Endoparasitic nematodes reduce multiplication of ectoparasitic nematodes, but do not prevent growth reduction of Ammophila arenaria (L.) Link (marram grass)

Several studies have suggested that plants are able to control the development of specialist herbivorous invertebrates, but not that of generalists. Plants are alleged to have evolved tolerance against specialists in order to suppress the development of more damaging generalists through competition. Here, we tested whether specialist plant parasitic nematodes in the root zone of the natural dune grass Ammophila arenaria are able to suppress the development of a generalist plant parasitic nematode and therewith protect the plant. We added a generalist ectoparasite (Tylenchorhynchus ventralis) and specialist endoparasites (Heterodera arenaria, Pratylenchus penetrans, Meloidogyne maritima) in different densities to A. arenaria. We also tested whether sequential inoculation of the specialists had an additional competitive effect on T. ventralis. Our results show that the specialist endoparasitic nematodes indeed suppressed the development of the generalist T. ventralis, but only when the specialists were added to the plant in relatively high densities that exceeded the field density of the specialist endoparasitic nematodes. Therefore, we conclude that competition by specialist nematodes is not a likely mechanism for the regulation of the generalist plant parasite T. ventralis. Sequential inoculation of endoparasites did not influence the development of T. ventralis more than inoculation at the same time. Despite their inhibiting effect on the development of T. ventralis, the endoparasites did not counteract the negative effect of T. ventralis on plant biomass. On the contrary, they themselves had a negative effect on shoot biomass of A. arenaria, although no effect was found on root biomass. We discuss our results in relation to other mechanisms that may regulate the population density of T. ventralis. [KEYWORDS: Competition; Generalist; Plant parasitic nematode; Population regulation; Specialist; Temporal nematode dynamics]