Predacious Activity of the Nematode-destroying Fungus, Arthrobotrys oligospora, on Preparasitic Larvae of Cooperia oncophora and on Soil Nematodes

Comparisons were made among larval stages of Cooperia oncophora and the soil nematodes Rhab- ditis wohlgemuthi and Panagrellus redivivus in their abilities to induce the nematode-destroying fungus Arthro- botrys oligospora Fres. to form traps in vitro. No difference in the potential of the various nematodes to induce fungal traps were found. Regardless of the nematode species or stage, the fungus trapped with same efficiency. In all tests the soil nematodes and first- and second-stage C. oncophora larvae were immobilized and killed soon after entrapments. However, the third-stage C. oncophora larvae continued to struggle vigorously in the traps for more than 20 hours. This is explained possibly by the inability of the fungus to penetrate the nematode's outer cuticle from the previous molt. The nematode-destroying fungus, Arthrobo- trys oligospora Fres, is possibly the most com- monly encountered of the predacious fungi in Danish agricultural soils (Shepherd, 1961). To ensnare nematodes the predacious fungi must develop the characteristic organs of capture, which in the case of A. oligospora are three-di- mensional networks of sticky hyphal anasto- mosing loops. Undoubtedly, the capturing devices of the pre- dacious fungi have primarily evolved to trap soil nematodes, but it has been demonstrated that some species and stages of animal-parasitic nematodes may be effectively trapped as well (Descazeaux, 1939; Roubaud and Deschiens, 1941; Soprunov, 1958; Pandey, 1973; Granvold et al., 1985; Gruner et al., 1985). This is perhaps not surprising in the light of prevailing views on the evolution of parasitism, which suggest that most zooparasitic nematodes have derived from free-living ancestors belonging to the soil-dwell- ing rhabditids (Anderson, 1984). Furthermore, as already pointed out by Drechsler (1941), cap- turing organs of many fungi are such that they allow little discrimination in their choice of prey. In pure culture, and probably also in a nema- tode-free natural environment, many predacious fungi do not form traps and they behave as sap- rophytes. In A. oligospora the transition from a saprophytic to a predacious phase of growth may be induced by the presence of live nematodes, nematode metabolites, peptides and amino acids,