Relative Toxicity of Five Insecticides to Alfalfa Weevil Larvae

The alfalfa weevil, Hypera postica (Gyllenhal), was first recorded in Missouri during 1964, and by 1966 it had to 50 counties in the State. Field comparison of insecticides for control of weevil larvae showed methyl parathion at 0.5 Ib, methoxychlor at 1.5 Ib, azinphosmethyl at 0.5 Ib, and malathion at 1.25 Ib per acre be equally effective. Laboratory comparisons using topical application techniques showed methyl parathion to be the most toxic compound tested against fourth-instar alfalfa weevil larvae, followed by azinphosmethyl, malathion, diazinon, and methoxychlor. The test population was more homogeneous in response to azinphos, methyl and methyl parathion than to diazinon, malatothion and methoxychlor after 48 hours.