Single 15-min injections of 0.18 to 0.24 p.p.m. of methoxychlor as an emulsifiable concentrate into the Saskatchewan River removed 75 to 99% of the larvae of Simulium arcticum Mall. from 23 to 34 km sections of the river. Stronger concentrations (0.30 to 0.31 p.p.m.) eliminated about 98% of the larvae from rapids 64 km distant and an estimated 46% at a distance of 139 km. Other kinds of aquatic larvae in exposed sites were also affected: Plecoptera to about the same degree as the Simuliidae, followed by Ephemeroptera, Trichoptera, and Chironomidae (least affected). Larvae in sand were not affected. Populations of Chironomidae, Trichoptera, and Ephemeroptera in many sites were completely restored within 7 to 14 days after a single treatment. Fish were apparently unharmed. Residues were negligible and short lived.
Potential outbreaks of S. arcticum were averted by all of the experimental injections of methoxychlor except that of 26 May 1970. In this test an outbreak that killed four animals occurred because a 15-min, 0.207 p.p.m. dosage eliminated only about 23% of the black fly larvae in an adjacent portion of the river.