Effects of insecticide formulations used in cotton cultivation in West Africa on the development of flat-backed toad tadpoles (Amietophrynus maculatus)

In the West African savanna zone, traditional subsistence farming increasingly shifts to cash crop farming and in particular to cotton cultivation, which is accompanied by application of pesticides. Increasing use of pesticides by smallholder farmers is suspected to have negative effects on non-target organisms. In this study, possible pesticide impact on the development of tadpoles was investigated. Two insecticide formulations used in cotton cultivation in Benin were compared: Cotofan® (active ingredient (a.i.): α- and β-endosulfan) and Tihan® (a.i.: spirotetramat and flubendiamide). Tadpoles of the widespread species Amietophrynus maculatus were kept in small water basins with increasing insecticide concentrations (1, 10 and 100 μg a.i./L) over a period of 28 days. Tadpoles showed reduced survival at the highest endosulfan concentrations (100 μg/L). Survival of tadpoles undergoing metamorphosis was not influenced by Tihan®. Endosulfan in concentrations of 10 and 100 μg/L negatively impacted the movement of the tadpoles which was not the case for the mixture of spirotetramat and flubendiamide. Time to metamorphosis was not significantly different in the various treatments. Tail length of tadpoles was significantly shorter in Cotofan® treatments compared to controls. Pesticide residues in the tadpoles were relatively low after keeping them in concentrations of 1 and 10 μg a.i./L (25 and 26 μg/kg wet weight (w/w) for the sum of α-endosulfan, β-endosulfan and endosulfan sulphate and below the detection limits for flubendiamide and spirotetramat). For the 100 μg a.i./L treatments, 1,600 μg/kg w/w was found of α-endosulfan, β-endosulfan and endosulfan sulphate together in the survived tadpoles and 21 μg/kg w/w of flubendiamide.