less time in sprayed plots relative to controls. Given the necessity of invasive grass control in natural areas with at-risk butterflies, we recommend strategies to minimize herbicide effects and next steps to assess effects of herbicides on at-risk butterflies.