Usavadee Thavara

Affiliations :
      National institute of Health, Department of Medical Sciences,
Ministry of Public Health, 88/7 Tiwanon Road, Muang, Nonthaburi 11000, Thailand

Source:
      การประชุมวิชาการ Bacillus thuringiensis จากการวิจัยพื้นฐานสู่การประยุกต์ใช้ ครั้งที่1 วันที่ 22-23 พฤษภาคม 2546 ณ สถาบันอณูชีววิทยาและพันธุศาสตร์
มหาวิทยาลัยมหิดล วิทยาเขตศาลายา จ.นครปฐม

Language :
     English

ABSTRACT :
      Bacillus sphaericus  (Bsph) has been recognized as a potential microbial larvicide against Culex quinquefasciatus  larvae since 1980s. This bacterium has been formulated for many formulations available commercially as mosquito larvicides. Although most formulations demonstrate good larvicidal efficacy under laboratory conditions, field evaluations is also needed for large-scale mosquito control program. Field trials to determine efficacy and persistence of the Water Dispersible Granule (WDG) formulation of the microbial larvicide, Bsph (strain 2362) against Cx. quinquefasciatus  larvae in highly polluted-water habitats were carried out in suburban areas of Nonthaburi Province, Thailand. At a dosage of 200 mg/m², complete control of the immature populations was achieved for a period of 5-6 weeks. To investigate the minimum effective dosage, subsequent treatments with lower dosages (50-100 mg/m²) were then carried out. However, the level of control declined over successive treatments, and a high level of resistance to Bsph was suspected at one site (Wat Pikul) after 5 treatments (about 12 weeks after the first treatment). The colony of this suspected population was established in the laboratory and resistance level was then assessed through larval bioassays. The larvae of this population showed resistance to Bsph for about 125,000-fold as compared to susceptible population. Therefore, this is the first report of resistance to Bsph in wild populations of Cx. quinquefasciatus  in Thailand subjected to successive treatments. We then embarked upon finding strategies to manage this high level of resistance by either using Bacillus thuringiensis israelensis  (Bti) alone or mixture of Bti and Bsph. Field studies were conducted in three low-income communities (Wat Pikul, Wat Lahan and Soi Jumpa) in Nonthaburi Province. At Wat Pikul, larvae of Cx. quinquefasciatus  that were highly resistant to Bsph were successfully controlled with applications of Bti alone or in combination with Bsph. As for the other two sites, Wat Lahan was treated with Bsph alone whereas Soi Jumpa was treated with a mixture of Bti and Bsph. At Wat Lahan, larvae treated with Bsph alone showed some resistance by the 9^th treatment and almost complete failure of control occurred by the 17^th treatment. At Soi Jumpa, no noticeable change in susceptibility to Bsph was detected after 9 treatments with the mixture over a 9-month period. The site treated with Bsph alone required 19 treatments, whereas the site treated with mixture 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. Using mixtures of Bti and Bsph in this study is then the first field evidence to prevent or delay resistance to Bsph larvicide of Cx. quinquefasciatus  larvae.