Efficacy of mixing vegetable oils with pirimiphos-methyl against the maize weevil, Sitophilus zeamais Motschulsky in stored maize

Sitophilus zeamais is a major insect pest of stored maize and grain products in the tropics. The toxicity of coconut, groundnut and soybean oils applied at 1, 2, 5 and 10 ml/kg and pirimiphos-methyl at 2.5, 5 and 10 μl active ingredient in 2 ml of water/kg of grain (116, 18 and 14 of the recommended dose), alone or in combination, to adults and immature stages of S. zeamais, the persistence of the treatments in maize grains and their effects on seed viability were evaluated in the laboratory. All the treatments caused significant mortality compared to untreated controls. Low dosages of the oils and pirimiphos-methyl when combined were highly toxic to adult S. zeamais. The mixtures completely inhibited the development of immature stages of the weevil. Only pirimiphos-methyl and the mixtures retained some activity 60 days after application. Pirimiphos-methyl can be used at reduced rates if combined with vegetable oils to control infestations of S. zeamais in stored maize.