Control of pasture Aedes mosquitoes by dripping larvicides into flowing water, with notes on residues in a pasture habitat.

Abstract

Emulsifiable concentrate formulations of several organophosphate insecticides were introduced continuously into flowing water for the control of Aedes melanimon Dyar and A. nigromaculis (Ludlow) in irrigated pastures. The irrigation water was spread over ¼- and ½-mile-long and 40- to 50-ft-wide checks. Abate® ( O, O -dimethyl phosphorothioate O, O -diester with 4,4’-thiodiphenol) at 0.1 ppm did not yield complete control of Aedes mosquitoes in a pasture, but produced 100% control at 0.17 ppm. Methyl parathion gave 100% control at 0.14 ppm and higher rates. Dursban® ( O, O -diethyl 0-3,5,6-trichloro-2-pyridyl phosphorothioate) gave 100% control at 0.07 ppm or higher rates. Fenthion gave excellent but not 100% control at 0.14 ppm, but produced complete control at 0.2 ppm rates. In general, efficacy for a given material was considered to be greater when applied by the drip method than the conventional topical applications by aircraft. The effective rate of application for ¼-mile checks was lower than those for ½-mile checks. Residues of Dursban applied to flowing water by the drip method were studied in a pasture habitat with ½-mile-long checks. Dursban introduced at the rate of 0.1 ppm was found to be 0.1-0.11 ppm in water coming out at the valves. The residue level declined as the water traveled away from the valves. Residues in water at 100 ft from the valve were 0.08 ppm and <0.01 ppm 2000 ft from the valve. The residues decreased in the water as the age of water in the field increased. Residues of Dursban in vegetation of the pasture treated with the chemical in irrigation water were rather low. The maximum residues of 0.22 ppm were found in grass sampled 100 ft from the valve 48 hr after treatment. Residues in grass declined with distance from the valves and increase in time after treatment. Residues in mud (soil) were also relatively low. The top 2 in. of soil samples had 3 to 8 times as much residues as did the next 2-in. section. Residues in soil were in general greater near the valve than away from the valve. The residues declined in time, but still 0.04 ppm were found in the top 2 in. 10 days after treatment at the 100-ft distance from the valve. The residues at the other stations were below 0.01 ppm 10 days after treatment.