Using the Bio-Insecticide Bacillus thuringiensis israelensis in Mosquito Control

Mosquito control is a major public health concern, as mosquitoes transmit many severe human diseases such as malaria, filariasis, dengue, yellow fever, West Nile virus and the chikungunya virus. These diseases represent a major health threat and economic burden in disease-endemic countries, and are currently in expansion due to increased worldwide exchanges, urbanization, and global warming. The only effective way of reducing the incidence of these diseases is to control the vector mosquitoes, mainly by application of insecticides to their breeding places. Since the 1950s, the massive use of chemical insecticides has led to undesired toxicity on non-target organisms and the selection of insecticide resistance mechanisms in mosquito populations (Hemingway & Ranson, 2000). A safe alternative to chemical insecticides is to spray toxins produced by the bacteria Bacillus thuringiensis subsp. israelensis (Bti) over mosquito breeding sites (Lacey, 2007). Bti represents today the best alternative to chemical insecticides in controlling mosquitoes. Bti toxins are safe for non-target species and human health, are believed to show low persistence in the environment, and so far no resistance was detected in mosquito populations. Bti is the only insecticide allowed against mosquito larvae in Europe. To insure a long-term efficiency of this bio-insecticide, it is however necessary to evaluate the risks associated to its intensive worldwide use. The two main risks are (1) the accumulation of spores and toxins in the environment, and possible proliferation of Bti a long time after spraying, which may have an impact on the whole ecosystem functioning, and (2) the evolution of resistance to Bti in mosquitoes, rendering the treatment inefficient. It is therefore necessary to develop monitoring tools to follow the fate of spores and toxins in the environment and the evolution of resistance in target mosquito populations. Here we review recent advances in our understanding of the mechanisms of Bti toxicity and of mosquito resistance. The chapter will be organized in three parts: the first part describes Bti structure and its fate in the environment, the second part describes the action of Bti toxins after ingestion by mosquito larvae and the diversity of mechanisms involved in mosquito resistance, and the third part is dedicated to the challenging objective of managing resistance in the field. We conclude in identifying issues that need further research.