Assessment of Sublethal Effects of Neonicotinoid Insecticides on the Life-History Traits of 2 Frog Species.

Neonicotinoid insecticides are used extensively in agriculture and, as a consequence, are now detectable in nearby aquatic environments. Few studies have evaluated the effects of neonicotinoids on amphibians in these aquatic environments. In the present study, we examined the effects of 2 commercial formulations of neonicotinoids (active ingredients clothianidin and thiamethoxam) on survival and life-history traits of wood frogs (Lithobates sylvaticus) and northern leopard frogs (Lithobates pipiens). We used artificial pond mesocosms to assess the effects of these neonicotinoids, at nominal concentrations of 2.5 and 250 µg/L, on amphibian larval development through metamorphosis. We found no differences between controls and neonicotinoid exposure for any of the endpoints assessed for either wood frogs or leopard frogs. The present study suggests that concentrations meeting or exceeding observed levels of clothianidin and thiamethoxam in surface waters will not directly affect metamorphosis in 2 amphibians. Environ Toxicol Chem 2019;38:1967-1977. © 2019 SETAC.

Macrophytes are highly sensitive to the herbicide diquat dibromide in test systems of varying complexity.

The herbicide diquat dibromide is used in North America to manage nuisance macrophytes. However, its effect on native macrophytes is less clear and it could cause indirect effects on other aquatic biota. This study determined the sensitivity of both native and non-native macrophytes grown in test systems with varying complexity to diquat dibromide applied directly to water following label directions. In an outdoor mesocosm experiment and single species greenhouse concentration-response tests, Elodea canadensis Michx., Myriophyllum spicatum L., Ceratophyllum demersum L. and Hydrocharis morsus-ranae L. were exposed to a range of diquat dibromide concentrations (4.7 - 1153 µg/L), corresponding to 0.4 - 100% of the recommended label rate of the formulated product. The mesocosm experiment contained all four plant taxa in the same system along with caged amphipods (Hyalella azteca Saus.), tadpoles (Lithobates pipiens Schreber), phytoplankton and periphyton; however, this study focuses on the macrophytes only. In both test systems, severe direct effects of diquat dibromide on macrophytes were detected, with almost 100% mortality of all macrophytes in both test systems at 74 µg/L. The most sensitive species in the single species tests, E. canadensis, showed almost 100% mortality at concentrations below the HPLC-based method detection limit of 5 µg/L. Effects occurred very rapidly and showed no difference in severity between native and non-native macrophytes or complexity of test systems. These results suggest that diquat dibromide could be applied at a considerably lower label rate, depending on the characteristics of the waterbody, while still achieving effective control of nuisance macrophytes.