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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.

We studied the population biology of the nematophagous fungus Hirsutella rhossiliensis to understand its potential as a biological control agent. Because the fungus is an infectious and transmissible parasite, we framed our study within an epidemiological context. Field observations, theory, and experiments demonstrated that (i) parasitism of nematodes by H. rhossiliensis is dependent on nematode density, (ii) local populations of the fungus will go extinct unless supplied with some minimum number of nematodes (the host threshold density), and (iii) natural epidemics of this fungus in populations of nematodes develop slowly and only after long periods of high host density. Additional in-depth research on population biology is needed to explain other biological control systems and to guide future research. The most effective research will combine field observation, theory, and experimentation.

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