Gamma Irradiation of the Tobacco Budworm: Sterilization, Competitiveness, and Observations on Reproductive Biology1

Abstract

Both sexes of adult tobacco budworms, Heliothis virescens (F.), were completely sterilized by 45 kilorads (krad) of gamma irradiation; 35 krad resulted in less than 1% egg hatch. Sterile females laid about ⅔ as many eggs as the control, and longevity of both sexes was reduced 0-10%. The mating ability of males sterilized as adults or pupae was not significantly less than that of the control males in most tests, the females sterilized as adults mated as frequently as the controls. Normally, neither sex mates more than once a night as determined by transfer of spermatophores. Males sterilized as pupae or adults were not fully competitive with untreated males as measured by egg hatch from ratio tests with untreated males and females. However, in a test in which the mating of sterile males was determined by labeling spermatophores with C¹⁴, males were found to be fully competitive in mating females. Females alternately mated to sterile and untreated males usually remained fertile after mating with an untreated male followed by a sterile male, or became fertile about 50% of the time if mated with a sterile male followed by an untreated male. Thus when mating to both types of males occurred, the female was more likely to be fertile than sterile. It was observed also that when second matings altered fertility, the alteration was complete—the result of sperm displacement, not sperm mixing. Sterile males mated to untreated females resulted in about 25% fewer eggs per female than controls.