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

## TL;DR

Short heatwaves reduce the brood size and affect development in two parasitoid wasp species that target ladybirds, which are important for controlling crop pests.

## Contribution

This study is the first to investigate the effects of heatwaves on fourth-trophic-level parasitoids.

## Key findings

- Heatwaves reduced brood size in both Oomyzus scaposus and O. spiraculus.
- Short heat exposure accelerated development in O. scaposus but reduced adult body mass with longer exposure.
- Daily heatwaves of a few hours can negatively affect the performance of both parasitoid species.

## Abstract

Climate change is causing more frequent and intense heatwaves, which can disrupt food webs by affecting insects at the top of food chains. This study examined how short-term heatwaves influence two species of parasitoids that attack the larvae and pupae of seven-spot ladybirds, which are important predators of crop pests. We simulated heatwaves of different lengths and measured the performance of the two parasitoids. We found that even brief heatwaves reduced the brood size in both wasp species. For Oomyzus scaposus, short heat exposure sped up development, but longer exposure led to smaller adults. O. spiraculus showed similar patterns, but these were not statistically significant. Overall, the study shows that even short daily heat events can influence the performance of both species. Understanding these effects is important for predicting how climate change may alter predator–prey interactions, which could indirectly affect the natural control of crop pests and the stability of ecosystems.

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, Oomyzus scaposus and O. spiraculus, parasitizing the seven-spot ladybird, Coccinella septempunctata L. Heatwave treatments negatively affected the brood size of both species. The heatwave also affected the sex ratio of O. scaposus, though it remained highly female-biased. Exposure to heatwaves for 1 h accelerated the development of O. scaposus. For O. scaposus, the body mass of both females and males was lowest under the 3 h heatwave treatment. In contrast, for O. spiraculus, 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 O. scaposus and O. spiraculus.