MIXTURES AS A TOOL FOR THE MANAGEMENT OF INSECTICIDE RESISTANCE TO THE BACTERIAL INSECTICIDE BACILLUS THURINGIENSIS ISRAELENSIS.

Abstract:
Mixtures of insecticides have been proposed as one tool for the management of insecticide resistance. The concept of mixtures relies on the hypothesis that organisms may be able to overcome a single toxicant but are unlikely to be able to overcome several independently acting toxicants. Mixtures have been successfully used to manage antibiotic resistance in medicine, and have been proposed to manage insecticide resistance in arthropods. Insecticide mixtures have been tested in theoretical models as well as in laboratory selection experiments, and the results are promising. The bacterial toxins of Bacillus thuringiensis israelensis (Bti) are toxic to mosquitoes and blackflies. Bti expresses four different toxin proteins: 1) CrylVA (125 kDa), 2) CrylVB (135 kDa), 3) CrylVD (65 kDa) and 4) CytA (28 kDa). Because resistance has been slow to develop to Bti, both in the field and in laboratory selection experiments, it has been hypothesized that the four toxins may act as a natural mixture, delaying the development of resistance. We undertook a series of experiments, designed to test whether resistance would develop more easily in response to selection pressure with a single Bti toxin, compared to selection with mixtures of two, three and four toxins. These experiments demonstrated that resistance developed more rapidly, and to a higher level, when mosquitoes were selected with a single Bti toxin. Further, we observed that no resistance developed in response to selection with four toxins, whereas resistance developed in lines selected with one, two and three toxins. The loss of a single toxin, the CytA toxin, from the original four toxins in Bti, resulted in significant levels of resistance in mosquitoes under selection pressure. The role of the CytA toxin was further investigated through bioassay tests, singly, and in combination with the other CryIV toxins. The results indicate that the CytA toxin, which differs significantly from the CrylV toxins in its gene sequence and mode of action, may play an important role in delaying resistance to Bti.