Insecticide susceptibility of mosquitoes invading tsunami-affected areas of Thailand.

In order to control the mosquitoes invading tsunami-affected areas of Thailand, the insecticide susceptibility status of field larvae and mosquitoes (Anopheles sundaicus and Culex sitiens) was tested under laboratory conditions. Larval bioassay tests were conducted using the WHO standard method. Three larvicides: temephos, malathion, and plant extract (ethanolic extract of the Southeast Asian long pepper. Piper retrofractum Vahl), were used in the experiments. The results revealed that Cx. sitiens was more susceptible to temephos than malathion and the plant extract, with LC50 ranges of 0.0008-0.0014 mg/l, 0.0046-0.0078 mg/l, and 5.3180-10.1030 mg/l, respectively. Cx. quinquefasciatus showed greater tolerance to every tested larvicide than Cr sitiens. Adult bioassay tests using a WHO test kit and diagnostic doses of 5% malathion, 0.75% permethrin, 0.05% deltamethrin, and 4% DDT were also conducted. The results revealed that Cx. sitiens and An. sundaicus were susceptible to all tested insecticides. The LT50 of 5% malathion ranged between 25.7-26.0 minutes for Cx. sitiens, and 44.7 minutes for An. sundaicus. In addition, Cx. quinquefasciatus showed susceptibility to malathion, with LT10 of 19.7 minutes. However, it showed resistance to both pyrethroid insecticides, with LT50 of 33.1 minutes for 0.75% permethrin, and 19.6 minutes. for 0.05% deltamethrin; it showed low percentage mortality at 24 hour post-exposure, of 48 and 32%, respectively. In conclusion, every tested larvicide could be used for controlling Cx. sitiens larvae, even in brackish water, pyrethroid insecticides for adult Cx. sitiens and An. sundaicus, and malathion for all three species.

Formulation of tablets from the crude extract of Rhinacanthus nasutus (Thai local plant) against Aedes aegypti and Culex quinquefasciatus larvae: a preliminary study.

Dried root powder of Rhinacanthus nasutus, Thong Phan Chang (Thai name) were extracted with methanol (MeOH) in a Soxhlet apparatus and made into 2 formulations of tablet containing the extract at 5% and 10% concentration. Due to the viscous and poor flow properties of the crude MeOH extract obtained, a wet granulation method was conducted in developing the tablets. Lactose was used as a filler. Polyvinyl pyrrolidone (PVP) K30 (15% w/w solution in alcohol) was used as the binding agent, while stearic acid (2% w/w) was used as a lubricant. Both formulas of prepared tablets had a smooth shiny surface with a round shape. Other physical properties of the tablets, such as weight variation, friability and disintegration time, met the requirements of the USP XX standard. The mosquito larvicidal activity of prepared tablets containing 5% and 10% R. nasutus extract against Aedes aegypti were not significantly different from each other (p > 0.05), with 48-hour LC50 values of 13.6 and 14.2 mg/I for the 5% and 10% tablets, respectively, while their activities against Culex quinquefasciatus were similar (p > 0.05) with LC50 values of 18.7 and 17.3, respectively. The larvicidal activity levels against Ae. aegypti and Cx. quinquefasciatus were also not significantly different from each other (p > 0.05). No larval mortality was observed in the two control groups: lactose solution and dechlorinated water. Toxicity to female and male fish (Poecilia reticulata) was tested with the prepared tablets. The toxicity of tablets containing 5% and 10% extracts were not significantly different from each other for the P. reticulata females with 48-hour LC50 values of 105.2 and 110.8 mg/I, respectively, and for P. reticulata males with LC50 values of 99.1 and 103.4 mg/I, respectively. Female and male P. reticulata were sensitive to the same dose of the extract. No fish died in the two control groups, with lactose solution and dechlorinated water. Acute-toxicity bioassay with fish showed that with an exposure of 48 hours the LC50 values of the tablets containing 5% and 10% were 5- to 10-fold higher than the LC50 of R. nasutus against mosquito larvae. These prepared tablets could possibly used to control mosquito vectors and be introduced into the mosquito control program.

