Applying captive ecotoxicological studies using surrogates to managing at-risk species in the field

Background/Question/Methods

Several Pacific Northwest prairie butterflies are considered at-risk of extinction. Invasive plants are considered one of the greatest threats to the persistence of many of these butterfly species. Herbicides are widely employed to manage invasives due to their potential high degree of selectivity, low cost, and efficacy at large spatial scales.  However, the impacts of herbicides on non-target species, such as butterflies, are not well understood. At the same time, it would be logistically impossible to study the impacts of every potential herbicide/surfactant combination on every at-risk non-target species. Using captive populations, we investigated the impacts of two grass-specific herbicides, fluazifop-p-butyl and sethoxydim, and the surfactants, Nufilm and Preference, on the survivorship, developmental time, morphology, and fecundity of a forb feeding butterfly, the Cabbage White (Pieris rapae), and a grass feeding butterfly, Ochre Ringlet (Coenonympha tullia). Using a transparent decision analysis approach, with our captive study results, we developed recommendations on the selection and use of grass-specific herbicides and surfactants in Pacific Northwest prairies. We compared decision analyses results for prairies occupied by Fender’s Blue butterfly (Icaricia icarioides fenderi), a federally endangered lupine (Lupinus spp.) feeding butterfly, and Mardon Skipper (Polites mardon), a state endangered fescue (Festuca spp.) feeding butterfly.

Results/Conclusions

We found that impacts on survivorship varied by development stage, with early instar Cabbage Whites seemingly unaffected by exposure to herbicides while late instar Cabbage Whites exposed to sethoxydim and the surfactant Nufilm demonstrating an 80% higher than expected mortality. Survivorship differences in late instar Ochre Ringlet larvae were primarily driven by surfactant types with those exposed to fluazifop-b-buytl and the surfactant Preference experiencing 60% higher mortality rates than expected whereas those exposed to sethoxydim or fluazifop-p-butyl with the surfactant Nufilm experienced 28% and 25% higher than expected mortality rates respectively. Responses in other monitored variables were not as extreme but did vary by life stage, species, and herbicide/surfactant combination. For example, Ochre Ringlet larvae exposed to herbicides generally had lower egg hatchability whereas this was not true for Cabbage Whites. By incorporating our greenhouse results into a decision analysis we demonstrated that site specific management objectives, such as managing for a particular butterfly species, can result in differing herbicide/surfactant use recommendations even when managing for the same invasive species. Additionally, we demonstrate the utility of transparent decision analyses approaches that can readily incorporate scientific data and site specific objectives and can be used to inform adaptive management strategies.