TOXIC EFFECTS OF TRIORGANOTIN COMPOUNDS ON THE ANOPHELES STEPHENSI MOSQUITO AND LARVAE

I N T R O D U C T I O N It is well documented that t r iorganotincompounds (RjSnX) have a wide range of biocidal activities and the toxicity is species specific. For example, trimethyltin compounds have a high toxicity towards insects and mammals." ' Changing the organic group to butyl results in compounds that are highly toxic towards grampositive bacteria, fish, fungi and mollusks. ' "' Due to their effectivenessas fungicides, there are numerousstudies of tr iorganotins as ag rochemica l s . 4 " The impetus forinsecticidal evaluation of organotin compounds is also primarily related to their role as agrochemicals. Mosquitoes are by far the most important blood sucking arthropods worldwide. They are responsible for the transmission of diseases to humans such as malaria and yellow fever. Malaria is one of the most widespread infectious diseases in the world. More than 40% of the wor ld ' s population lives in tropical areas where they are at risk for malaria transmission. Approximately 400 million people are infected each year, with an estimated I 2 million deaths per year, primarily among children under five years of age. Mosquitoes in the genus Anopheles are responsible for the transmission of malaria. Because the use of insecticides remains the primary method of most countries' mosquito control programs, the development of a more effective larvicide/insecticide to combat the Anopheles mosquito would be of worldwide interest. Triorganotins. as a class, have been found to be effective larvicides against the Anopheles stephensi mosquito when compared to some synthetic larvicides. To evaluate the total effect ivenessoftr iorganotinsas possible larvicides/insecticides, residual activity and delayed effect studies were conducted since these studies would give an indication of the long term effectiveness of the compounds Delayed effects and residual activities of 19 triorganotins are reported herein. Also reported are adult mosquito contact studies for the triorganotins. M A T E R I A L S A N D M E T H O D S Preparation of the test compounds All the t r iorganot incompounds wereobtained commercially and used without further purification. The triorganotin stock solutions, which ranged from 25 1000 parts per million (ppm), were prepared by dissolving the compound of interest in either acetone, d imethylsulfoxide(DMSO) or 95% ethanol. The dissolution of the triorganotins in the organic media was to facilitate the dispersion of the compounds in water. The acetone and D M S O was spectrograde quality while the 95% ethanol was reagent grade. Anopheles stephensi Mosquito larvae and adult mosquitoes were obtained from the laboratory of Dr. Daniel Str ickman, Chief , Entomology Department at the Walter Reed Army Institute of Research, Washington, DC. All mosqui to and larvae were maintained in an environment chamber at 27 28 °C with a humidity of 80 90%. Adult mosquitoes were maintained on a 10% sucrose solution and growing larvae were fed finely ground dog food. Residual activity studies Twenty-five larvae were used in each experiment to test the residual effects of the triorganotin compounds . The larvae were placed in 200 mL of the triorganotin solution of interest at a concentration that represented the LC50 value. After 24 h of exposure, the mortality of larvae was determined by visual counting.