BACILLUS THURINGIENSIS SEROVARIETY ISRAELENSIS AND BACILLUS SPHAERICUS FOR MOSQUITO CONTROL

Gram-positive spore-forming entomopathogenic bacteria can utilize a large variety of protein toxins to help them invade, infect, and finally kill their hosts, through their action on the insect midgut. These toxins belong to a number of homology groups containing a diversity of protein structures and modes of action. In many cases, the toxins consist of unique folds or novel combinations of domains having known protein folds. Some of the toxins display a similar structure and mode of action to certain toxins of mammalian pathogens, suggesting a common evolutionary origin. Most of these toxins are produced in large amounts during sporulation and have the remarkable feature that they are localized in parasporal crystals. Localization of multiple toxin-encoding genes on plasmids together with mobilizable elements enables bacteria to shuffle their armory of toxins. Recombination between toxin genes and sequence divergence has resulted in a wide range of host specificities.

The entire 127,923-bp sequence of the toxin-encoding plasmid pBtoxis from Bacillus thuringiensis subsp. israelensis is presented and analyzed. In addition to the four known Cry and two known Cyt toxins, a third Cyt-type sequence was found with an additional C-terminal domain previously unseen in such proteins. Many plasmid-encoded genes could be involved in several functions other than toxin production. The most striking of these are several genes potentially affecting host sporulation and germination and a set of genes for the production and export of a peptide antibiotic.

Insecticides are the primary means to control Culex pipiens, an enzootic vector of West Nile virus, in the USA. To better understand how the evolution of resistance might impact control of this insect, we investigated the levels of resistance in populations collected from 2 metropolitan areas (Albany and Syracuse, NY) to 4 larvicides (methoprene, phenothrin, Bacillus sphaericus [Bs], and Bacillus thuringiensis israelensis [Bti]) and 1 adulticide (phenothrin) registered for mosquito control in New York State. High levels of resistance were found only to Bti, and only at 1 site (Syracuse). Resistance levels to the other insecticides were less than 10-fold. Given the large difference in Bti resistance between Syracuse and Albany, it appears these populations of Cx. pipiens do not rapidly mix, leading to localization of resistant populations.