[Applications of bacterial pathogens to the mosquito control]

Both Bacillus thuringiensis subsp. isrealensis (Bti) and Bacillus sphaericus are common bacteria found in a variety of soil and aquatic habitats. Some strains of them can produce a protein crystal, called parasporal body (or crystal) beside the spore in the cell during sporulation, and it is toxic if eaten by mosquito larvae. As the parasporal bodies are ingested into a mosquito digestive tract, they will be digested and activated by certain enzymes in alkaline digestive juice. The activated toxins can destroy midgut epithelium and other tissues, and then the mosquitoes will be intoxicated and killed. There are some factors affecting efficiency of the toxins produced by these two mosquitocidal bacteria. First, the toxicity of the toxins will decrease as larval age and density increase. Second, difference in mosquito species will demonstrate a different tolerance to both toxins. However, larvae of Anopheles are more tolerant to Bti than those of Aedes and Culex; whereas larvae of Aedes are more tolerant to B. sphaericus than Culex and Anopheles. Environmental factors are as important as the mosquito hosts in influencing the efficiency of the toxins. For examples, raising temperature under suitable ranges may increase the toxicity of the toxins. The UV light of sunlight could inhibit activity of the toxins and therefore decrease their efficiency. Quality and depth of the water are also important environmental factors affecting toxicity of the toxins to the mosquito larvae. The precipitation of the crystal and organic materials absorbing the toxins decreases their toxicity to the mosquito larvae as well.