Effects of Bacillus thuringiensis var. israelensis on Target and Nontarget Organisms: a Review of Laboratory and Field Experiments

Since the discovery of Bacillus thuringiensis var. israelensis (Bti) in 1976, extensive literature has proved its efficacy to control mosquitoes and black flies, of which many species are known as important vectors of diseases or simply as pests of humans and animals. Since 1978, Bti has been used in many countries on all continents and numerous studies have been made on target mosquitoes and black flies, as well as nontarget organisms (NTO). This review analyses the results of 75 studies on these organisms covering approximately 125 families, 300 genera and 400 species. Different factors such as species, instar, feeding behaviour and environmental parameters (larval density, water temperature, suspended matter etc.) may drastically affect the efficacy of the Bti products. This is addressed in detail by reviewing the main factors affecting mosquitoes as well as black flies. The results of a wide range of laboratory and field experiments using different target and nontarget species, various preparations and formulations of Bti and different biotic or abiotic factors are present in the literature, making the data difficult to compare on a common basis. Our analysis shows that, under different application conditions, the effects of Bti on target and nontarget organisms may be hard to predict. Although Bti has been proclaimed to be relatively highly specific, some studies show that some NTO are affected either by single or repeated Bti treatments. Present use against black flies seems ecologically acceptable. High frequencies of application and/or overdosages against mosquitoes may result in some persistence of the toxin crystals and ultimately this may have adverse effects on the food web. A long-term study (published in 1998) in mosquito habitats has shown that intensive Bti treatments over three years did in fact produce an impact on the food web in wetlands. This raises questions, for the first time, on Bti environmental specificity. The importance of this impact is discussed and the alternatives for practical pest control are considered. Some modifications of Bti use against mosquitoes, guided by research, is probably the best of these alternatives.