MIR S. MULLA¹
USAVADEE THAVARA²
APIWAT TAWATSIN²
JAKKRAWARN CHOMPOSRI²
TIANYUN SU¹

Affiliations :
      1 Department of Entomology,University of California,Riverside, CA 92521.
      2 NIH, Department of Medical Sciences,Ministry of Public Health, 88/7 Tiwanon Road, Nonthaburi 11000,Thailand.

Source:
      Journal of the American Mosquito Control Association. 19(1):39-46,2003

Language :
     English

ABSTRACT :
      In recent years, highly potent mosquitocidal strains of the microbial agent Bacillus 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 1 of the communitieds, larvae of Culex quinquefasciatus  that were highly resistant (>125,000-fold) to Bsph strain 2362 were successfully controlled with applications of Bacillus thuringiensis  var. israelensis  (Bti ) alone or in combination with Bsph.  To prevent or delay resistance to Bsph, 2 other sites were selected, 1 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, on 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 mixture took only 9 treatments becuase of slower resurgence of larvae at the site treated with mixture than at the site treated with Bsph  alone. This is the 1st field evidence for delay or prevention of resistance to microbial agents in larval Cx. quinquefasciatus  by using mixtures of Bti  and Bsph . Further studies on the use of mixtures for the management of field resistance are warranted.