Abstract
Insecticide resistance level of two different colonies of the larva of Simulium ornatum was compared. One colony was taken at Chino, Nagano Prefecture, where frequent control by DDT or diazinon was made for more than five years, another colony was taken at Saku, Nagano Prefecture, where no insecticide control was made due to the regulation of local fish culture. A rather simple test method was adopted and satisfactory results were obtained. Larvae are dipped in each the emulsions for ten minutes and mortality was observed after 24 hours in aerated water. The Chino-colony showed in LC_<50> about 10 fold greater in DDT, diazinon and fenthion than that of Saku-colony. These results may be due to the fact that the Chino-colony has experienced past insecticide pressure. At present the following sequence in the efficiency of these insecticides can be mentioned for this species. fenthion=fenitrothion>diazinon>trichlorfon>DDT
