The nematode-trapping fungus Arthrobotrys oligospora in soilof the Bodega marine reserve: distribution and dependenceon nematode-parasitized moth larvae

Of the 13 nematode-trapping fungi previously detected at the Bodega Marine Reserve (BMR, Sonoma County, CA, USA), Arthrobotrys oligospora is by far the most abundant. Why A. oligospora is so abundant is unclear, but the answer may involve bush lupines (Lupinus arboreus), ghost moth larvae (Hepialus californicus), and insect-parasitic nematodes (Heterorhabditis marelatus). Previous research documented a dramatic increase of A. oligospora in BMR soil with the addition of an H. marelatus-parasitized moth larva. The current study tested two predictions based on the hypothesis that the H. marelatus-parasitized ghost moth larva is a unique and important resource for A. oligospora at BMR. First, because ghost moth larvae are concentrated in soil under bush lupines, we predicted that A. oligospora numbers would be greater under lupines than away from lupines. Second, we predicted that A. oligospora would be enhanced more by moth larvae containing living H. marelatus than by moth larvae containing dead H. marelatus or no H. marelatus or by nematodes alone. The first prediction was supported by data from a field study (A. oligospora population density was greater beneath lupines than in grasses 2 m away), but the difference was small. The second prediction was not supported by data from a laboratory experiment (dead moth larvae caused dramatic increases in A. oligospora numbers whether or not the dead moth larvae contained living nematodes). While H. marelatus are clearly unnecessary for the large increase in A. oligospora numbers, the importance of nematodes in general remains unclear because addition of dead moth larvae always resulted in large increases in bacterivorous nematodes and because addition of nematodes alone enhanced A. oligospora in one trial but not in two others.