Laboratory and field experiments with methoxychlor as a larvicide for simuliidae (diptera)

Abstract

The dispersion of methoxychlor and Rhodamine B dye in the Chalk River, Canada, was measured by chromatography and fluorometry. The accumulation of methoxychlor was studied in laboratory and field experiments. The river was treated for 15 min with an oil solution of methoxychlor (0·79 μg/litre). The numbers of drifting simuliid larvae reached maximum values 60 min after the start of treatment at stations 275 m and 550 m downstream from the point of application. The peak numbers decreased with distance downstream, and most of the larvae began to drift after the methoxychlor had passed by. No methoxychlor could be detected in water, larvae or moss collected from the Chalk River before the experiment, but drifting larvae caught after the treatment contained residues ranging from 0·24 to 2·57 mg/kg. The larvae which began drifting later generally contained more methoxychlor than those drifting soon after treatment. S. venustum began drifting sooner than S. vittatum, and the mean sizes of drifting larvae of both species tended to increase as time passed. Residues of methoxychlor ranging from 7·4 to 34·6 μg/kg were detected in moss and grasses in the river for up to 8 weeks after the treatment. Laboratory experiments indicated that simuliid larvae concentrate particulate formulations of methoxychlor more efficiently than ethanol ones. Residue values in the larvae reached 2310 μg/kg. Trichoptera larvae concentrated methoxychlor to levels up to 1563 μg/kg, and ethanol formulations to higher levels than particulate ones.