The effect of seston on the susceptibitlity of Simulium vittatum larvae (Diptera: Simuliidae) to insecticidal proteins produced by Bacillus thuringiensis subsp. israelensis

Proteins produced by Bacillus thuringiensis subsp. israelensis are highly toxic against black fly larvae. Occasional control problems in the North Branch of the Susquehanna River in Pennsylvania have been reported following application of Bti. Experiments conducted in 2009 and 2010 with water from problem areas revealed that seston is responsible for the reduction in larval mortality. Seston from sites exhibiting less than optimum larval mortalities was analyzed using scanning electron microscopy and x-ray diffraction.  Individual components of the seston were exposed to Simulium vittatum larvae prior to Bti exposure. Clay minerals were obtained to represent those found in the seston with x-ray diffraction. These minerals had no effect on larval mortality in the concentrations used. An abundance of diatoms and frustules were observed from the SEM of the seston. Similar diatoms were cultivated in the laboratory and exposure of larvae to the viable cells prior to Bti resulted in a significant decrease in mortality. Exposure to purified diatom frustules resulted in the most severe impairment of mortality following Bti exposure.