Studies on the culicine mosquito host range of Bacillus sphaericus and Bacillus thuringiensis var. israelensis with notes on the effects of temperature and instar on bacterial efficacy

Toxicity tests of three strains of Bacillus sphaericus against late instars of 12 culicine mosquito species indicated a wide range of susceptibility. Culex pipiens and C. salinarius were highly susceptible (LC₅₀s < 10⁴ spores/ml) to strain 1593, and C. pipiens and C. restuans were highly susceptible to strain 2013-4. The potency of strain SSII-1 was approximately one-tenth that of strains 1593 and 2013-4 against C. pipiens. Susceptibility of Aedes species to strain 1593 was highly variable. At temperatures ≥ 20°C, A. fitchii, A. intrudens, A. stimulans, and A. vexans were moderately to highly susceptible (LC₅₀s 6 × 10³−4 × 10⁴ spores/ml), A. triseriatus was only slightly susceptible (LC₅₀ > 10⁶ spores/ml), and A. aegypti was refractory. Susceptibility of Aedes mosquitoes to strain SSII-1 was less variable, with LC₅₀s against A. aegypti, A. canadensis, A. stimulans, and A. triseriatus all being between 10⁴ and 10⁶ vegetative cells + spores/ml. All species of mosquitoes tested were, in general, highly susceptible to B. thuringiensis var. israelensis (LC₅₀s 2.3 × 10³−2.5 × 10⁴ spores/ml). In B. sphaericus toxicity tests, decreased temperatures resulted in up to a 16-fold increase in LC₅₀ and a substantial reduction in probit line slope. First-instar A. aegypti larvae were more susceptible to B. sphaericus strain SSII-1 than the three later instars, which were approximately equally susceptible; however, no significant difference was observed in the susceptibility of the four instars of A. triseriatus.