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Numerous insect species and their associated microbial pathogens are exposed to elevated CO 2 concentrations in both artificial and natural environments. However, the impacts of elevated CO 2 on the fitness of these pathogens and the susceptibility of insects to pathogen infections are not well understood. The yellow mealworm, Tenebrio molitor, is commonly produced for food and feed purposes in mass-rearing systems, which increases risk of pathogen infections. Additionally, entomopathogens are used to control T. molitor, which is also a pest of stored grains. It is therefore important to understand how elevated CO 2 may affect both the pathogen directly and impact on host-pathogen interactions. We demonstrate that elevated CO 2 concentrations reduced the viability and persistence of the spores of the bacterial pathogen Bacillus thuringiensis. In contrast, conidia of the fungal pathogen Metarhizium brunneum germinated faster under elevated CO 2 . Pre-exposure of the two pathogens to elevated CO 2 prior to host infection did not affect the survival probability of T. molitor larvae. However, larvae reared at elevated CO 2 concentrations were less susceptible to both pathogens compared to larvae reared at ambient CO 2 concentrations. Our findings indicate that whilst elevated CO 2 concentrations may be beneficial in reducing host susceptibility in mass-rearing systems, they may potentially reduce the efficacy of the tested entomopathogens when used as biological control agents of T. molitor larvae. We conclude that CO 2 concentrations should be carefully selected and monitored as an additional environmental factor in laboratory experiments investigating insect-pathogen interactions., (© 2024. The Author(s).)