<b>Heat stress has immediate and persistent effects on immunity and development of </b><b><i>Tenebrio molitor</i></b>

<p dir="ltr">The yellow mealworm (<i>Tenebrio molitor</i>) is a promising insect species for mass-rearing for the production of feed and food. In mass-production systems, insects may be exposed to abiotic stressors such as heat stress as well as potentially lethal pathogens. To ensure mass-reared <i>T. molitor</i> populations are healthy and productive there is a need to understand both the risks, and potential benefits of heat stress, on the fitness of insects and their susceptibility to pathogens.<b><i> </i></b>In this study, we investigated the effects of a short (2 h) or a long (14 h) heat stress (38 ℃) exposure on the susceptibility and the immune responses of <i>T. molitor</i> larvae exposed to a fungal pathogen (<i>Metarhizium brunneum</i>). Larvae were exposed to the pathogen either immediately or five days after the heat stress treatments. The development of heat stressed larvae and their offspring was also assessed.<b><i> </i></b>A short heat stress immediately before exposure to <i>M. brunneum </i>increased the survival probability of <i>T. molitor</i> larvae, which correlated with increased antibacterial activity in the hemolymph. The exposure of larvae to short, or long heat stresses five days before pathogen exposure did not affect their survival, despite a temporary lowered body mass gain of heat stressed larvae. However, heat stressed larvae showed decreased hemocyte concentrations when exposed to <i>M. brunneum</i>. We also found an increased body weight in larval offspring of females that had been exposed to a short heat stress as larvae themselves. These findings demonstrate the importance of understanding the effects of heat stress in the long-term.<b><i> </i></b>The beneficial effects of heat stress on pathogen susceptibility in <i>T. molitor</i> and the negative effects on body mass gain are only transient, whereas negative effects on immune response (hemocyte concentrations) persist over an extended period.</p>