Biological effects of glyphosate and emamectin benzoate based pesticides on the freshwater prawn Macrobrachium macrobrachion juveniles

Emamectin benzoate based insecticide and glyphosate based herbicides are widely used in developing countries for agricultural purposes. The problem lies in the total lack of knowledge of data on their uses as well as the absence of scientific data on the potential effects on local biodiversity. This study evaluates the acute and chronic effects of two commercial formulations EMACOT^® and SUNPHOSATE-G^® on juveniles of a prawn species (Macrobrachium macrobrachion) native of large watersheds in Benin. Juveniles with an average weight of 1.49 ± 0.2 g were subjected to six and seven lethal doses of EMACOT^® and SUNPHOSAT-G^® respectively. Thus, swimming behavior, loss of mobility and mortality were monitored every hour for 72 h. Then, sub-lethal effects on molting effectiveness, weight and size gain as well as survival were evaluated on juveniles individually exposed to nominal concentrations of 0.06 and 0.11 mg.L^-1 of EMACOT, and 13.99 and 27.98 mg.L^-1 of SUNPHOSATHE-G. The results showed that the LC₅₀-72 h of the herbicide formulation on juveniles is 279.76 mg.L^-1 which corresponds to 211.78 mg.L^-1 of glyphosate. That of the insecticide is 1.14 mg.L^-1 corresponding to 0.06 mg.L^-1of emamectin benzoate. As for the chronic effects, both concentrations of emamectin benzoate prevented the prawn from molting (1 molt at the start of the experiment against 3 molts for the control subjects in 28 days). As a result, weight gain in these two treatments was very low (0.06 and 0.19 g, respectively) compared to 0.66 g in the controls. For glyphosate, findings with 13.99 mg.L^-1 are similar to those of both doses of emamecin benzoate. On the other hand, no significant effect was observed on the effectiveness of molts within 27.98 mg.L^-1 treatment (3 molts in 28 days). The weight gain was 0.11 g and 0.46 g respectively for both glyphosate sub-lethal doses. The results of this study indicate a significant sensitivity of the prawn juveniles to emamectin benzoate (EMACOT) and a relatively low sensitivity to glyphosate (SUNPHOSATE-G). But the risk is significant when these two pesticides are used in the flood plains of large rivers. In developing countries such as Benin, measures must be taken to review current pesticide use practices in large watersheds.