Degradation of isofenphos in cornfields with conservation tillage practices

Isofenphos (Amaze, Oftanol, 1-methylethyl 2-[[ethoxy-[(1-methylethyl)amino] phosphinothioyl]oxy]benzoate) was applied at planting to study the effects of four different tillage treatments (no-tillage, fall chisel plow, Paraplow^®, and fall moldboard plow) on isofenphos degradation rates and routes in cornfields over two growing seasons. Soil samples were taken at intervals extending over 69 days each growing season. Tillage treatment had no significant effect on isofenphos degradation rates and products. However, the repeated application of isofenphos had a very significant effect on isofenphos degradation. Degradation was much more rapid the second year. A laboratory experiment comparing sterile and nonsterile soils, with and without isofenphos history, confirmed enhanced microbial degradation resulting from two consecutive years of isofenphos application. In the first year, isofenphos oxon was found at greater amounts relative to the second year. Soil bioassays conducted on soils collected from the field at different dates in the second year showed that effective control of larvae was no longer present 21 days after isofenphos application in the field.