Various Levels of Cross-Resistance to Bacillus sphaericus Strains in Culex pipiens (Diptera: Culicidae) Colonies Resistant to B. sphaericus Strain 2362

Abstract

We studied the cross-resistance to three highly toxic Bacillus sphaericus strains, IAB-59 (serotype H6), IAB-881 (serotype H3), and IAB-872 (serotype H48), of four colonies of the Culex pipiens complex resistant to B. sphaericus 2362 and 1593, both of which are serotype H5a5b strains. Two field-selected highly resistant colonies originating from India (KOCHI, 17,000-fold resistance) and France (SPHAE, 23,000-fold resistance) and a highly resistant laboratory-selected colony from California (GeoR, 36,000-fold resistance) showed strong cross-resistance to strains IAB-881 and IAB-872 but significantly weaker cross-resistance to IAB-59 (3-to 43-fold resistance). In contrast, a laboratory-selected California colony with low-level resistance (JRMM-R, 5-fold resistance) displayed similar levels of resistance (5-to 10-fold) to all of the B. sphaericus strains tested. Thus, among the mosquitocidal strains of B. sphaericus we identified a strain, IAB-59, which was toxic to several Culex colonies that were highly resistant to commercial strains 2362 and 1593. Our analysis also indicated that strain IAB-59 may possess other larvicidal factors. These results could have important implications for the development of resistance management strategies for area-wide mosquito control programs based on the use of B. sphaericus preparations.

TwoBacillus sphaericusbinary toxins share the midgut receptor binding site: implications for resistance ofCulex pipienscomplex (Diptera: Culicidae) larvae

“…In fact, previous data showed that colonies highly resistant to strain 2362 do not show cross-resistance to IAB59, in contrast to the high cross-resistance observed among the strains 1593, 2362, C3-41 and 2297 [25,30]. Data demonstrate that strain IAB59 has different features from other highly toxic strains and its action does not rely exclusively on the Bin toxin.…”

“…The mode of action currently known relies on the interaction of a single toxin to one binding site, as a consequence, populations may attain high levels of resistance when exposed to strong selection pressure with bioinsecticides based on those strains [21][22][23][24][25]. However, screening of strains able to overcome resistance show that strain IAB59, that produces Bin1 toxin, is active against Culex larvae highly resistant to the strains 2362, 1593 and C3-41 [25,30]. Those strains, the most successfully commercialized for mosquito control, produce Bin2 toxin with identical amino acid composition and cross-resistance in Culex quinquefasciatus was detected among them.…”

A Strain ofBacillus sphaericusCauses Slower Development of Resistance inCulex quinquefasciatus

“…Vector Ecol., P17, 1994), respectively. Laboratory studies have shown that the resistance developed to certain strains of B. sphaericus confers more or less cross-resistance to other strains of the same species of toxin-producing organisms (16,22,27). Therefore, the resistance of mosquito populations to B. sphaericus Bin toxin will seriously threaten the sustainability of current mosquito control programs using this microbial insecticide.…”

“…israelensis formulation in the formerly B. sphaericus-treated area, the resistant mosquito population became susceptible to B. sphaericus C3-41; The resistance ratio of the field-collected larvae dropped from 22,600-to 5.76-fold (31). Recent studies (16,27) have shown that B. sphaericus strain IAB59, which produces Bin1 during its sporulation and belongs to serotype H6, had not only a very high toxicity to susceptible C. quinquefasciatus colonies but also a moderate toxicity to a B. sphaericus-resistant Culex colony.…”

Improving the Insecticidal Activity against Resistant Culex quinquefasciatus Mosquitoes by Expression of Chitinase Gene chiAC in Bacillus sphaericus

“…However, high-level resistance against B. sphaericus binary toxin after intensive treatment has developed in mosquitoes (6), with resistance ratios ranging from 35-to 150,000-fold in the laboratory (5,8) and from 10-to 100,000-fold in the field (15). Experiments testing binary toxin binding to the mosquito larval midgut in vitro have demonstrated that resistance of mosquitoes to B. sphaericus binary toxin has occurred mainly because of elimination of the toxin-binding site on the midgut brush border membrane fraction (1,6). The appearance of high-level of resistance in mosquitoes is a threat to the future application of B. sphaericus as a mosquito control agent.…”