1987 Volume 38 Issue 4 Pages 303-316
An insect growth regulator diflubenzuron was applied to the Kokawa River at the concentration of 1.25ppm for an hour to control simuliid larvae (Ogata et al., unpublished). On that occasion, the effects of the chemical on both invertebrate communities and fish were assessed. Weekly sampling of invertebrates and fish was conducted till the 4th week after the application in both treated and untreated region. Most of invertebrates were eliminated within 2 weeks, while Hydropsycidae died out gradually. Adults of Elmidae, which had not found before, appeared 1 week after in large number at the uppermost of the treated region. Fast recovery of Baetis at the same place was recognized. Downstream drifts from untreated region accounted for this result. While recolonizations by newly hatched larvae of Baetis, Chironomidae, Antocha and Simuliidae were prominent 3-4 weeks after in all the treated region. These fast recolonizers reached abnormally higher densities. On the other hand, most of caddisflies and mayflies had not recovered by the 4th weeks. Since application of diflubenzuron induces an enormous increase in target dipteran larvae including simuliids, once this chemical is used in a river, frequent applications are required subsequently. No fish mortality was observed on Phoxinus lagowski f. steindachneri and Leuciscus hakonensis. Both adults and fry of the former were commonly found during the study periods. Their condition factors increased after the application, suggesting that this species fed on attached algae which became abundant in the treated region.
