Daily Rhythm in the Susceptibility of an Insect to a Toxic Agent

Abstract

Adult boll weevils exhibited a daily rhythm in their susceptibility to standardized doses of the insecticide, methyl parathion. The mortality produced by the insecticide was intimately related to the time of day at which the toxicant was applied. The rhythm appeared to be photoperiodically entrained and, regardless of the length of day or "clock time-of-day of treatment," a period of greatest resistance always occurred at dawn and recurred at 6-hour intervals throughout the 24-hour cycle. The greatest difference in response occurred in a photoperiod having 10 hours of light per 24-hour cycle. Here, the same dose of methyl parathion killed approximately 10 percent of the weevils treated at dawn but almost 90 percent of those treated only 3 hours later.