Resistance Frequencies in Overwintering Pupae and the First Spring Generation of Helicoverpa armigera (Lepidoptera: Noctuidae): Selective Mortality and Immigration

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Abstract
Since the introduction of a resistance management program in 1983, frequencies of pyrethroid resistance in Helicoverpa armigera (Hiibner) in areas of the Namoi and Gwydir valleys, New South Wales, in which cotton is grown, have increased each summer then declined between autumn and late spring. We examined two possible causes for this decline—immigration of susceptible adults and selective mortality of resistant pupae. Diapausing pupae were collected during winter and spring, and larval populations were collected in spring 1987 and 1988, in and around the Namoi-Gwydir region. In winter 1987, large populations of pupae with resistant frequencies >53% were found under crop residues of cotton. Smaller populations of pupae were observed under maize and pigeon pea; the resistance frequencies were <55%. Selective mortality in pupae could not account for the presence of populations of larvae with resistant frequencies <35% in spring 1987. We propose that immigration of susceptible moths is the most likely cause in the annual decline in resistance frequencies. Results from the 1988 season are consistent with this hypothesis.