Effects of grass-specific herbicides on butterflies: an experimental investigation to advance conservation efforts

Encroachment by invasive plants is a leading threat to rare butterflies. Restoration plans increasingly recommend herbicides to control invasive plants within butterfly habitats. Few studies address the effects of these herbicides on at-risk butterflies. The effects of two graminicides (fluazifop-p-butyl and sethoxydim) and a surfactant (Preference(A (R))) were evaluated on Icaricia icarioides blackmorei and Pieris rapae. The effects on butterfly larvae were assessed by mimicking recommended timing and mixture rates of field applications. Differences in survival to adult eclosure, development time, biomass, sex ratio and adult morphology were assessed. Survival of P. rapae was reduced by 32% with sethoxydim and 21% with fluazifop-p-butyl. Wing size and pupal weights of P. rapae were reduced by herbicide treatments. Icaricia icarioides blackmorei experienced a 21% reduction in development time from the date of treatment to eclosure. These results highlight the importance of careful consideration in the use of herbicides in habitats harboring at-risk butterfly populations.