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[The activity of Bacillus thuringiensis var. israelensis against black fly larvae under field conditions in South Bohemia].

The effectiveness of Bacillus thuringiensis var. israelensis spore suspension against black fly larvae was tested in two regulated brooks in the Ceské Budĕjovice district, South Bohemia. The sites under study were small, regulated, paved brooks of trapezoidal profile, with vegetation of different density, and with stream flow rates ranging from 40 to 90 cm/sec. Lyophilized spores (produced by Roger Belon) at a final concentration of 1 X 10(5) spores per ml were used. The spore efficacy was determined in both natural and artificial substrates in the field and under laboratory conditions, on the larvae collected at the control sites. A 90-100% mortality of the larvae was recorded in all parts of the brooks examined both in the natural samples studied in laboratory and in those kept on artificial substrates. In the area of fading spore suspension action, a higher percentage of the last-instar larvae was found in larval population which may be ascribed, in agreement with other authors, to an increased susceptibility of younger larvae to the preparation. The results of the tests demonstrated that Bacillus thuringiensis var. israelensis is an effective agent to control the black fly species occurring in Central Europe and may be of value mainly in the control of some species of veterinary importance overpopulated in regulated brooks.