Tests for Density-Dependent Parasitism of Nematodes by Nematode-Trapping and Endoparasitic Fungi

The relationship between parasitism by fungi and density of host nematodes (juveniles of Meloidogyne javanica) was examined in soil microcosms. One of five fungi (Hirsutella rhossiliensis, Arthrobotrys dactyloides, A. oligospora, Monacrosporium ellipsosporum, or M. cionopagum) was added to untreated loamy sand at the start of each of five trials, host nematode density was controlled at four levels, and parasitism of assay nematodes (juveniles of Heterodera schachtii) was assessed after 15 weeks at 20°C. Increases in percentage parasitism with host density were greatest for H. rhossiliensis, intermediate for M. ellipsosporum and A. dactylooides, and least for A. oligospora and M. cionopagum. Although background fungi (especially H. rhossiliensis) in the loamy sand confounded the results, the relative strengths of the parasitic responses to host density were confirmed in an additional experiment in a loam soil that did not contain H. rhossiliensis. The failure of A. oligospora and M. cionopagum to respond to nematode density provides indirect evidence that substrates other than nematodes are important to these and probably other trapping fungi.