The plant hormone salicylic acid (SA) is recognized as an effective defense against biotrophic pathogens, but its role as regulator of beneficial plant symbionts has received little attention. We studied the relationship between the SA hormone and leaf fungal endophytes on herbivore defenses in symbiotic grasses. We hypothesize that the SA exposure suppresses the endophyte reducing the fungal-produced alkaloids. Because of the role that alkaloids play in anti-herbivore defenses, any reduction in their production should make host plants more susceptible to herbivores. Lolium multiflorum plants symbiotic and non-symbiotic with the endophyte Epichloë occultans were exposed to SA followed by a challenge with the aphid Rhopalosiphum padi. We measured the level of plant resistance to aphids, and the defenses conferred by endophytes and host plants. Symbiotic plants had lower concentrations of SA than did the non-symbiotic counterparts. Consistent with our prediction, the hormonal treatment reduced the concentration of loline alkaloids (i.e. N-formyllolines and N-acetylnorlolines), and consequently decreased the endophyte-conferred resistance against aphids. Our study highlights the importance of the interaction between the plant immune system and endophytes for the stability of the defensive mutualism. Our results indicate that the SA plays a critical role in regulating the endophyte-conferred resistance against herbivores.