Influence of thermal stress on the cellular immunity of Galleria mellonella F. (Lepidoptera: Pyralidae), and the biological traits of Bracon hebetor (Hymenoptera: Braconidae) in a host–parasitoid interaction: implications under climate change

Abstract

Extreme temperatures, the most evident indicator of climate change, threaten the delicate balance among par asitoids, key components of the ecosystem, and their agricultural pest hosts. Understanding the effects of thermal stress on parasitoids is essential for improving the mass production of Bracon hebetor (Hymenoptera: Braconidae), and predicting how the climate change will affect host-parasitoid relationship. The immune system of the host Galleria mellonella (Lepidoptera: Pyralidae) varied with both temperature and duration of exposure. Total he mocyte count peaked at 40 ◦C after 24 h due to elevated granulocyte, plasmatocyte, spherulocyte, and other cell types. The mitotic index peaked at 38 ◦C (24 h) before dropping sharply at 40 ◦C. Strong encapsulation responses rose significantly at 40 ◦C compared to 38 ◦C. Re-exposing adult females of B. hebetor to thermal stress, along with its previously heat-stressed hosts, altered biological traits in both adult females and F1 offspring, especially the reduced egg fecundity. Rising temperatures initially reduced the female sex ratio, but ultimately it nearly balanced out. At 36 ◦C, 95 % of the 24 eggs per female developed into adults, and prolonged adult longevity increased parasitoid numbers and extended their active period, a key finding. Overall, B. hebetor displayed high thermal tolerance, yet temperature-driven changes in host immunity and parasitoid traits may reshape their interactions under future climates.