Abstract
The LD50 for topical applications of methyl parathion to larvae of the tobacco budworm, Heliothis virescens (F.), reared from a field collection made near Brownsville, Texas, was 0.44 mg/g, 20 times higher than that for tobacco budworms collected earlier in the season from the same field (0.022 mg/g), in 1967. The LD50 for topical applications of monocrotophos to larvae of the tobacco budworm reared from larvae collected from the Brownsville field was 1.21 mg/g. This value was about 12 times greater than that obtained for a laboratory strain (0.10 mg/g). Parathion was more toxic than methyl parathion to larvae of the beet armyworm, Spodoptera exigua (Hubner), but not to larvae of the bollwonn, H. tea (Boddie). An attempt to select for resistance in laboratory strains of the bollwonn with monocrotophos and methyl parathion, the tobacco budworm with parathion and monocrotophos, and the beet armyworm with parathion by topical applications was not successful; the strains demonstrated increased susceptibility.
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