Resistance Development To Mosquitocidal Mtx1 Toxin From Bacillus Sphaericus In Culex Quinquefasciatus (Diptera: Culicidae)

Mtx1 toxin is a mosquitocidal toxin produced by Bacillus sphaericus during its vegetative stage, with different mode of action to target mosquitoes than binary toxin, thus it is considered to be a potential candidate for future mosquito control. This study focused on evaluation of toxicity of Mtx1 toxin against different insects, its delayed effect on Culex quinquefasciatus (Diptera: Culicidae), Mtx1 resistant C. quinquefasciatus colony selection and characterization, for providing informative data for further delaying the resistance development in mosquitoes. Bioassasy results showed that Mtx1 had little or no toxicity to the tested lepidopteran species, but had moderate-level toxicity to Aedes albopictus (Diptera: Culicidae), and high-level toxicity to both susceptible and binary toxin-resistant C. quinquefasciatus. The LC50 values of E-TH21 lyophilized powder, a strain expressing Mtx1 toxin, against a susceptible colony SLCq, two resistant colonies RLCq1/C3-41 and RLCq2/IAB59 were 0.508, 0.854 and 0.675 mg/l, respectively, indicating that Mtx1 could be an alternative toxin to delay or overcome the resistance to binary toxin developed in C. quinquefasciatus. Mtx1 not only has specific activity against C. quinquefasciatus larvae, but also induced stronger delayed effects on the development of mosquitoes. After the 2-3 instar larvae of C. quinquefasciatus treated with different concentration (LC₁₀, LC₃₀, LC₅₀, LC₇₀, LC₉₀, and LC₉₈) for 48h, the Mtx1 and binary toxin preparation could induced significant continued mortalities in larval, pupal and emergence stages, with the cumulative preadult mortalities of 85.0%, 97.5%, 98.6%, 99.5%, 100%, 100% and 23.1%, 44.4%, 61.6%, 83.2%, 95.1%, 100% respectively. The finding that Mtx1 induced stronger delayed effects than binary toxin on C. quinquefasciatus indicates the two toxins might have different modes of action on the target mosquito. This information is significant in understanding the mechanism of action of these two toxins and for future mosquito control studies. Besides, we selected a low-level Mtx1 resistant C. quinquefasciatus colony after 30 generations'selection with high dose E-TH21 lyophilized powder, with a resistance ratio of about 223.2. The Mtx1 resistant colony (RLCq4/Mtx1) had no cross resistance to B. thuringiensis subsp.israelensis, B. sphaericus strains LP1-G and 47-6B, partial cross resistance to B. sphaericus IAB59 strain, but high-level cross resistance to binary toxin producing strains such as C3-41 and B-CW-1. Furthermore, the biology fitness of the three C. quinquefasciatus colonies SLCq, RLCq1/C3-41 and RLCq4/Mtx1 has also been investigated. The results showed that the lower fecundity of RLCq1/C3-41 and RLCq4/Mtx1 colonies was not very significantly, but the longevity of the female adult of the two resistant colonies was much longer than that of SLCq colony.