Non-target effects of grass-specific herbicides differ among species, chemicals and host plants in Euphydryas butterflies

Habitat modification by invasive species places numerous butterfly species at risk of extinction. Grass-specific herbicides, graminicides, are a sought-after tool to reduce invasive grasses, augment forbs and enhance butterfly populations. However, possible non-target effects raise concern. We investigated non-target effects of graminicides on three species of caterpillars in the genus Euphydryas (Nymphalidae), a taxon with numerous species in decline worldwide. In one experiment, we compared the effects of three graminicides (clethodim, sethoxydim and fluazifop-p-butyl) on E. colon . In a second experiment, we assessed the effects of fluazifop-p-butyl, the most commonly used graminicide in prairies in the Northwest USA, on three Euphydryas species ( E. colon, E. editha and E. phaeton ), each raised on two different hostplants. In the first experiment, fluazifop-p-butyl did not affect survivorship of pre-diapause larvae, sethoxydim reduced survivorship by 20 % relative to controls, and effects of clethodim were inconclusive. Graminicides did not change the total concentration of iridoid glycosides in the caterpillars, but all three graminicides increased the concentration of aucubin to almost double the level in control treatments. In the second experiment, the effects of fluazifop-b-butyl were not consistent across butterfly and host plant species. However, fluazifop-b-butyl reduced group size of gregarious pre-diapause larvae under all conditions. Our results suggest that if managers use graminicides over the short-term to control invasive grasses, fluazifop-p-butyl has the most promise for minimal effects. However, efforts should be paired with demographic and behavioral monitoring to quantify context-dependent impacts.