SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 62 Vol 37 No. 1 January 2006 Correspondence: Usavadee Thavara, National Institute of Health, Department of Medical Sciences, Ministry of Public Health, 88/7 Soi Bumrasnaradul, Tiwanon Road, Amphoe Mueang, Nonthaburi 11000, Thailand. Tel: 66 (0) 2589 9850-8 ext 99245 E-mail: usavadee@dmsc.moph.go.th INTRODUCTION Currently, dengue fever is considered to be the most important arboviral disease of humans in terms of its public health impact (Gubler, 1989). The disease incidence and distribution have steadily increased with more than 2 to 3 billion people at risk of infection, and an esti- mated 20 million dengue cases annually (WHO, 1997). Aedes aegypti (Linnaeus) and Ae. albopictus (Skuse) serve as the primary and sec- ondary vectors, respectively. Unlike the mosqui- toes that cause malaria, dengue mosquitoes bite during the day. Although the viruses are related, antibodies obtained after infection with one se- rotype are not cross-protective for the other se- rotypes (Beaty and Marquardt, 1996). At present, dengue control measures include the use of LABORATORY AND SEMI-FIELD EVALUATION OF MOSQUITO DUNKS ® AGAINST AEDES AEGYPTI AND AEDES ALBOPICTUS LARVAE (DIPTERA: CULICIDAE) Thanyalak Fansiri 1 , Usavadee Thavara 2 , Apiwat Tawatsin 2 , Somporn Krasaesub 3 , and Ratana Sithiprasasna 1 1 Department of Entomology, US Army Medical Component, Armed Forces Research Institute of Medical Sciences, Bangkok; 2 National Institute of Health, Department of Medical Sciences, Ministry of Public Health, Nonthaburi; 3 Department of Administration, US Army Medical Component, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand Abstract. Laboratory bioassays and semi-field studies were conducted on the efficacy and longevity of Mosquito Dunks ® (7,000 ITU/mg Bti) in order to determine the concentration-response relation- ship and the effectiveness on the potency of the Bti product against Aedes mosquito species based on the WHO protocol standard methods and to determine the longevity of release for this product against Ae. aegypti mosquito larvae in water storage containers. This bio-potency study with the late 3 rd instar larvae of Ae. aegypti and Ae. albopictus was carried out according to WHO standard protocols. The six concentrations of the Bti product used in each test were replicated 4 times with 25 mosquito larvae. Probit analysis was then used to determine the LC 50 and LC 95 which was equated with dosages of 1.02 and 1.86 ppm for Ae. aegypti; and 0.39 and 0.84 ppm for Ae. albopictus, which reveals a potency of 382.95 and 303.74 ITU/mg, respectively. The semi-field evaluation of this product in 200-liter earthen jars against 3 rd instar larvae of Ae. aegypti showed satisfactory control of greater than 80% at 11 weeks post-treatment. chemicals for larvicide and space spraying, per- sonal protection, health education and source reduction with the aim of immediate removal of infected mosquitoes. Insecticide use still remains a major component of any control strategy, es- pecially during an outbreak. Chemicals frequently used are those belonging to the organophos- phate and pyrethroid classes of insecticides (WHO, 1997). With the current trends in dengue incidence worldwide and without an effective vaccine or treatment, it is expected that the wide- spread use of insecticides will continue. This practice will likely lead to the selection of resis- tant strains, rendering current insecticides less effective, leading to the need to identify replace- ment control strategies. Bacillus thuringiensis var. israelensis ( Bti) can be used to prevent these vectors from breeding (Marin/Sonoma Mosquito and Vector Control District, 2005). Controlled semi-field studies show that Bti can be effective for about 7 to 12 weeks (Mulla et al, 2004; Vilarinhos and Monnerat, 2004) in undisturbed conditions. We conducted laboratory investiga- tions to determine the concentration-response