The mass-production of insects is expanding globally and the industry of producing insects as feed and food predicted to grow rapidly in the coming years. Insects kept in closed rearing systems at high densities may be exposed to various abiotic and biotic stressors, often occurring simultaneously. It is crucial to understand the impact of these stressors because they can reduce the productivity of reared insects. In manuscript 1 of this thesis, I review the literature on environment-host-parasite interactions in mass-reared insect species and produce a workflow to optimise environmental conditions to reduce the impact of parasite infection in mass-production systems. In my experimental studies, I explore the impact of multiple stressors on host-pathogen interactions, focusing on Tenebrio molitor, which is one of the most promising insect species massreared for protein production. In manuscript 2, I find that a short heat stress of two hours can increase the survival probability of larvae exposed to the fungus Metarhizium brunneum, which correlates with the antimicrobial activity measured in the hemolymph. Nevertheless, these beneficial effects wane within five days after the heat stress, accompanied by reduced larval weight gain. Elevated carbon dioxide (CO2) reduces the viability and persistence of the spores of the bacterium Bacillus thuringiensis, but it accelerates the germination of M. brunneum conidia. Interestingly, T. molitor larvae reared at elevated CO2 conditions have a reduced susceptibility to both pathogens (manuscript 3). However, there is no evidence that elevated CO2 affects the interactions between the two pathogens during co-exposure of T. molitor larvae. Furthermore, in manuscript 4, I find mostly antagonistic or additive interactions between the two pathogens when using a mixture toxicity model, indicating that the risk during co-exposure to the two tested pathogen species does not increase in early-stage larval populations of T. molitor. Lastly, I discuss how this novel understanding of different abiotic stressors interacting with parasites of mass-reared insects, may be used to optimise the environmental conditions to simultaneously enhance productivity, and mitigate the risk of parasite infections.