Effect of CO₂ Concentrations on Entomopathogen Fitness and Insect-Pathogen Interactions

Numerous insect species and their associated microbial pathogens are exposed to elevated CO₂ concentrations in both artificial and natural environments. However, the impacts of elevated CO₂ 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 CO₂ may affect both the pathogen directly and impact on host-pathogen interactions. We demonstrate that elevated CO₂ 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 CO₂. Pre-exposure of the two pathogens to elevated CO₂ prior to host infection did not affect the survival probability of T. molitor larvae. However, larvae reared at elevated CO₂ concentrations were less susceptible to both pathogens compared to larvae reared at ambient CO₂ concentrations. Our findings indicate that whilst elevated CO₂ 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 CO₂ concentrations should be carefully selected and monitored as an additional environmental factor in laboratory experiments investigating insect-pathogen interactions.