MULTIPLE INSECTICIDE TREATMENTS AFFECT HARVEST RESIDUES OF LETTUCE

Multiple treatments of mevinphos, methomyl, and permethrin were applie d to head lettuce (Latuca sativa L.) under commercial field conditions , and harvest residues compared with those of single treatments made a t identical periods before harvest. Single treatments resulted in harv est residue levels below current legal tolerances for these materials on head lettuce. Two consecutive treatments of all insecticides 8 d ap art resulted in similarly low harvest residues. However, at shorter tr eatment intervals of 4 and 2 d, the same materials showed extensive re sidue accumulation. This trend was greatest at successive treatment in tervals of 2 d, where mevinphos increased by 79%, methomyl by 161%, an d permethrin by 242%. The importance of this residue loading phenomeno n is discussed in light of management of insecticide residues on lettu ce in commercial production.