Timing of Treatments for Control of the Alfalfa Weevil in Northern California1

Abstract

Field trials were conducted during 1961-63 in mountainous northern California for control of the alfalfa weevil, Hypera postica (Gyllenhal), by pre-ovipositional applications of granulated and liquid insecticides. Excellent control was afforded by heptachlor granules at 0.25 Ib active material per acre and by granulated Telodrin® (1,3,4,5,6,7,8,8-octachloro -1,3,3a,4,7,7a hexahydro -4,7-methanoisobenzofuran) at 0.125 lb. With these low dosages, the optimum time for treatment was prior to any sign of new growth of alfalfa in the spring. Neither General Chemical4072 (2-chloro-l- (2,4-dichlorophenyl) vinyl diethyl phosphate), lmidan® (O,O-dimethyl S-phthalimidomethyl phosphorodithioate), nor Bidrin® (3-hydroxy- N,N-dimethyl.cts-croton-amide dimethyl phosphate) provided appreciable control when applied in the early spring. A population of the alfalfa weevil highly resistant to heptachlor was found in Lassen County and there were indications that this population was resistant also to certain other chlorinated hydrocarbon insecticides.