Cyt1A from Bacillus thuringiensis restores toxicity of Bacillus sphaericusagainst resistant Culex quinquefasciatus (Diptera : Culicidae)

The 2362 strain of Bacillus sphaericus, which produces a binary toxin highl y active against CytlA mosquitoes, has been developed recently as a commerc ial larvicide. It is being used currently in operational mosquito control p rograms in several countries including Brazil, France, India, and the Unite d States. Laboratory studies have shown that mosquitoes can develop resista nce to B. sphaericus, and low levels of resistance have already been report ed in field populations in Brazil, Fiance, and India. To develop tools for resistance management, the Cyt1A protein of Bacillus thuringiensis subsp, i sraelensis De Barjac was evaluated for its ability to suppress resistance t o B. sphaericus in a highly resistant population of Culex quinquefasciatus Sap. A combination of B. sphacricus 2362 in a 10:1 ratio with a strain of B . thuringiensis subsp, israelensis that only produces CytlA reduced resista nce by >30,000-fold. Resistance was suppressed completely when B, sphaericu s was combined with purified CytlA crystals in a 10:1 ratio. Synergism was observed between the CytlA toxin and B. sphaericus against, the resistant m osquito population and accounted for the marked reduction in resistance. Ho wever, no synergism was observed between the toxins against a nonresistant mosquito population. These results indicate that CptlA could be usefull for managing resistance to B. sphaericus 2362 in Culex populations, and also p rovide additional evidence that CytlA may synergize toxicity by enhancing t he binding to and insertion of toxins into the mosquito microvillar membran e.