A field evaluation of Bacillus thuringiensis var. israelensis as a biological control agent for Simulium chutteri (Diptera: Nematocera) in the middle Orange River

Abstract

Bacillus thuringiensis Berliner var. israelensis de Barjac (Serotype H-14) (B.t.i.) at a concentration of 1,6 ppm/10 min and a toxicity of 1500 AAU/mg was tested against Simulium chutteri Lewis larvae in the Orange River near Prieska, South Africa. Samples of benthic fauna from the stones-in-current biotope were collected before application of the product and at various intervals up to 80 h afterwards at 4 stations from 200 m to 11 km downstream of the application site. Faunal drift increased slightly after the arrival of the Bacillus at 2 stations 1,4 and 6 km respectively downstream of the application site. Large numerical decreases in benthic simuliid larval numbers after the application of B.t.i. in the Orange River were not statistically different (P>0,05). This indicated that the size of replicated samples that showed significant decreases (P<0,05) of simuliid numbers in the Vaal River was not adequate to show statistical differences in the Orange River. The quantity of dead larvae on stones collected from rapids after application of the B.t.i., and the numerical decreases found by comparing median values of larval counts on stones indicated that B.t.i. effectively killed simuliid larvae. Three days after application of the Bacillus, recruitment of small simuliid larvae on stones 1,4 km downstream of the application site was discernible again. Tanytarsini were also numerically reduced after B.t.i. application. At a flow rate of 38 m³/s B.t.i. was visibly effective in killing S. chutteri up to 6 km downstream of the application site and statistically significant decreases (P<0,05) in numbers of larvae were seen at a site 11 km downstream of the application site. The use of B.t.i. in Simulium control is preferable to organophosphate and organochloride formulations because it has a more specific action against blackfly and because there is no known immunity to B.t.i. in any Simulium species. However, we must advise that B.t.i. should be screened against all co-existing fauna in each situation where new community structures of animals are encountered. Several methods for improving the efficacy of B.t.i. are suggested.