The sex ratio in the natural population of most Hymenopteran parasitoids is 1 : 1. Females of these parasitoids, however, can regulate the sex of their offspring in response to environmental and biological factors. Under certain circumstances, this may lead to an overproduction of males, which can result in a very negative impact on any control program using these parasitoids. For these reasons, understanding the effect of these factors is very important. In this study, five biological and environmental factors known to affect sex ratio in parasitic Hymenoptera were investigated for Ascogaster quadridentata Wesmael, a parasitoid of Cydia pomonella L. Results showed that food, ambient temperature, and parasitoid age had significant effects on the sex ratio of the offspring of Hymenopteran parasitoids. The percentage of females decreased with lack of food, parasitoid age (when it became 10-days-old or older), and at low (15°C) and high (35°C) temperatures. The effect of host age and density, however, was not significant.
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