Two trials. to test the efficacy of Bacillus thuringiensis Berliner var. israelensis de Barjac (serotype H-14) against target simuliid and non-target aquatic invertebrates were undertaken in the Vaal River near Warrenton in South Africa. In the 1st trail an application of 1,6 ppm/10 min of B. thuringiensis resulted in a significant (P<0,05) reduction of simuliid larvae in rapids 70 m below the treatment point 40 hours after its application. Further downstream the larvicide was ineffectual because the low flow of the river (6 m³/s) allowed the Bacillus pores to settle out in calmer stretches. The 2nd trial was carried out upstream of small rapids with a calculated flow of 0 ,5 m³/s at a spore concentration of 2,3 ppm/7 min. The effect of B. thuringiensis on the benthic population density and drift activity of the benthos was recorded. A high mortality of simuliid larvae and Ephemeroptera was observed 35 m below the application point 9 hours after the application of the larvicide. The mortality in Ephemeroptera was partially due to the handling of these animals. Population densities of simuliid larvae in the treated rapids decreased 18 hours after application of the larvicide, but small simuliid larvae showed a numerical increase again after 72 hours, indicating rapid recolonization from drifting larvae. Tanytarsine Chironomidae decreased after the application of B. thuringiensis, but most other fauna either increased or did not decrease significantly (P>0,05). Within 43 minutes after treatment of the rapids with the larvicide, simuliid drift increased more than sixtyfold, revealing the immediate irritating effect of the product on the target organisms. Drift of other non-target organisms was not noticeably influenced. Populations of Simulium adersi Pomeroy, S. chutteri Lewis, S. hargreavesi Gibbins, S. mcmahoni de Meillon. and S. damnosum s.l. Theobald were all significantly reduced (P<0,05) after treatment of rapids with B. thuringiensis, but S. damnosum s.l. showed the lowest mortality and appeared less affected by the product than the other species.
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