Effect of chronic exposure to neonicotinoid insecticides on metamorphosis of juvenile African clawed frogs (Xenopus laevis)

Abstract/Summary

This study examined effects of neonicotinoid insecticides on the growth and development of juvenile African clawed frogs, Xenopus laevis. Larvae were exposed to thiamethoxam (THX) and clothianidin (CLO), both at 20 ppm and 100 ppm. Biological end points were evaluated throughout two exposure periods. Data were first recorded over 44 days of exposure period 1 during pre-metamorphic development. Incremental total length measurements were taken at days 0, 5, 19, 30 and 44, and instantaneous total length and final wet weight and developmental stage (Nieuwkoop-Faber scale; NF) were determined (n=~400) at day 44. During a second exposure period, tadpole developmental stages were recorded daily on 80 individuals until they reached a baseline developmental stage of NF 66, just prior to tail resorption. Both THX and CLO retarded growth during exposure period 1, with THX significantly reducing total length over time, wet weight and developmental stage. The higher 100 ppm THX treatment significantly slowed the rate of development during exposure period 2, when compared to the NEO-free treatment. Among all treatments, THX induced the significant reduction in tadpole growth and development, whereas the higher treatment of THX (100 ppm) showed the greatest effect on survival (P<0.006). These negative effects of THX and CLO on the whole-organism fitness characteristics are a finding useful for predictive risk assessments.

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