Weight Versus Toxicity: a Need for Revision of the Standard Method of Testing for Resistance of the Tobacco Budworm to Insecticides1

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Abstract
Toxicity studies with methyl parathion and permethrin topically applied to several weight classes of the larvae of Heliothis virescens (F.) revealed that LD50 expressed as μg of toxicant per g of body weight was not constant among the weight classes treated for both insecticides. Specifically, an increase in susceptibility was noted near the larval weight of 30 mg, which also corresponds to the average larval weight at the third molt. Because an Insect’s body is undergoing rapid and dramatic changes in structure and enzyme activity during a molt, the weight range, selected as a standard for testing insects for resistance to insecticides, should exclude molting larvae. The accepted standard weight range for H. zea (Boddie) and H. virescens set by the Second Conference on Test Methods for Resistance in Insects of Agricultural Importance is 35 ± 5 mg. This weight range includes a significant number of larvae at or near molt. To avoid time of molt, acceptance of 20 ± 3 mg as a standard weight range for testing insecticide resistance in H. virescens is suggested.

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