Multiple Insecticide Treatments Affect Harvest Residues of Lettuce

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