HORTICULTURALENTOMOLOGY Multiple Insecticide Treatments Affect Harvest Residues of Lettuce

ABSTRACT Multiple treatments ofmevinphos, methomyl, and pennethrin 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 succes-sive treatment intervals of 2 d, where mevinphos increased by 79%, methomyl by 161%, and pennethrin by 242%. The importance of this residue loading phenomenon is discussed in light of management of insecticide residues on lettuce in commercial production. KEY WORDS lettuce, pesticide residues, insecticide contamination THE REQUIREMENTFORmultiple insecticide ap-plications in commercial lettuce (Latuca sativa L.) production is well documented (University of California 1987, Toscano et al. 1990). Major pests, such as Helicoverpa zea (Boddie), He