Blackfly control with the microbiological preparation VectoBac 12AS was started in Lithuania in 1999. This preparation is based on bacterium Bacillus thuringiensis var. israelensis. The VectoBac 12AS is applied at a single point from a bank prominence from the year 2000. Investigations were carried out in April–June of 2006 and 2007. The bacteria were found in the Nemunas River 164 km downstream the point of application of the larvicide 3 days after the application. This showed that VectoBac 12AS reached this segment of the river in 3 days. The density of B. thuringiensis bacteria decreased downstream from the point of application of the preparation. The highest density of bacteria was found in blackfly larvae, lower densities were found in the ground and on water plants. The effect of VectoBac 12AS on nontarget invertebrates was estimated in study sites from the point of application up to Druskininkai. Using the method of application from one point, this distance of the river was affected by the highest doses of preparation, and its effect on nontarget organisms could be seen in this part of the river. The usage of the preparation had no effect on nontarget invertebrates in the Nemunas River. Significant differences in Chironomid density were detected only in one study site, 6 km downstream the point of application of the larvicide. At a distance of 14 km from the point of application and downstream the river, no differences in the density of Chironomid larvae were detected.

Keywords: bacterium, Bacillus thuringiensis var. israelensis, blackflies, aquatic invertebrates