Pulse-exposure effects of selected insecticides to juvenile Australian crimson-spotted rainbowfish (Melanotaenia duboulayi).

Laboratory toxicity studies were conducted in southeastern Queensland, Australia, to determine the acute lethal effects of a 1-h pulse exposure of selected insecticides to adult and juvenile (<72 h old) crimson-spotted rainbowfish, Melanotaenia duboulayi (Castlenau). In addition, to its ecological significance, this native fish is a predator of mosquitoes. Two organophosphate (OP) compounds (temephos and pirimiphos-methyl), an entomophathogenic bacterium (Bacillus thuringiensis variety israelensis [Bti]) and two insect growth regulators (IGRs) (s-methoprene and pyriproxyfen) were evaluated. Although none of the five insecticides were acutely toxic to adult M. duboulayi under the test conditions, temephos and pirimiphos-methyl were found to be toxic to juveniles, with 24 h pulse exposure LC50 values of 27 and 15 microg/liter (ppb), respectively. Of the two OPs, pirimiphos-methyl was the most toxic, with a lethal dose ratio (pulse exposure LC50 temephos/ pulse exposure LC50 pirimiphos-methyl) of 1.8 (95% CL 0.5-6.4). These pulse exposure LC50 values represented 40 and 4.5% of the estimated environmental concentrations (EEC) for a 15 cm deep water body, respectively. Bti and the two IGRs had no acute toxic effects at up to 10 and 12.5 times the EEC, respectively. Accordingly, in the interests of environmental conservation and integrated pest management (IPM), preference should be given to the latter three insecticides for control of mosquito larvae in juvenile M. duboulayi habitat.