BASIS FOR THIFENSULFURON-INSECTICIDE SYNERGISM IN SOYBEANS (GLYCINE-MAX) AND CORN (ZEA-MAYS)

Fresh weight reductions of greenhouse-grown soybeans and corn treated with postemergence thifensulfuron at 4.4 g al ha(-1) were synergistica lly enhanced when the herbicide was mixed with formulated chlorpyrifos or malathion insecticides, bur the enhancement: was not observed with formulated methomyl insecticide. Thifensulfuron plus formulants of ei ther chlorpyrifos or malathion reduced fresh weights no more than did the herbicide applied alone. Growth rare of hydroponically grown soybe ans was reduced by root-applied thifensulfuron in combination with a f oliar-applied formulation of chlorpyrifos or malathion, but not methom yl. Postemergence-applied thifensulfuron reduced the growth rate of hy droponically grown soybeans and corn with chlorpyrifos, malathion, or methomyl applied postemergence 1 d before thifensulfuron and with proc edures identical to those used for absorption, translocation, and meta bolism experiments. None of the insecticides applied 1 d before thifen sulfuron altered foliar absorption of C-14-thifensulfuron or its trans location in unifoliolate soybeans or three-leaf corn. Thin-layer chrom atography of soybean extracts revealed one primary thifensulfuron meta bolite, presumably the deesterified free acid. C-14-thifensulfuron met abolism in corn produced about five unidentified metabolites in apprec iable amounts. Levels of unmetabolized C-14-thifensulfuron 24 h after herbicide application were highest in insecticide-treated soybeans and corn. Over all experiments, enhancement of injury and inhibition of t hifensulfuron metabolism generally were greatest in soybeans with chlo rpyrifos but were greatest in corn with chlorpyrifos or malathion. Syn ergistic enhancement of thifensulfuron injury to soybeans and corn by chlorpyrifos and malathion appears to result from insecticide inhibiti on of thifensulfuron detoxication.