Laboratory evaluation of environmental risk assessment of pesticides for mosquito control: toxicity of dimilin on a larvivorous fish, Gambusia affinis.

Mosquitoes are medically and veterinary important vectors. They are generally controlled by conventional insecticides which possess strong secondary effects on the environment. The insect growth regulators such as chitin synthesis inhibitors seem promising because of their specific mode of action on insect pests. Dimilin (25% WP), the trade name of diflubenzuron (DFB), is a chitin synthesis inhibitor belonging to the class of benzoylphenylurea. It was found a potent insecticide against mosquito larvae. In this context, there is a need for tools to monitor the toxicity risk of these pesticides to environment and natural predators. Therefore, the aims of the present study was to investigate the probable toxicity of this insecticide on morphmetric measurments, and to measure acetylcholinesterase (AChE) and catalase (CAT) activities in adult females of a larvivorous fish, Gambusia affinis (Cyprinodontiformes, Poeciliidae) extensively used in biological control of mosquitoes. The compound was added in rearing water at two concentrations corresponding to LC50 and LC90 previously determined against the fourth instar larvae of Culex pipiens (Diptera, Culicidae). The adult females were exposed in a continuous treatment for 28 days and morphometric measurements (length, body weight, hepatosomatic index and condition factor), and environmental stress biomarkers (AChE and CAT), respectively determined at different exposure times (0, 7, 14, 21 and 28 days). The results showed that the compound had no significant (P>0.05) effects on growth, hepato-somatic, gonadal-somatic indexes and condition factor. In addition, this compound had no significant (P>0.05) effect on AChE activity during the experimental period. Furthermore, treatment affected significantly (p<0.05) the CAT activities starting day 21 of exposure. The observed effects varied as function the exposure time and the concentration. Conclusively, the overall data obtained suggested that G. affinis is a suitable biological model which can be used in toxicological and biomonitoring studies.