Temporal density—dependent parasitism and a host threshold density are important features of disease induced by infectious parasites in populations of aboveground, macroscopic organisms. We determined whether these features also occur in soil microcosms containing a microscopic host (the nematode Heterodera schachtii) and its parasite (the nematophagous fungus Hirsutella rhossiliensis). Soil microcosms are especially interesting because (1) the environment and scale are quite different from conventional host—parasite systems and may result in considerably different disease dynamics, (2) the small size of the soil microcosms, although biologically appropriate, facilitates experimentation and parameter estimation, and (3) some soil—borne, microscopic organisms (such as H. schachtii) are important agricultural pests. Temporal density—dependent parasitism was directly assessed with laboratory experiments in which host density and environment were controlled. A theory, which complements and extends the experiments, was developed to enable direct comparison of observed and predicted dynamics and to provide a stringent test of our understanding of processes underlying the dynamics. The theory was simple, yet explicitly described the essential biology. Parameters for the theory were measured with short—term experiments. We found that the disease dynamics in soil microcosms exhibited both temporal density—dependent parasitism and a host threshold density. However, epidemics were slow to develop. Observed and predicted dynamics were quite similar, indicating that our understanding of the underlying biology was correct.

The feeding biology of Enchytraeus crypticus and other enchytraeids is poorly understood as is their effect on nematophagous fungi. Because enchytraeids had been associated with nematophagous fungi in the field and had suppressed these fungi in soil microcosms, we tested the hypothesis that exclusion of enchytraeids, largely E. crypticus, would improve establishment of certain nematophagous fungi in field plots. The fungi, Hirsutella rhossiliensis and Monacrosporium gephyropagum, are being studied as potential control agents of plant-parasitic nematodes and were formulated as hyphae in alginate pellets. The pellets were mixed into soil without enchytraeids and placed in cages (PVC pipe, 80 cm volume) with fine (20 μm) or coarse (480 μm) mesh; cages were buried 15 cm deep in field plots and then recovered after 6-52 days. When fine mesh was used, enchytraeids were excluded and the fungi increased to large numbers. When coarse mesh was used, enchytraeid numbers in cages increased rapidly and the fungi did poorly. Although mesh also affected other potential fungivores, including collembolans and large dorylaimid nematodes, we suspect that enchytraeids were more important because large numbers were consistently found in cages with coarse mesh soon after the cages were placed in soil. Organisms smaller than enchytraeids (bacteria, fungi, and protozoa) also appeared to be important because the fungi did better in heat-treated soil than in non-heat-treated soil, regardless of mesh size. The rapid increase in enchytraeid numbers in cages with hyphal pellets and coarse mesh was probably caused by movement of enchytraeids toward the pellets with hyphae: increase in enchytraeid numbers was minimal when movement into cages was blocked (or when cages contained pellets without hyphae). Overall, the data were consistent with the hypothesis that enchytraeids, or other meso- or macrofauna, contributed to suppression of nematophagous fungi in our field plots.

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