Abstract
Changes in benthic macro-invertebrates and concentrations of pesticide residues were surveyed at a station in the Hinuma River from April 1990 to December 1991, focusing especially on the periods before and after transplantation of rice plants into paddy fields bordering the river. Diazinon, fenthion, butachlor and oxadiazon reached their peak concentrations (2.3-3.1 μ g L-1 for insecticides, 5.2-5.5 μ g L-1 for herbicides) soon or one month after rice transplantation. The other pesticides (e. g. fenitrothion, pretilachlor) were detected with peak concentrations below 1 μ g L-1 at the site. The density of Cheumatopsyche brevilineata (Trichoptera) larvae increased markedly, but larvae of Hydropsyche orientalis (Trichoptera) decreased, after diazinon had reached its maximum concentration at the site. The 48-h LC50 values of insecticides and herbicides for the final instar larvae of both species were higher than those of concentrations detected in the river, However, H. orientalis was more sensitive than C. brevilineata to diazinon and fenthion (48-h LC50, 6.8 and 280 μ g L-1, 4.4 and 420 μ g L-1, respectively).
