Rs. Bennett et al., TEMPORAL AND SPATIAL-DISTRIBUTION OF AZINPHOS-METHYL APPLIED TO ALFALFA, Archives of environmental contamination and toxicology, 27(4), 1994, pp. 534-540
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.