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Unique Methods Used to Understand Frog Exposure to Pesticides in Agricultural Settings

Close up of a Northern Leopard frog - Lithobates pipiens - in a wetland
Northern Leopard frog (Lithobates pipiens) in a wetland in Worth County, Iowa. Scientists track 72 northern leopard frogs in two wetlands in an agricultural setting in Iowa for insights into where and when individual adult frogs are likely exposed to pesticides. Photo Credit: Clay L. Pierce, USGS.

Adult frog exposure to pesticides in aquatic and terrestrial habitats was quantified using a novel combination of radio telemetry and passive sampling techniques to better understand factors affecting frog health and survival in agricultural landscapes.

Pesticides are one of multiple physical, chemical, and microbial stressors that have been reported to contribute to the decline of frog populations. Pesticides accumulate in frog tissues and have been associated with mortality and a range of sub-lethal effects including immune suppression, and reproductive changes. A clear understanding of the role that pesticides have relative to other stressors on frog health and survival is currently limited partly because the timing and magnitude of pesticide exposure is not well defined.

Many frog species have complex life cycles and use aquatic and terrestrial habitats during different periods of their life. Their exposure to pesticides may fluctuate depending on habitat use, and their permeable skin increases the likelihood of pesticide accumulation directly from their surrounding habitat. Previous U.S. Geological Survey (USGS) research has focused on exposure in aquatic habitats early in the breeding season, leaving questions about the full range of exposures as they move from aquatic to terrestrial habitats.

Scientists from the USGS Amphibian Research and Monitoring Program, the USGS Toxic Substances Hydrology Program, and Iowa State University collaborated on a unique field study designed to track 72 northern leopard frogs (Lithobates pipiens) in two wetlands in an agricultural setting in Iowa for insights into where and when individual adult frogs are likely exposed to pesticides.

Scientist deploying a sampler near a corn field
Scientist deploying a silicone passive sampler near a corn field in Cerro Gordo County, Iowa. The science team used the passive samplers to quantify pesticide exposure in wetland, grassland, and agricultural habitats. Photo Credit: Clay L. Pierce, USGS.

The most unique aspect of this study is the combination of traditional and novel methods used to help determine if habitat choice by adult frogs has implications for pesticide exposure and accumulation in their tissues. Scientists combined radio telemetry to track frog movement to determine habitat use; novel silicone passive samplers to quantify pesticide exposure in wetland, grassland, and agricultural habitats; and tissue analyses to measure pesticide accumulation in adult frogs.

The results of this research indicate that habitat and time of year affect exposure and accumulation of pesticides in adult frogs and support the hypothesis that adult frogs are exposed to pesticides in both the aquatic and terrestrial environment.

Habitat use changed throughout the summer, but telemetry confirmed that frogs predominantly utilized surrounding grasslands (58 percent) and wetlands (34 percent) and occasionally were found in agricultural fields (less than 10 percent).

Pesticides were detected in the silicone passive samplers placed in grasslands, wetlands and agricultural areas sampled and during all sampling periods including the winter. Pesticide concentrations were greatest in agricultural settings followed by wetlands and then grasslands—all habitats that are used by the frogs to varying degrees.

Pesticide concentrations in frog tissues were greater in May when frogs were utilizing wetlands for breeding compared to August when they were occupying more terrestrial habitats. These results indicate that frogs more readily accumulate pesticides from wetland habitats during breeding season or in the winter during hibernation. Northern leopard frogs hibernate in wetland bottom sediments during the winter and no studies to date have assessed pesticide exposure during hibernation.

The results of this research can inform conservation strategies by providing information about when and where the frogs are most likely exposed to pesticides. The unique approach that the scientists used in this study can be utilized in other settings with other organisms and contaminants to better understand if contaminant exposure affects growth, development, fitness, and survival.

The USGS Toxic Substances Hydrology Program funded this study, as well as the USGS Amphibian Research and Monitoring Program.

References

Swanson, J.E., Muths, E., Pierce, C.L., Dinsmore, S.J., Vandever, M.W., Hladik, M.L., and Smalling, K.L., 2018, Exploring the amphibian exposome in an agricultural landscape using telemetry and passive sampling: Scientific Reports, v. 8, no. 1, p. 10045, doi:10.1038/s41598-018-28132-3.

Swanson, J.E., Muths, E., Pierce, C.L., Vandever, M.W., and Smalling, K.L., 2018, Amphibian occupancy and effects of habitat use on pesticide exposure in Iowa wetlands: U.S. Geological Survey Data Release.

Smalling, K.L., Reeves, R., Muths, E., Vandever, M., Battaglin, W.A., Hladik, M.L., and Pierce, C.L., 2015, Pesticide concentrations in frog tissue and wetland habitats in a landscape dominated by agriculture: Science of the Total Environment, v. 502, p. 80-90, doi:10.1016/j.scitotenv.2014.08.114.

Battaglin, W.A., Smalling, K.L., Anderson, C., Calhoun, D., Chestnut, T., and Muths, E., 2016, Potential interactions among disease, pesticides, water quality and adjacent land cover in amphibian habitats in the United States: Science of the Total Environment, v. 566-567, p. 320-332, doi:10.1016/j.scitotenv.2016.05.062.

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