Top‐down and bottom‐up regulation of herbivores: Spodoptera frugiperda turns tables on endophyte‐mediated plant defence and virulence of an entomopathogenic nematode

.  1. The fungus Neotyphodium lolii forms a symbiotic relationship with its grass host Lolium perenne (perennial ryegrass). The fungus benefits from access to plant nutrients and photosynthate, whereas the plant benefits from acquired chemical defence against herbivory. 2. This study examined the potential for endophyte‐mediated plant defences to influence interactions between fall armyworm Spodoptera frugiperda , and the entomopathogenic nematode Steinernema carpocapsae and clarified biological mechanisms underlying the observations made. 3. In laboratory and greenhouse experiments, S. frugiperda larvae were fed endophytic or non‐endophytic L. perenne then exposed to S. carpocapsae or injected with the nematodes' symbiotic bacteria Xenorhabdus nematophila . 4. In all instances, S. frugiperda larvae fed endophyte‐infected grass suffered significantly lower mortality than those fed non‐endophytic plants. Although larvae fed endophyte‐infected grass often had significantly lower biomass than those fed uninfected grass, these differences did not account for altered susceptibility to S. carpocapsae . 5. Endophyte‐mediated reductions in herbivore susceptibility to the nematode pathogen represent a herbivore adaptation that effectively turns the tables on both plant and natural enemy by reducing the virulence of the nematodes' symbiotic bacteria while expanding the temporal window of herbivory.