Heat stress affects male reproduction in a parasitoid wasp

In insects, reproductive success and survival are affected by temperature. Reproduction is more sensitive than other physiological traits. While the effects of heat stress on females are well known, the effects on males are less clear. Hymenopteran parasitoids are valuable for studying the consequences of heat stress on male reproduction. In these species, through arrhenotoquous parthenogenesis, the sex ratio of the offspring is directly dependent on the sperm stock acquired by females during copulation. In the lab, heat temperature treatments (32–44 °C) were applied for 3 days in the pupal stage of Anisopteromalus calandrae males, and development was completed at 30 °C. Three different effects were observed depending on the temperature: mortality above 42 °C, sterility of emerging males at 40 °C, and sub-fertility at 38 °C. This sub-fertility is characterized by a dramatic decrease in male sperm supplies, of up to 7% compared to control males. In the course of ageing, the sperm stock of sub-fertile males increases but never reaches the level of control males. Survival was significantly higher in control (30 °C) males than those treated at 38 °C. Male mating ability was similar whatever the treatment (control and 38 °C), but females mated with 38 °C-treated males stored 100 times less sperm on average than those mated with control males. The offspring sex ratio of females mated with 38 °C-treated males was strongly male biased. The physiological mechanisms are as yet unknown. The relationship between temperature, sperm stock and sex ratio should be taken into account in the management of parasitoids for integrated pest management.