The degradation/persistence of residues of the organophosphorus insecticide pirimiphos-methyl was determined in corn and popcorn grains and in some of their processed products such as grain, bran, hominy and corn flour from corn and prepared popcorn and grain from popcorn. The action of the insecticide on the control of the corn weevil, Sitophilus zeamais Mots., was also evaluated. The grains were treated with the concentration of 12 mg.kg-1 (ppm) a.i. pirimiphos-methyl for the control of the weevils. Samples were taken at zero, 15, 30, 60, 120 and 240 days after treatment. Quantitative measurements were made by gas chromatography, by using a flame photometric detector. For pirimiphos-methyl residual action studies, samples of both types of grain were taken at 15, 30 days after treatment and subsequently at monthly intervals up to one year. Treated and non-treated (control) grains were infested with weevil adults. Mortality was checked 15 days after exposure. Pirimiphos-methyl residues were not persistent in either types of grains or their processed products. Residues dropped 5- 8-fold in corn bran and popcorn. They were stable in hominy and corn flour, but the concentration levels were low. Higher residues in corn bran than in grain are related to the higher oil content in that processed product. The insecticide remained effective against the pest over the entire period of observation (1 year) for both types of grains.
Pesticide residue; degradation; corn weevil
