SHORT-TERM IMPACTS OF FORMULATIONS OF BACILLUS THURINGIENSIS VAR. ISRAELENSIS DE BARJAC AND THE ORGANOPHOSPHATE TEMEPHOS, USED IN BLACKFLY (DIPTERA: SIMULIIDAE) CONTROL, ON RHEOPHILIC BENTHIC MACROINVERTEBRATES IN THE MIDDLE ORANGE RIVER, SOUTH AFRICA

Summary The impacts of larvicides used in the control of blackflies (Diptera: Simuliidae) on macroinvertebrates in the stones-in-current biotope were assessed during 8 field trials in the middle Orange River, South Africa. Two Bacillus thuringiensis var. israelensis (B.t.i.) products (VectobacR 12AS and TeknarR HP-D) and the organophosphate temephos (AbateR 200EC) were applied at recommended and high dosages to simulate “operational” and “worst-possible” scenario's respectively. Mortality was evaluated either by direct counting of invertebrates on stones before and after application, or by ranking invertebrates on a 4-point relative abundance scale before and after application. In addition, the re-appearance of benthic invertebrate population densities after temephos application was examined. At the recommended dosage (1.2 ppm/10 min), B.t.i. significantly reduced blackfly larval numbers (P<0.001) and those of the chironomid Rheotanytarsus fuscus Freeman (P<0.05). At high dosage (20 ppm/10 min), numbers o...