Larvicidal and adulticidal activities of castor oil against the dengue vector, Aedes aegypti

@#Plants contain numerous bioactive compounds that can be used to develop environmentally-safe insect control agents. Castor oil is a vegetable oil derived from the seeds of castor bean and is widely used as an industrial lubricant and medicinal purgative. In search of an alternative natural insecticide, the objective of this study was to evaluate the larvicidal and adulticidal activities of castor oil against the important dengue vector, Ae. aegypti. Larvicidal and adulticidal bioassays were conducted following the World Health Organization methods. Larvicidal activity was observed at castor oil concentrations of 10, 25, 50, 75, and 100 ppm; larval mortality was checked after 48 h of exposure and the lethal concentration (LC) at LC50 and LC90 were 51.38 and 116.26 ppm, respectively. Adulticidal activity was determined by topical application at the concentrations of 1, 5, 10, 15, and 20 µg/ mg on female mosquitoes and the mortality was checked after 24 h of exposure. The effective adulticidal activity was apparent with the LD50 and LD90 values of 6.03 and 25.07 µg/mg against female mosquitoes. The results indicated that castor oil has potential in the practical control of both immature and adult stages of the mosquito vector.

Seasonal distribution of phlebotomine sand flies (Diptera: Psychodidae) in Tham Phra Phothisat temple, Saraburi province, Thailand

Phlebotomine sand flies have long been incriminated as vectors of leishmaniasis in various parts of both the Old and New World. Prompted by recent indigenous cases of leishmaniasis in Thailand, a bionomic study of sand flies was undertaken in Tham Phra Phothisat temple, Saraburi province. In this study, sand flies were collected using Centers for Disease Control (CDC) light traps, to clarify the activity patterns and species composition of the sand flies. Traps were laid from August 2005 to July 2006. The insects were collected monthly between 1800-0600 hours. A total of 8,131 sand flies were collected with a female:male ratio of 1.9:1. Sixteen species were identified, of which 5 belonged to the genus Phlebotomus, 9 to Sergentomyia and 1 to Chinius. Species comprised the abundant species (Sergentomyia silvatica 35.6%, Sergentomyia barraudi 18.1%, Sergentomyia anodontis, 17.1%, Sergentomyia iyengari 11.9%, and Sergentomyia gemmea 11.2%); the less common species (<2%) were Sergentomyia dentata 1.8%, Phlebotomus stantoni 1.1%, Sergentomyia indica 1.0%, Phlebotomus argentipes 0.8%, Sergentomyia perturbans 0.4%, Chinius barbazani 0.3%, Phlebotomus asperulus 0.2%, Phlebotomus philippinensis gouldi 0.1%, Phlebotomus major major 0.1%, Sergentomyia quatei 0.1% and Sergentomyia bailyi 0.1%. The results revealed seasonal variation in sand fly prevalence, with the highest peak in July. Soil samples collected were characterized by alkaline (pH 7.6).

Seasonal variation of cave-dwelling phlebotomine sandflies (Diptera:Psychodidae) in Phra Phothisat Cave, Saraburi Province, Thailand.

Phlebotomine sandflies are widely distributed in Thailand. In view of recent occurrence of indigenous cases of leishmaniasis in Thailand, a bionomic study of sandflies was undertaken in Phra Phothisat cave, Saraburi Province, Thailand from August 2005 to July 2006. The insects were collected monthly by CDC light traps between 06:00 PM and 06:00 AM. They were preserved in 80% alcohol and mounted with Hoyer's medium for species identification. A total of 5,514 sandflies were collected with a female:male ratio of 2.5:1. The collected sandflies belong to 13 species in the genera Phlebotomus and Sergentomyia, namely S. silvatica (43.5%), S. anodontis (31.5%), S. dentata (15.3%), S. barraudi (3.3%), P. argentipes (2.0%), P. philippinensis gouldi (1.0%), P. stantoni (0.5%), S. gemmea (0.5%), P. major major (0.1%), S. perturbans (0.1%), S. iyengari (0.1%), S. bailyi (0.1%), and P. teshi (0.1%). The results revealed seasonal variation in sandflies with the highest peak in July (436 sandflies/trap-night). Some of the sandflies could not be identified and were assumed to be new species. Soil samples inside the cave were analyzed for chemical characteristics. The soil was characterized by moderate acidity (pH 5.8) with various amount of chemicals and nutrients.

Three indigenous Thai medicinal plants for control of Aedes aegypti and Culex quinquefasciatus.

