Influence of Stream Profile and Abiotic Factors on the Performance and Residual Dosages of Two Commercial Formulations of Bacillus thuringiensis subsp. israelensis During a 2-Year Experiment

Recently, results of a 3-year study showed the importance of abiotic factors such as water temperature, stream discharge and the hyporheic zone on the behaviour, performance and loss of the residual dosages (amount of the injected dosage left at each station) of two commercial liquid formulations of Bacillus thuringiensis subsp. israelensis ( Bti ), Teknar HP-D and Vectobac 1200L. Experiments were performed in the same stream and under environmental conditions that allowed comparisons of results between formulations. To improve our understanding of the influence of abiotic factors on the behaviour and the performance of these formulations, a 2-year experiment using the same products was conducted in another river characterized by a much different profile, higher discharges and colder temperatures. A field procedure based on a system of gutters located on the bank of the river already used in the previous 3-year experiment was utilized. Black fly larval mortalities were recorded along the river at different distances (stations) to evaluate the behaviour and the performance of the products while water samples taken at the stations were tested in the laboratory against mosquito neonate larvae to evaluate the residual dosages of Bti at each station. Results showed that higher residual dosages of the Vectobac 1200L were recovered compared to Teknar HP-D along the river. Although higher dosages were recovered, higher mortality was observed only for the Vectobac 1200 L in high discharge conditions. As seen before, the hyporheic zone (interstitial water between the streambed and groundwater) produced a major loss of the dosages in the first meters of the river. But because of the river profile, the hyporheic zone had a lesser effect on the loss of the dosages further in the river resulting in very long carries for both Teknar HP-D and Vectobac 1200L.     

Factors Affecting Black Fly Larval Mortality and Carry of Two Formulations of 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 black flies in abatement programs. However, products were tested in different streams and under different environmental conditions, thus rendering the evaluation of the performance of liquid formulations of Bti very arbitrary or even impossible. Using a new field procedure, two commercial liquid formulations of Bti, Teknar HP-D and Vectobac 1200L, were for the first time tested within short time intervals in the same stream and under environmental conditions that allowed better comparison of the behavior and the performance (carry) of both products. Field results from tests using the products tested in either the same (replicate) or different environmental (abiotic) conditions were analyzed and compared. Results of the 3-year experiment showed 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°C) and similar discharge (70 l s -1 ), Teknar HP-D and Vectobac 1200L did not show a significant difference in their carry50 (distance where 50% of black fly larvae mortality is recorded). However, an increase in water temperature (20- 22°C) or in the discharge (250-350 l s -1 ) enhanced significantly the carry50 of both products with Vectobac 1200L having a longer carry than Teknar HP-D. Other abiotic elements such as the hyporheic zone and adsorption onto periphyton were also identified as factors affecting the behavior and the carry of the products. However, 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 resulting field mortality that would lead to suggestions on how to improve Bti formulations.     

A New Field Procedure and Method of Analysis to Evaluate the Performance of Bacillus thuringiensis subsp. israelensis Liquid Formulations in Streams and Rivers

Many field tests have shown Bacillus thuringiensis subsp. israelensis ( Bti ) to be an effective simuliid larvicide. However, literature indicates that an effective evaluation and comparison of Bti -based formulations when tested in streams or rivers is difficult. Most field trials have been conducted in different rivers (different discharge, river profile, water temperature, suspended matter, larval species, etc.), thus rendering the evaluation of the performance of liquid formulations of Bti very arbitrary or even impossible. A new field procedure is proposed to evaluate the performance of liquid formulations of Bti in the same lotic environment and under similar abiotic and biotic conditions. The procedure, based on a system of gutters located on the bank of a stream, showed very good reproducibility of the mortality levels of the target pest(s) recorded at various distances (stations) along the stream over a two-year field trial, proving the efficacy of the system. The system allows either a single formulation or different formulations to be tested repeatedly in the same portion of a stream, thereby providing a more accurate evaluation of the performance of a Bti formulation or a much better comparison between different formulations. The use of the probit model (allowing comparison of slopes and intercepts) gives a reliable statistical value for the analysis of the results. Moreover, this system is not expensive and can be transferred easily to other streams or rivers.