Persistence of isofenphos (Amaze®) soil insecticide under laboratory and field conditions and tentative identification of a stable oxygen analog metabolite by gas chromatography

Field plots in central and northern Illinois were treated with isofenphos (Amaze 20G) at a dosage of 1.12 kg Al/ha in an 18 cm band over the corn row at planting time. Soil samples were collected at approximately 2‐week Intervals for 2 months after application. In the central Illinois field, isofenphos concentration was virtually unchanged after 35 days in spite of heavy rains, and the DT‐50% (the time interval until 50% of the initial insecticide residue disappears) was about 43 days. In the northern illinois field, a shorter lag period was observed, and the DT‐50% was about 31 days. Isofenphos oxon was detected in both fields within 2 weeks after treatment, and by 2 months its concentration had increased to 0.35 and 1.34 μg/g, or 17 and 21% of the initial parent insecticide recovered in the northern and central Illinois fields, respectively. Isofenphos oxon is a stable degradation product that was produced in nearly quantitative amounts from the parent insecticide in laboratory studies. The rate of isofenphos loss from soil in laboratory experiments was dependent on initial insecticide concentration. The toxicological significance of isofenphos and its oxon to corn rootworm control is discussed.