Larvicidal Efficacy of Temephos against Aedes Aegypti

Three new formulations of temephos (LAVIFOS SG 1%, MOSQ SG 1% and AZAI-SS ZG 1%) were evaluated for larvicidal efficacy against larvae of Aedes aegypti (L.) in waterstorage jars under field-simulated conditions. LAVIFOS SG 1% and MOSQ SG 1% are sand granule formulations, whereas AZAI-SS ZG 1% is zeolite granule formulation. Each formulation contained 1% temephos as an active ingredient. Each formulation was packed in a non-woven sachet at quantity of 20 g per sachet and placed in a 200-liter glazed clay jar to obtain a dosage of 1 mg/l (one sachet per jar). Each treatment and control (jar without larvicide) was replicated four times. A concurrent set of treatments and controls were carried out in parallel, but the water in each treated and control jars was removed and refilled weekly. All jars (treatment and control) were challenged weekly by adding 25 third-instar larvae per jar and assessment was made of larval mortality by counting pupal skins one week after the addition of larvae. The three formulations provided complete larvicidal efficacy (100%) for at least 24 weeks post-treatment (the length of this study). In the jars where all the water was removed and refilled weekly, LAVIFOS SG 1%, and MOSQ SG 1% provided complete larvicidal efficacy for at least 24 weeks post-treatment, whereas AZAI-SS ZG 1% showed complete larvicidal efficacy for 16 weeks post-treatment. AZAI-SS ZG 1% still demonstrated a high degree of larvicidal activity (93-99%) from 17 to 24 weeks post-treatment. The present study reveals an excellent residual efficacy of the three new formulations of temephos against larvae of Aedes aegypti in water-storage jars lasting for at least 16 to 24 weeks post-treatment. These new formulations will make the control of DHF vectors in Thailand more cost effective as they are removable and retrievable sachets that can be reused after cleaning the water-storage containers. SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 642 Vol 38 No. 4 July 2007 storage containers (Bang and Tonn, 1969a,b; Bang et al, 1972). Until now, the temephos sand granule formulation (1%) is still an effective larvicide against Ae. aegypti larvae. In practice, temephos sand granules possess two negative features: they cause an objectionable odor and water turbidity (Thavara et al, 2001). These factors cause objection by household members regarding the use of temephos sand granules in their water-storage containers. As a result, a new formulation of temephos zeolite granules was recently developed to overcome these disadvantages. Mulla et al (2004) revealed the newly developed formulation of temephos zeolite granules lacked these undesirable characteristics and possessed high residual efficacy against Ae. aegypti larvae for more than six months under field-simulated conditions. This formulation also provided high larvicidal activity for at least three months in village-scale trials under normal water-use practices with high acceptance by villagers (Thavara et al, 2004). However, loss of granules during the process of cleaning water-storage containers is another problem with granular formulations of temephos larvicide found in the field (Thavara et al, 2004). As a result, Thavara et al (2005) initiated an experiment to show that temephos 1% sand granules enclosed and tied in a muslin cloth and placed in 200-liter water storage jars then transferred sequentially to new sets of jars four time successively provided high larvicidal efficacy (92-100% emergence inhibition) for about five months or longer. It was therefore necessary to develop a new effective formulation of temephos larvicide that is retrievable during the cleaning process, which is more practical and desirable. This study was carried out to evaluate the larvicidal efficacy of three new formulations of temephos larvicides packed in non-woven sachets against the larvae of Ae. aegypti in water-storage jars under field-simulated conditions. The results obtained from this study could provide information regarding a cost effective strategy for the control of Ae. aegypti larvae in Thailand and elsewhere. MATERIALS AND METHODS