Tobacco budworm (Lepidoptera: Noctuidae): variations in response to methyl parathion and permethrin in the subtropics [Heliothis virescens ]

To determine the variations in the effect of methyl parathion on the tobacco budworm, Heliothis virescens (F.), the progeny of insects collected in the larval stage from cotton in the subtropical Lower Rio Grande Valley (LRGV) of Texas were topically treated from 1966 to 1975 and from 1978 to 1981. LD sub(50) values for 80% of the collections exceeded those of a susceptible laboratory-reared strain. However, the values for the field strains varied widely within and among years. Peak resistance to methyl parathion occurred in 1971. By 1981, however, resistance was at about the level found in 1966. Neither field data nor LD sub(50) values indicated that H. virescens larvae in northeastern Mexico and the LRGV had developed resistance to permethrin between 1974 and 1981.