The potential larvicidal activity and insect growth regulator (IGR) properties of three selected indigenous medicinal Thai plants were tested against two species of mosquito with special reference to the late 3rd and early 4th instar larvae (L3 and L4, respectively). In case of larvicidal activity, Thevetia peruviana was the most potent, followed by Pueraria mirifica, and Butea superba was the least effective. In all cases, the late 3rd instar was more susceptible than the early 4th instar larvae, and the 48-hours exposure yielded more potent larvicidal activity than 24-hours exposure. However, at sublethal dosages, both P. mirifica and B. superba showed some dispersed effects interfering with ecdysis. A variety of toxic effects were observed and recorded in eight categories according to the stage of metamorphosis when death occurred. P. mirifica rendered the main deleterious effects in the pupa-adult period in both instar of Ae. aegypti and Cx. quinquefasciatus, whereas B. superba showed highest effect in black-pupa period of the late 3rd instar larval stage. The results were reversed for the early 4th instar larvae of both species of mosquito as the main effect appeared in the pupa-adult category. The overall results indicated that T. peruviana did not show any IGR properties; whereas, P. mirifica and B. superba seemed to exhibit the juvenile hormone type activity which resulted in abnormal death at various stages of development. B. superba was more promising than P. mirifica, and Ae. aegypti was about 2 times more susceptible than Cx. quinquefasciatus. In addition, L3 was always more susceptible than L4 with both mosquito species.

Anopheles barbirostris/campestris as a probable vector of malaria in Aranyaprathet, Sa Kaeo Province.

As a result of dramatic increase in malaria cases in Sa Kaeo Province from 666 cases in 1995 to 4,381 in 1997, a brief entomological study was carried out during January 1998 to December, 1999 in Pa Rai subdistrict where most malaria cases were reported. Of fourteen species of mosquitos found, only Anopheles barbirostris group was the most abundant species throughout the year. Adult identification was not able to confirm species within An. barbirostris group, particularly between An. barbirostris and An. campestris because of morphological resemblance. Therefore, the barbirostris group captured in this study is reported to be either An. barbirostris or An. campestris. The seasonal prevalence of barbirostris/campestris was bimodal in distribution (September and November) and coincided well with malaria occurrence in this area. Human landing collections revealed high adult densities with 20 bites/person-night for mean indoor density and 53.5 bites/person-night for mean outdoor density. The biting peak was during 21.00-24.00 hours. Among 223 barbirostris/campestris dissected for oocysts and sporozoites only one gut from the outdoor collection in November was infected with oocysts. There were no sporozoites detected in salivary glands of all mosquitos collected. This appears to indicate that in the absence of major vectors local species may serve as potential transmitters of malaria in Thailand.

A brief survey of phlebotomine sandflies in Thailand.

Sandflies were found in widely scattered localities in Thailand in varied numbers. Daytime resting places for the adults included caves, termite hills, abandoned houses, ancient stone sanctuaries, air-raid shelters, tree hollows, loose bark of dead standing trees and rock crevices. Of fifteen species, P. major major, P. teshi, S. anodontis, S. gemmea, S. hodgsoni hodgsoni, S. perturbans, S. punjabensis and N. vietnamensis were here recorded for the first time in Thailand. P. argentipes and P. major major are interesting in view of their potential as disease vectors.

Population genetic studies on the Anopheles minimus complex in Thailand.

The population genetic studies on the Anopheles minimus complex revealed that An. minimus is a species complex evident by IK value were less than 0.0534 and 0.4163 for the morphotaxonomy and isozyme studies. There are two sibling species in the An. minimus species complex. The first is the typical minimus and its variations described by Harrison. Esterase-2 alleles 100 and 102 are predominant with a few other alleles of 96 and 104. It is exophagic and zoophilic. It is widely distributed in all provinces where minimus breeds. The second of the two sibling species is An. minimus species C, with 2 pale spots on the humeral dark band and Est-2 allele 98. It is found most frequently in Pu Toei District of Kanchanaburi Province, Thailand. An. minimus probably species B showing M1 + 2 wing vein entirely dark is found in China.

Esterase patterns of Anopheles dirus (Bangkok strain) and Anopheles balabacensis (Perlis form) in the laboratory.

Esterase isoenzyme were performed in Anopheles balabacensis (Perlis Form) and Anopheles dirus (Bangkok Strain and Kanchanaburi Strain). The zymogram showed four positions, E1, E2, E3 and E4. Seventy-five percent of An. balabacensis (Perlis Form) or An. dirus (Bangkok Strain) can be correctly diagnosed by the presence of fast band in E3 or super slow band in E3 respectively. Homozygous slow band of E3 cannot be fully distinguished in both species, but by the combination of E2 and E3, 99.42% can be correctly identified to differentiate species.