Studies on chemical control of a nuisance chironomid midge (Diptera : Chironomidae) : 3. Susceptibility of Chironomus yoshimatsui collected from various rivers to the three kinds of organophosphorus larvicides and larval control trials with chlorpyrifos-methyl in the fields

In the laboratory, three organophosphorus insecticides, fenthion, temephos, and chlorpyrifos-methyl were bioassayed against 4th instar larvae Chironomus yoshimatsui collected from ten polluted rivers in the areas of Tokyo, Chiba, Kanagawa, Saitama, Shizuoka, Aichi, Ehime, and Fukuoka. In most of these rivers, practical control of the midge has been achieved by dripping the larvicides at 1ppm concentration maintained for 60min, but recently the resistance to fenthion in C. yoshimatsui was suspected because of the failure to achieve practical control especially in the Kanda River of Tokyo. The LC_<90> level (48-hr exposure) by fenthion was >0.4ppm against larvae inhabiting most of the rivers; the larvae in four rivers showed similar resistance levels to temephos. In all rivers, the larvae were relatively susceptible to chlorpyrifos-methyl (LC_<50><0.015ppm). Field treatments of a WP formulation of chlorpyrifos-methyl by continuous dripping at rates of 1ppm for 30min, 1.5ppm for 30min, and 1ppm for 60min in seven polluted rivers resulted in excellent larval control of Chironomus yoshimatsui. The larvae were observed to drift downstream immediately after the insecticidal application. The insecticide was carried downstream to more than 800m by the water current. The treatment rate of 1ppm for 60min had no adverse effects on oligochaete worms, Tubifex tubifex and Branchiura sowerbyi, and the gastropod, Physa acuta.