Sub-lethal and lethal effects of a neonicotinoid pesticide on the development of Northern leopard frog tadpoles.

Well-known global declines in amphibian populations have sparked decades of studies into potential causes (Stuart et al. 2004). Pesticides are a suspected contributor to declining populations (Bruhl et al. 2013). Imidacloprid is the most widely used insecticide in the world, but few studies have considered its potential effects on anurans. I conducted a static-renewal experiment to monitor the lethal and sub-lethal, developmental effects in Northern leopard frog tadpoles exposed to three concentration levels (250 ng/L, 8.5 mg/L, and 85 mg/L) of imidacloprid in a laboratory setting. Survivorship was 0% by day 23 of exposure to imidacloprid at the previously lowest known LC50 value for frogs of 85 mg/L. This served as the high concentration level in this study. Tadpoles exposed to imidacloprid had reduced length at metamorphosis compared with the control group (one-way ANOVA, p<0.001). Imidacloprid exposure concentration was inversely related to the rate of development of tadpoles (Somers’ d, p=0.009). Imidacloprid concentration level was positively associated with frequency of nuclear abnormalities. Exposure to imidacloprid may cause sub-lethal effects. Tadpoles exposed to 250 ng/L imidacloprid (a concentration found in a Canadian wetland area) exhibited sub-lethal effects (e.g. binucleated, blebbed, lobed and notched nuclei), suggesting that these effects may be observed in the environment with wild populations of frogs. More research is necessary to understand the lethal and sub-lethal effects of imidacloprid on Northern leopard frog tadpoles, but these results offer a basis for further research.