Patterns of spread in insect‐pathogen systems: the importance of pathogen dispersal

Abstract

A general model of insect‐pathogen dynamics is presented which includes explicit host and pathogen dispersal. Four distinctive, wave‐like, patterns of dispersal are produced which can be categorised by two universal parameters – the speed of advance, and the position of the leading edge, of the wave of dispersal of the host relative to that of the disease. These patterns are (1) the pathogen becomes extinct, allowing the host to disperse at the carrying capacity across the land surface, (2) the host disperses more rapidly than the pathogen, producing host densities at the carrying capacity in a region behind the leading edge of the wave, with these densities reduced due to interaction with the pathogen in the wave interior, (3) the host and pathogen disperse at the same speed but the leading edge of the host extends beyond that of the pathogen, allowing the host to reach ‘high’ density at the leading edge only, and (4) the host and pathogen disperse at the same speed but the leading edge of the pathogen extends beyond that of the host, producing ‘low’ density host dispersal across the land surface. A biological description explaining the causes of these patterns has important consequences regarding the use of pathogens for biological control of insect pests. The model is modified to represent a specific insect‐pathogen system, the winter moth, Operophtera brumata, and its nuclear polyhedrosis virus. The same patterns, categorised by the same universal parameters, are observed. Thus, it is suggested that the strength of infection and the relative dispersal rates of the host and pathogen are influential in determining the patterns of host outbreaks observed in this insect.