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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.

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