Crossing thermal boundaries: Quantifying the impact of sublethal heat stress on growth in black soldier fly (Hermetia illucens)

Abstract

Ectothermic responses to permissive temperatures that support growth, maintenance and repair are typically modelled by thermal performance curves (TPC). In contrast, thermal death time (TDT) models are well suited to describe the potent, exponential accumulation of injury that occurs at stressful temperatures. While each model is well understood, complexity arises when model boundaries are crossed. Using larvae of the black soldier fly (Hermetia illucens) we characterise a thermal performance curve for growth. We then apply the optimal temperature (TPCopt = 35.6°C) as a recovery point to assess growth impairment in larvae following controlled, dose-specific exposure to thermal stress beyond the upper boundary of the TPC (TPCmax ~ 42°C). Above 42°C, injury accumulates additively towards a lethal dose, with higher intensities accelerating injury accumulation and shortening survival time (Lt50), as predicted by the TDT model. Using TDT parameters, we apply fixed doses of thermal stress (e.g. 10%, 20%, 30% of the lethal dose) to demonstrate a negative linear relationship between stress dose and subsequent reduction in specific growth rate when larvae are returned to permissive temperature. Specifically, sublethal heat injury corresponding to 1% of Lt50 reduces the relative growth rate by 1.41% day−1 when the growth rate is evaluated after 1 day. Over time, the injured larvae recover, and the apparent growth impairment is, therefore, reduced when evaluated after 7 days (0.14% day−1) and 10 days (0.09% day−1). Instead, the initial reduction in growth is gradually manifested as delayed development, with larvae reaching the prepupal stage 0.0247 days later for each 1% of lethal dose. Collectively, these findings demonstrate how a consequent and model-based approach can quantify the cost of sublethal heat stress in animals that experience temperature fluctuations between the permissive and stressful range. We show that the costs of sublethal stress have lasting fitness consequences, increase linearly with stress dose and are additive across the stressful temperature range. Acknowledging and parametrising these effects is essential for forecasting climate change impacts in natural systems and quantifying heat stress costs in large-scale insect production. Read the free Plain Language Summary for this article on the Journal blog.