Abstract
Since 1972 the chironomid midges, prevalent at the near renovated urban worst rivers, have been becoming a serious nuisance in this country. By the light traps run at the infested sites of the river Kanda, Tokyo, 3 species of the midges referable to 2 genera were recognized. The dominant species was always Chironomus yoshimatsui Martin & Sublette. According to the results obtained in 1972-1973 C. yoshimatsui emerge out during a long period from late March to mid December showing definite peaks of emergence in every 2-3 weeks, the maximum population size reaching in late August-September. One adult emergence cycle is approximately in accord with that obtained in laboratory. In 1974 higher peak in prevalence showing in late June-early July and following peak appeared in September. On the other hand large numbers of the midges were prevalent successively from mid June through October and a curve of plateau has been showing in the trend of emergence since 1975. It was also confirmed experimentally that the range of infested area is within 100 meter distance along the river side.
