Effect of Chlorpyrifos-Methyl on Oat Ecosystems in Farm Granaries

Abstract

The insecticide chlorpyrifos-methyl was toxic to certain mite species when applied at 3.8 to 7.7 ppm on hulled oats ( Avena sativa L.) or 3.1 to 5.0 ppm on hulless oats in a farm granary in southern Manitoba. Insecticide residues decreased by about 65% in hulled oats and 50% in hulless oats in 1 year and by 80% in hulled oats and 60% in hulless oats in 2 years. Bioassay of treated oats with Tribolium castaneum (Herbst) adults for 24 h indicated 100% mortality above 4.7 ppm and negligible mortality below 2 ppm of insecticide. Chlorpyrifos-methyl virtually eliminated the predatory mites Androlaelaps casalis (Berlese) and Blattisocius keegani Fox in treated compared with untreated hulled and hulless oats. Populations of another predatory mite, Cheyletus eruditus (Schrank) were depressed but began to recover when the insecticide decreased below 0.6 ppm on hulled oats. In both types of oats, populations of prey mites Tarsonemus granarius (Lindquist) and Acarus siro L. were unaffected by the insecticide. Other prey mite populations included Aeroglyphus robustus (Banks), which were moderately reduced, and Lepidoglyphus destructor (Schrank) and Paratriophtydeus coineaui Andre, which were sharply reduced. The insecticide treatment had no effect on seed germination or microfloral infection, although these variables changed during 2 yr. The lack of impact of the insecticide on microflora, seed germination, and several mite species and the rate of insecticide degradation indicated that the original ecosystems could largely reestablish themselves in 2 yr under western Canadian storage conditions, although predator diversity was reduced.