HOST-PATHOGEN POPULATION-DYNAMICS, BASIC REPRODUCTIVE RATES AND THRESHOLD DENSITIES

Populations of the Indian meal moth, Plodia interpunctella, were maint ained in the laboratory in the presence of 2 pathogens, P. interpuncte lla 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 combinatio n. The dynamics of the two pathogens were very different, with PiGV pe rsisting within the host population and B. thuringiensis usually becom ing extinct. This behaviour can be explained by the host threshold den sity and the relationship between host density and the basic reproduct ive rate (R-o) for each pathogen. The host threshold density was consi derably less for PiGV than for B. thuringiensis, indicating that PiGV would be able to persist in much lower density populations than B. thu ringiensis. None of the populations of P. interpunctella showed the ge neration-length cycles which have previously been described from simil ar experiments. This may be due to the quality, and possibly also the quantity of food available to the larvae.