Evaluation of Insecticides as Protectants against Pests of Stored Grain and Seeds1

Abstract

Results are presented from a series of tests with varying dosages of each of 86 insecticides in acetone solution sprayed on wheat for protection against adults of the rice weevil, Sitophilus oryza (L.), granary weevil, Sitophius granaries (L.), and confused flour beetle, Tribolium confusum Duv., and larvae of the khapra beetle, Trogoderma granariun Everts. Mortalities of adult insects recorded after an exposure of 14 days and of khapra larvae after an exposure of 28 days to treated grain was the only criterion used to assess the effectiveness of insecticide deposits of 1.25, 2.5, 5, 10, 15, 25, 50, 100, and 200 p.p.m. Malathion, owing to its present use as a grain protectant under existing residue tolerances, was considered the standard insecticide in making comparisons between compounds. Because of the resistance of khapra larvae to insecticides, only organic phosphates, but not all of those tested, were effective against all the species of test insects. Methyl parathion, phorate, parathion, and Phosdrin@ (2-methoxycarbonyl-1-methylvinyl dimethyl phosphate) were more effective than malathion (10 p.p.m.) as indicated by lower dosages required for 100% mortality of all species of the test insects, while Am. Cyanamid 12009 ( O,O -diethyl S-n -propylthiomethyl phosphorodithioate), DDVP, Diazinon® ( O,O -diethyl O -(2-isopropyl-6-methyl-4-pyrimidinyl) phosphorothioate), and Dibrom®(1,2-dibromo-2,2-dichloroethyl dimethyl phosphate) were equally as effective. Although less efficient than malathion in preliminary tests, the desirability for further evaluation of Guthion® ( O,O -dimethyl S -4-oxo-l,2,3-benzotriazin-8 (4H)-ylmethyl phosphorodithioate), ronnel, and dimethoate was indicated by the relatively high mortalities of khapra larvae exposed to wheat treated with these compounds. By excluding khapra larvae and considering only the results from tests with the rice weevil, granary weevil, and confused flour beetle, more compounds are favored. Methyl parathion, phorate, parathion, dieldrin, Guthion, Am. Cyanamid 12009, Chlorthion® ( O -(3-chloro-4-nitrophenyl) O,O -dimethyl phosphorothioate), Dibrom, endrin, and Phosdrin were more effective than malathion (10 p.p.m.) as indicated by lower dosages required for 100% mortality of these three species of insects. DDVP, Diazinon, and dimethoate were equally as effective, while lindane and ronnel were slightly less effective than malathion. DDT and related compounds were among the least toxic to all insects, and Sevin® (1-naphthyl methylcarbamate) was the least effective of the 36 compounds tested, failing to cause 100% mortality of test insects of any species with the maximum dosage of 200 p.p.m.