Indirect effects of beneficial fungal treatments on pest insects can vary with plant genotype and spore viability

• Dead fungal entomopathogen spores increased aphid numbers on one cotton genotype. • Viable spores had no effect on aphid populations in either cotton genotype. • Dead spores and defense elicitors both boosted aphid reproduction. • Chitosan caused genotype-specific plant responses to fungal MAMPs similar to spores. • Non-viable spores can shift plant defenses, sometimes helping insect pests. Fungal entomopathogens as biopesticides are typically applied as foliar sprays and assessed for their insecticidal activity. However, plant responses to these microbial treatments remain poorly understood. Additionally, environmental conditions such as desiccation and UV exposure can reduce spore viability, resulting in both live and dead spores serving as potential elicitors of plant responses. Whether foliar treatments with viable or dead fungal spores influence cotton defenses against the cotton aphid, Aphis gossypii , was studied. Spores of Beauveria bassiana , Phialemonium inflatum , and Chaetomium globosum were tested on two cotton genotypes for effects on aphid reproduction. Viable spores did not alter aphid population growth on either genotype. In contrast, dead spores significantly increased aphid populations on one genotype. Treatments with plant defense elicitors, Methyl Jasmonate and Actigard 50WG, increased aphid populations to levels observed with dead spore treatments, suggesting that dead spores may activate plant defense pathways that inadvertently benefit aphids. Foliar application of chitosan, a proxy for fungal chitin, further indicated that plant genotype-specific responses to Microbe-Associated Molecular Patterns (MAMPs) likely underlie the observed differential aphid responses. Together, these findings demonstrate that non-viable fungal spores can modulate plant defenses, thereby influencing the performance of insect herbivores. Our results highlight how beneficial fungal applications may alter host plant suitability by inducing plant defense pathways, potentially with unintended consequences for insect pest populations.