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Abstract: Plant secondary chemicals can alter herbivores’ susceptibility to pathogens by weakening the host, by affecting exposure through feeding, or by synergizing or deactivating pathogenic microbes in the gut. This study examined how feeding on perennial ryegrass (Lolium perenne) with (E+) or without (E−) its alkaloid-producing fungal endophyte, Neotyphodium lolii, affects susceptibility of a grass-feeding caterpillar to a baculovirus. In separate experiments, black cutworms (Agrotis ipsilon) were reared on E+ or E− grass that had been sprayed with different rates of A. ipsilon multiple nucleopolyhedrovirus (AgipMNPV), or were droplet-fed a single dose of the virus while developing on those grasses. Larvae grew more quickly on E− than on E+ grass, and mortality from virus increased at higher doses, but there was no endophyte×virus interaction in either trial. Despite larger size, larvae that fed ad libitum on virus-sprayed E− grass died more quickly than counterparts feeding on virus-sprayed E+ ryegrass, but such an effect was not seen when the virus was consumed as a single dose. Together, the results suggest that feeding on E+ grass neither compromises nor synergizes infectivity of AgipMNPV in the A. ipsilon midgut. Reduced consumption or avoidance of less-palatable E+ grass could, however, decrease ingestion of virus and rates of subsequent mortality in the field. Net benefits of integrating endophytic grasses and baculoviruses for managing A. ipsilon may depend on the extent to which reduced intake of virus on E+ grass is counterbalanced by slower growth and increased mortality from other natural enemies. [Copyright &y& Elsevier]