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 teste d in streams or rivers is difficult. Most Weld trials have been conducted i n different rivers (different discharge, river profile, water temperature, suspended matter, larval species, etc.), thus rendering the evaluation of t he performance of liquid formulations of Bti very arbitrary or even impossi ble. A new Weld procedure is proposed to evaluate the performance of liquid formulations of Bti in the same lotic environment and under similar abioti c and biotic conditions. The procedure, based on a system of gutters locate d on the bank of a stream, showed very good reproducibility of the mortalit y levels of the target pest(s) recorded at various distances (stations) alo ng the stream over a two-year Weld trial, proving the efficacy of the syste m. 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 muc h better comparison between different formulations. The use of the probit m odel (allowing comparison of slopes and intercepts) gives a reliable statis tical value for the analysis of the results. Moreover, this system is not e xpensive and can be transferred easily to other streams or rivers.