Insecticidal Control of the Alfalfa Weevil1 in Northern Utah and Some Resulting Effects on the Weevil Parasite Bathyplectes curculionis2

Abstract

Since 1962, annual experiments for control of Hypera postica (Gyllenhal) have been conducted in northern Utah. During most of these seasons, the effects on the major parasite, Bathyplectes cuculionis (Thomson), were also evaluated. Only 2 materials, carbofuran and phorate, showed promise when applied at the time alfalfa growth started in the spring. These early-season treatments had little or no effect on the parasite. Many materials controlled weevil larvae when applied during late May about 2 to 3 weeks before the 1st cutting. Carbofuran, methyl parathion, parathion, diazinon, GS-13005 ( O,O -dimethyl phosphorodithioate S-ester with 4- (mercaptomethyl) -2-methoxy ∆2-1,3,4thiadiazolin-5-one), azinphosmethyl, and methoxychlor usually gave 80 and 95% reduction in larval numbers, evaluated just before harvest. All treatments applied before cutting reduced the total adult parasite numbers, but only a few longer residual insecticides lowered the percentage of parasitized larvae. The parasite adults returned quickly in plots treated with the shorter residual materials. No treatments killed the parasitized larvae more readily than the unparasitized. Postharvest treatments applied to the stubble during late June or early July caused alfalfa growth to respond quickly, but the actual control was hard to measure. Carbofuran, Imidan® (O, O-dimethyl S -phthalimidomethyl phosphorodithioate), malathion, parathion, methyl parathion, and GS-13005 were all effective. Damage to the parasites was slight, although there was sometimes significant reduction in numbers following stubble treatments. Imidan, methoxychlor, and trichlorfon were generally less damaging to the parasites than the other insecticides tested. Trichlorfon was the least toxic to the parasites, but was ineffective in weevil control.