TEMPORAL AND SPATIAL-DISTRIBUTION OF AZINPHOS-METHYL APPLIED TO ALFALFA

The U. S. Environmental Protection Agency uses a simple exposure nomog ram to make preliminary assessments of the pesticide residues concentr ations on foods of terrestrial wildlife. This study was conducted to m easure the spatial and temporal distribution of the organophosphorus i nsecticide azinphos-methyl (Guthion (R) 2S) in a dense alfalfa crop to evaluate the assumptions used in this nomogram and to determine the i nfluences of application rate and spatial distribution of residues on the potential dietary exposure to herbivores. Concentrations of azinph os-methyl were measured on spray cards, soil, and alfalfa plants (top 15 cm and bottom 15 cm) on plots treated at 0, 0.77, 1.55, 3.11, and 4 .67 kg active ingredient/ha (4 enclosures/treatment) at 2 h and 2, 6, 14 and 28 days after application. Although the concentrations measured on spray cards were very close to predicted, the concentrations on al falfa canopy vegetation were higher than expected from the nomogram an d increased at a faster rate with increasing application rate than exp ected. Concentrations were 1.5-2.4 times higher in the alfalfa canopy than on alfalfa near soil level. Variability among alfalfa samples wit hin treatments was high, with part of the variation in canopy samples explained by the distance to the end of the spray boom. Only 16-32% of pesticide reached the soil surface. The calculated half-life of azino phos-methyl was 2.5-4.5 days on vegetation and 19.3 days on soil. Diet ary exposure to small herbivores would be expected to exceed nomogram predictions, but be highly variable for individuals due to the patchy nature of the pesticide distribution.