Humidity modifies age-dependent heat wave effects in an insect host-parasitoid interaction

Abstract

1. Climate change is projected to increase the likelihood of extreme heat events, but it may also alter humidity levels, leading to the potential for coupled thermal and hydric stress. While increasing frequency and intensity of extreme heat events have been well-documented for their negative effects on species and their interactions, how humidity modulates the impacts of heat waves is currently unknown. 2. We investigated how humidity interacted with heat waves of different timings and durations to affect the life histories in an insect host-parasitoid interaction, comprising the Indian meal moth, Plodia interpunctella, and its endoparasitoid wasp, Venturia canescens. Hosts parasitised as 4th instar larvae and unparasitized hosts were maintained in a high humid (60.8% RH) or low-humid (32.5% RH) environment at a constant temperature of 28℃. They were then exposed to 38℃ heatwaves with a duration of 6 or 72 hours in either the 4th or 5th instar. 3. Humidity and heat waves did not affect the adult emergence of unparasitized hosts, but longer heat waves and lower humidity increased the probability that host adults emerged from parasitized hosts, indicating the negative effect of these conditions on the survival of parasitoids. Furthermore, juvenile development time and body size of hosts and parasitoids responded differently to timing and duration of heat waves, and high humidity decreased larval development time of unparasitized hosts and increased the size of both hosts and parasitoids. In addition, humidity negatively interacted with larval stage and duration of heat waves to affect the size of unparasitized hosts, but this effect was not found in parasitoids. 4. Our results show that humidity modulates the life history of hosts and parasitoids, highlighting the importance of humidity in maintaining host-parasitoid interactions. Humidity should be considered when predicting the impact of temperature extremes on species’ population dynamics and their interactions.