THE EFFECT OF MULTIPLE SOIL APPLICATIONS OF DISULFOTON ON ENHANCED MICROBIAL-DEGRADATION IN SOIL AND SUBSEQUENT UPTAKE OF INSECTICIDAL CHEMICALS BY POTATO PLANTS

Potatoes were grown during 1992 in 2 m(2) plots of loam which had rece ived 1, 2 or 3 annual treatments of Di-Syston 15G, equivalent to 3.36 kg AI/ha, in furrow at planting. The presence of enhanced degradative activity to the sulfoxide and sulfone metabolites of disulfoton in the soil treated in the previous two years was confirmed by laboratory te sts prior to the 1992 treatments. Soil, seed potato and foliage from t he three treatments were analyzed for disulfoton and its sulfoxide and sulfone metabolites for 12 wk following planting/treatment. Disulfoto n was the major insecticidal component of the soil, a minor component of the seed piece and was not detected (<0.02 ppm) in potato foliage. Disulfoton concentrations in each of the three substrates sampled were similar for the three treatments. Disulfoton sulfoxide and sulfone we re the major insecticidal components of the seed piece and foliage. Th eir maximum concentrations in 1st year soil, seed pieces and foliage w ere 2x, 2x and 6x, respectively, those measured in the 2nd and 3rd yea r treatments. The results demonstrate that enhanced microbial degradat ion of relatively minor insecticidal compounds in the soil can profoun dly affect insecticide levels in the plant when these compounds are th e major insecticidal components accumulated. The broader implications for crop protection using soil-applied systemic insecticides are discu ssed.