To date, no study has explicitly addressed effects of variation in species diversity of root-feeding herbivores on host plant biomass. Root-feeding nematodes typically occur in multi-species communities. In a three-year field experiment, we investigated how variation in species diversity of root-feeding nematodes affected nematode dynamics and response of the dune grass Ammophila arenaria to root-feeder activity. This plant species needs regular burial by fresh beach sand to remain vigorous, suggesting that A. arenaria benefits from a temporary escape from root-feeding soil organisms and that root-feeders are involved in plant degeneration in stabilized dunes. We created series of ceased and continued sand burial and added the endoparasitic nematodes Meloidogyne maritima, Heterodera arenaria and Pratylenchus penetrans alone or in combination to A. arenaria. We included treatments with and without the whole soil community, measured plant biomass and quantified numbers of nematodes. Addition of H. arenaria and P. penetrans decreased numbers of M. maritima juveniles and delayed the first appearance in time of both juveniles and females, while numbers of males only decreased when plants had been buried. Burial with sand and addition of the other two endoparasites affected numbers of H. arenaria juveniles, while numbers of P. penetrans were low and not affected. Shoot biomass of A. arenaria was lower when M. maritima had been added alone than when the three species had been ad Our results point at idiosyncratic effects of nematode diversity on A. arenaria biomass. Heterodera arenaria and P. penetrans protected their host by reducing numbers and delaying activity of M. maritima to a later stage in the growth season, when root-feeding activity was less harmful for plant biomass development.