<i>Epichloë</i> endophytes impair performance and preference of <i>Rhopalosiphum padi</i> by disrupting aphid feeding behaviour in tall fescue

Mutualistic symbioses between plants and fungi are widespread in nature and increasingly being adopted in agroecosystems to enhance economic and environmental sustainability. In temperate forage grasses, associations with Epichloë endophytic fungi can confer resistance to herbivores, particularly aphids. While their impact on aphid preference and performance are well documented, the underlying behavioral mechanisms remain poorly understood. We investigated interactions between two tall fescue cultivars (INIA Aurora and INIA Fortuna), the AR584 strain of Epichloë coenophiala , and the bird cherry‐oat aphid Rhopalosiphum padi , focusing on feeding behaviour using the electrical penetration graph (EPG) technique. Plants (both adult and seedling stages) with and without the endophyte were tested in both choice and no‐choice experiments under greenhouse conditions. Aphid populations grew ~10% faster on endophyte‐free plants, with significantly higher numbers of apterae and nymphs after three weeks. In choice tests, aphids preferred endophyte‐free plants, and aphid‐free plants showed greater biomass. Daily fecundity was consistently lower on endophyte‐infected plants, regardless of cultivar, and aphids performed best on wheat controls. EPG analysis revealed that on endophyte‐infected plants, aphids exhibited shorter phloem ingestion phases, delayed time to sustained ingestion, and increased probing activity, particularly in mesophyll. In seedlings, similar feeding disruptions were observed, including increased xylem ingestion and reduced duration of sustained phloem feeding. These findings demonstrate that Epichloë endophytes disrupt aphid feeding behavior, particularly access to phloem sap, leading to reduced performance and host preference. This supports their potential role in integrated pest management strategies via enhancement of plant resistance, offering insights into the functional mechanisms behind this symbiosis.