Transmission of Plodia interpunctella granulosis virus does not conform to the mass action model

1. Transmission of insect pathogens is traditionally described by a term which states that transmission is proportional to the densities of the susceptible hosts and the infectious units, multiplied by a constant, the transmission coefficient. Theoretical studies suggest chat deviations from this can be important in host-pathogen population dynamics, but little is known of how commonly pathogen transmission conforms to the conventional model. 2. We describe a test of the traditional assumption for the Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae) (Hubner) and its granulosis virus using a modification of the previous methods, which allows for unpredictable declines in the amount of infectious material present. 3. The estimated transmission coefficient increased with the density of susceptible hosts and showed a marked decline with density of infectious cadavers. This suggests that the usual assumption does not adequately describe transmission in this system. 4. The reasons for this deviation from the usual assumption are likely to be a combination of behavioural and physiological changes at high host density, and differential susceptibility to the pathogen leading to an effect analogous to pseudo-interference in parasitoids.