SYNERGISM OF MALATHION AND INHIBITION OF MIDGUT ESTERASE-ACTIVITIES BY AN EXTRACT FROM MELIA TOOSENDAN (MELIACEAE)

A refined bark extract from the tree Melia toosendan (Meliaceae) reduced malathion toxicity to the Asian armyworm Spodoptera litura during the first 2 days posttreatment but synergized the toxicity on the subsequent 2 days when combined at a ratio of 16:1 (extract:malathion). These phenomena may be associated with increased midgut esterase activity during the first 2 days and decreased esterase activity thereafter in S. litura fed on the extract-treated food. In vitro experiments indicated that a minor ingredient(s) in the bark extract inhibited esterase activity, rather than toosendanin, a limonoid constituting 75% of the extract, itself. The extract inhibited midgut esterase activity in both S. litura larvae and adult migratory grasshoppers, Melanoplus sanguinipes, but not whole-body esterase activity in green peach aphids, Myzus persicae. (C) 1995 Academic Press, Inc.