Outbreak of chironomid midge in a polluted river and the chemical control of the larvae

Abstract

Some investigations on the chemical control of the larvae of chironomid midges were carried out in the Suimon river which runs through the center of Ohgaki City, Gifu Prefecture. The river was so heavily polluted due to drainage from the factories that no midge larvae could bread there before the Water Pollution Control Law was enforced in 1971. The midge-swarm problems arose in Autum 1973,because of the enforcement of regulation of the drainage from the factories which improved the biological oxygen demand in the water from 85ppm in 1970 to 12ppm in 1974. The density of the larvae per 40(cm)^3 (20(cm)^2×2cm in depth) of the bottom mud was estimated from 126 to 186 before the river was treated with temephos. In the laboratory test, 45% of the tested larvae was killed and the survivals were intoxicated at 12 hours after the treatment when they were dipped into 0.8ppm (AI) of temephos WP for 60 minutes. In the field test, the river was treated with the temephos WP for 60 minutes at 1ppm of the flowing water volume. At 9 days after the treatment, no midge larvae were found at about 1.6km downstream from the application spot. The temephos showed no poisonous effects on a loach up to 10ppm in the laboratory test. A loach, carp, crucion and top minnow habitating in the river were not affected by the temephos treatment.