Environmental risks of two chitin synthesis inhibitors on Gambusia affinis: Chronic effects on growth and recovery of biological responses

Abstract

To reduce the impact of conventional insecticides, new more selective chemicals need to be developed. Insect growth regulators such as chitin synthesis inhibitors seem promising because of their specific mode of action on insect and their lower toxicity against non-target organisms than conventional insecticides. Dimilin (25% WP) and andalin (25% EC), the respective trade names of diflubenzuron (DFB) and flucycloxuron (FCX), are two chitin synthesis inhibitors belonging to the class of benzoylphenylurea. In the present study, the toxicity of these insecticides was evaluated on growth and metric indexes in adult females of a larvivorous fish, Gambusia affinis (Cyprinodontiformes, Poeciliidae), extensively used in biological control of mosquitoes. In addition, the compounds were tested on amount of glutathione (GSH) and on glutathione S-transferase activity (GST). The exposure regime included an insecticide exposure period (28days) and a recovery period in untreated water (8days). The compounds were tested separately. Each compound was added in rearing water at two concentrations corresponding to LC50 and LC90 against fourth instar larvae of Culex pipiens (Diptera, Culicidae). The adult females were exposed in a continuous treatment for 28days. The growth, metric indexes, GSH and GST activities were determined at different exposure times (0, 7, 14, 21 and 28days). The results showed that DFB and FCX had no significant (p>0.05) effects on growth, hepato-somatic (HSI) and gonadal-somatic (GSI) indexes, while only FCX at LC90 reduced significantly (p<0.05) the condition factor starting day 21 of exposure. Furthermore, treatment affected both GSH amounts and GST activities. The observed effects varied as function of tested insecticide, exposure time and concentration. The response appeared more marked with FCX. The recovery study showed that this fish species was able to overcome relatively rapidly the stress induced by these insecticides. The overall results suggested that FCX and to a lesser extent also DFB exhibited slight toxic effects on this non-target fish species, and can be used for controlling of mosquitoes in an integrated manner.