Host-pathogen population dynamics, basic reproductive rates and threshold densities

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Populations of the Indian meal moth, Plodia interpunctella, were maintained in the laboratory in the presence of 2 pathogens, P. interpunctella Granulosis Virus (PiGV) and Bacillus thuringiensis, for between 9 and 12 generations. Neither pathogen had any discernible effect on the population dynamics of the host, either individually or in combination.

The dynamics of the two pathogens were very different, with PiGV persisting within the host population and B. thuringiensis usually becoming extinct. This behaviour can be explained by the host threshold density and the relationship between host density and the basic reproductive rate (R0) for each pathogen. The host threshold density was considerably less for PiGV than for B. thuringiensis, indicating that PiGV would be able to persist in much lower density populations than B. thuringiensis. None of the populations of P. interpunctella showed the generation-length cycles which have previously been described from similar experiments. This may be due to the quality, and possibly also the quantity of food available to the larvae.

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