Simulated Heatwaves Affect Development of Two Congeneric Gregarious Larval–Pupal Endoparasitoids

Ongoing climate change is increasing the frequency and magnitude of high-temperature events, and extreme weather events such as heatwaves are expected to become more common. The impacts of extreme temperatures are likely even more severe at higher trophic levels, which depend on the adaptive capacity of lower trophic levels. However, compared with parasitoids at the third trophic level, less is known about these effects on parasitoids at the fourth trophic level, which occupy the terminal end of food chains. This study investigates the effects of experimental heatwave duration variability on the development of two congeneric gregarious larval–pupal endoparasitoids, <i>Oomyzus scaposus</i> and <i>O. spiraculus</i>, parasitizing the seven-spot ladybird, <i>Coccinella septempunctata</i> L. Heatwave treatments negatively affected the brood size of both species. The heatwave also affected the sex ratio of <i>O. scaposus</i>, though it remained highly female-biased. Exposure to heatwaves for 1 h accelerated the development of <i>O. scaposus</i>. For <i>O. scaposus</i>, the body mass of both females and males was lowest under the 3 h heatwave treatment. In contrast, for <i>O. spiraculus</i>, heatwaves had a marginally negative effect on the body mass of both female and male adults, though this effect was not statistically significant. Our results indicate that both species respond similarly, and even daily heatwave exposure of a few hours can affect the performance of <i>O. scaposus</i> and <i>O. spiraculus</i>.