Abstract
Larvicidal effects of some organophosphorus insecticides against the larvae of the biting midge, Culicoides circumscriptus KIEFFER, were tested at the tideland of Lake Saroma, Hokkaido. The insecticides used in this test were ABATE 5% emulsifiable concentrate (EC) and 1% granule (GR), Dursban 5% EC and 3% GR and Dowco-214 10% EC and 3% GR. In this field test, Dowco-214 was the most effective. There were little larvae at 35 days after treatment of Dowco-214 EC and GR when applied at the rate of 50mg and 90mg of active ingredients per square meter respectively. Application of Dursban 5% EC was fairly effective. Effect of ABATE against the larvae of this species was insufficient. In spite of dosage as much as 120mg per m^2 applied, heavy infestation of the larvae was ovserved as before. In mid-summer, vertical distribution of the larvae in the mud was observed. The majority of the larvae was found from the mud close to the tideland surface, and there were little larvae below the depth of 2cm.
