Degradation and biological impact of chlorpyrifos-methyl on stored wheat and pirimiphos-methyl on stored maize in western Canada

Abstract

Insecticide degradation and pest survival were monitored in treated stored wheat and maize in southern Manitoba. Wheat stored in two wooden bins each holding 2.4 tonnes of grain was treated with the insecticide chlorpyrifos-methyl at calculated levels of 5 ppm; untreated wheat was placed in two other identical bins in the same granary. Maize was simultaneously stored in four identical bins each holding 2.3 tonnes of grain in another granary and the maize in two of these bins was treated with the insecticide pirimiphos-methyl at calculated levels of 6 ppm. Monthly samples were taken from the top and at 1 m depths of the grain bulks for 18 months. Grain surface temperatures fluctuated with ambient air conditions, but at 1 m depth lagged behind top temperatures. Moisture content of the wheat ranged from 12.2 to 15.1% and the maize from 12.1 to 15.3% over 18 months. Chlorpyrifos-methyl residues on wheat declined 51 and 38% at the top and 1 m depths, respectively. Pirimiphos-methyl residues on maize did not decrease; however, after 8 months, pirimiphos-methyl began to appreciably lose effectiveness against the insect Tribolium castaneum (Herbst) in 24 h laboratory bioassays, and after a year against T. castaneum and the mite Tarsonemus granarius Lindquist within the granaries. Fat acidity values increased in both wheat and maize and insecticide treatments were not directly related to the increases. Under western Canadian conditions chlorpyrifos-methyl applied on wheat degraded with time, but at 4.6 ppm, offered effective insect control (>90%) for 10 months. Pirimiphos-methyl (5.6 ppm) on maize killed > 90% of insects for 8 months. It did not degrade at either the surface or 1 m depth, but became less effective with time, based on short-term exposure of adults, possibly because of translocation into the seed.