Emergence of resistance and resistance management in field populations of tropical Culex quinquefasciatus to the microbial control agent Bacillus sphaericus
Mulla, M.S.; Thavara, U.; Tawatsin, A.; Chomposri, J.; Su, T.
Journal of the American Mosquito Control Association 19(1): 39-46
2003
ISSN/ISBN: 8756-971X PMID: 12674533 Document Number: 554242
In recent years, highly potent mosquitocidal strains of the microbial agent B. sphaericus (Bsph) have been isolated and developed for the control of mosquito larvae around the world. Laboratory selection experiments with the most active strains and their use in large-scale operational mosquito control programs resulted in the emergence of resistance in larvae of the Culex pipiens complex. This generated great concern among vector control agencies around the world, who feared reduced efficacy of this highly active larvicidal agent. To address this issue, the current studies were started to find practical strategies for controlling resistant mosquitoes and more importantly to develop resistance management strategies that would prevent or delay development of resistance. We initiated field studies in 3 low-income communities in Nonthaburi Province, Thailand. In one of the communities, larvae of C. quinquefasciatus that were highly resistant (>125 000-fold) to Bsph strain 2362 were successfully controlled with applications of B. thuringiensis var. israelensis (Bti) alone or in combination with Bsph. To prevent or delay resistance to Bsph, two other sites were selected, one treated with Bsph 2362 alone and the other treated with a mixture of Bsph 2362 and Bti. Mosquitoes treated with Bsph 2362 alone showed some resistance by the 9th treatment and almost complete failure of control occurred by the 17th treatment. After 9 treatments with the mixture over a 9-month period at another site, no noticeable change in susceptibility to Bsph was detected. During this period, the site treated with Bsph alone required 19 treatments, whereas the site treated with mixtures took only 9 treatments because of slower resurgence of larvae at the site treated with the mixture than at the site treated with Bsph alone. This is the first field evidence for delay or prevention of resistance to microbial agents in larval C. quinquefasciatus by using mixtures of Bti and Bsph. Further studies on the use of mixtures for the management of field resistance are warranted.