Factors affecting black fly larval mortality and carry of two formulationsof Bacillus thuringiensis subsp israelensis tested in the same stream during a 3-year experiment

Over the last 20 years, many formulations of Bacillus thuringiensis subsp. israelensis (Bti) have been commercialized for the biological control of bl ack flies in abatement programs. However, products were tested in different streams and under different environmental conditions, thus rendering the e valuation of the performance of liquid formulations of Bti very arbitrary o r even impossible. Using a new field procedure, two commercial liquid formu lations of Bti, Teknar HP-D and Vectobac 1200L, were for the first time tes ted within short time intervals in the same stream and under environmental conditions that allowed better comparison of the behavior and the performan ce (carry) of both products. Field results from tests using the products te sted in either the same (replicate) or different environmental (abiotic) co nditions were analyzed and compared. Results of the 3-year experiment showe d that both water temperature and discharge had an important effect on the performance of both products (Teknar HP-D and Vectobac 1200L). In temperate water temperature (16 degreesC) and similar discharge (70 l s(-1)), Teknar HP-D and Vectobac 1200L did not show a significant difference in their car ry50 (distance where 50% of black fly larvae mortality is recorded). Howeve r, an increase in water temperature (20-22 degreesC) or in the discharge (2 50-350 l s(-1)) enhanced significantly the carry50 of both products with Ve ctobac 1200L having a longer carry than Teknar HP-D. Other abiotic elements such as the hyporheic zone and adsorption onto periphyton were also identi fied as factors affecting the behavior and the carry of the products. Howev er, more work should be done on the loss of the residual dosages along the stream to better understand the relation between the dosages and the result ing field mortality that would lead to suggestions on how to improve Bti fo rmulations.