Estimation of pesticide mixture interaction in Nile tilapia (Oreochromis niloticus) using survival analysis
The acute toxicity of the pesticides atrazine, mancozeb, chlorpyrifos and lambda-cyhalothrin, acting singly and jointly, was assessed on Nile tilapia (Oreochromis niloticus) fingerlings. Median lethal concentration (LC50), median lethal time (LT50), and mixture interaction were estimated, whereas survival analysis was used to model time-todeath. The most toxic single and joint mixture was lambda-cyhalothrin and chlorpyrifos-lambda cyhalothrin, respectively. The risk of death (RoD) of fingerlings exposed to 9.22 mg l−1 atrazine-mancozeb mixture was 1.76 times higher than fingerlings exposed to 9.0 mg l−1 atrazine (p > 0.05). However, RoD of fingerlings exposed to 9.95 mg l−1 atrazine-chlorpyrifos was 5.59 times higher than fingerlings exposed to 9.0 mg l−1 atrazine (p < 0.01). The risk of death of fingerlings exposed to 20.8 mg l−1 atrazine-lambda cyhalothrin was 2.81 times higher than 21.0 mg l−1 atrazine. The toxicity of 2.3 mg l−1 mancozeb-chlorpyrifos was 254.25 higher than 2.2 mg l−1 mancozeb (p < 0.01). Fingerlings exposed to 4.33 mg l−1 mancozeb-lambda cyhalothrin and 0.177 mg l−1 chlorpyrifos-lambda cyhalothrin mixture were 0.02 and 0.14 times less likely to die than those exposed to 4.8 mg l−1 mancozeb and 0.17 mg l−1 chlorpyrifos, respectively (p < 0.01). Atrazine-mancozeb, atrazine-chlorpyrifos, atrazine-lambda cyhalothrin, and mancozeb-chlorpyrifos interaction were synergistic, and their relative risk was >1. Both mancozeb-lambda cyhalothrin and lambda-cyhalothrin-chlorpyrifos mixtures were antagonistic, and their relative risk was less than 1. Survival analysis can show interaction in complex pesticide mixtures.