Efficacy of repellent products against caged and free flying Anopheles stephensi mosquitoes.

The efficacy of 9 repellents (8 commercial repellents and one product under development) was evaluated on the skin at dosages of 0.65 and 1.7 mg of product/cm2, the latter dosage being the industrial standard for deet based repellents. The repellents were applied to the arm or lower leg of a human subject and tested against Anopheles stephensi in a cage or flying freely in a mosquito-proof room. In the cage tests, a product with 20% p-menthane-3, 8-diol (PMD) active ingredient provided complete repellency for 7-8 hours, while with 10% PMD had complete repellency for only 30 minutes. The natural oils of clove (Syzygium aromaticum) (10% active ingredient) plus makaen (Zanthoxylum limonella) (10% active ingredient) gave protection for 4-5 hours. In the case of free flying mosquitoes, products with 20% and 30% PMD gave complete protection for 11-12 hours at a dosage of 1.7 mg/cm2 or 6 hours at half the dosage, while the product with 10% PMD afforded protection for less than 2 hours. At the higher dosage rate 40% citronella and hydroxyethyl isobutyl piperidine carboxylate, a new synthetic compound, provided complete repellency for 7 hours. Fifty percent deet (N,N-diethyl-3-methylbenzamide) was effective for 30 hours if left undisturbed on the skin.

Screening for larvicidal activity in some Thai plants against four mosquito vector species.

Ninety-six ethanolic extracts from various parts of 84 Thai plant species were tested for their larvicidal activity against Aedes aegypti mosquitoes. Extracts from Rhinacanthus nasutus, Derris elliptica, Trigonostemon reidioides, Homalomena aromatica, Stemona tuberosa and Acorus calamus possessed high larvicidal activity, with LC50 values between 16.0 and 48.2 mg/l. Petroleum ether (PE) and methanol (MeOH) extracts were tested for their larvicidal activity against 4 mosquito vector species. The PE extract of R. nasutus exhibited larvicidal effects against Ae. aegypti, Culex quinquefasciatus, Anopheles dirus and Mansonia uniformis with LC50 values between 3.9 and 11.5 mg/l, while the MeOH extract gave LC50 values of between 8.1 and 14.7 mg/l. D. elliptica PE extract showed LC50 values of between 11.2 and 18.84 mg/l and the MeOH extract exhibited LC50 values between 13.2 and 45.2 mg/l.

Laboratory and field trial of developing medicinal local Thai plant products against four species of mosquito vectors.

Oils of Syzygium aromaticum (clove) and Zanthoxylum limonella (makaen), widely used essential oils for dental caries or flavoring of food in Thailand, were prepared as 10 experimental repellent products in gel or cream form against Aedes aegypti, Culex quinquefasciatus, and Anopheles dirus under laboratory conditions, using the human-arm-in-cage method. Two products that gave the longest-lasting complete protection were selected to examine their repellency against a variety of mosquito species under field conditions. In laboratory tests, 0.1 g of each product was applied to 3x10 cm of exposed area on a volunteer's forearm, while in field trials, 1.0 g was applied to each volunteer's leg (from knee to ankle). In the laboratory, the gel dosage form contained 20% clove oil (Gel B) or 10% clove plus 10% makaen oil mixture (Gel E) were promising plant-based repellents against three mosquito species and gave significantly longer complete protection times of 4-5 hours than all other developing products. Therefore, their efficacy in the field was evaluated. Under field conditions, Gel E showed complete protection for 4 hours and gave 95.7% repellency after 5 hours application, whereas Gel B and 20% deet (di-methyl benzamide) provided only 86.8 and 82.7% repellency after treatment, respectively against Ae. aegypti, daytime-biting mosquitos. For nighttime-biting, the 3 repellents under development yielded equally excellent (average 97.1%) repellency for 5 hours against the predominant Cx. quinquefasciatus and Mansonia uniformis, but they gave 89.0% repellency against Cx. tritaeniorhynchus and Cx. gelidus. This finding demonstrated the effectiveness of Gel B and Gel E products for possible use by low-income rural communities against various mosquito species.