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

A general model of insect-pathogen dynamics is presented which includes exp licit host and pathogen dispersal. Four distinctive, wave-like, patterns of dispersal are produced which can be categorised by two universal parameter s - 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 a re (1) the pathogen becomes extinct, allowing the host to disperse at the c arrying capacity across the land surface, (2) the host disperses more rapid ly 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 exten ds beyond that of the pathogen, allowing the host to reach 'high' density a t 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 biologi cal description explaining the causes of these patterns has important conse quences regarding the use of pathogens for biological control of insect pes ts. The model is modified to represent a specific insect-pathogen system, the w inter moth, Operophtera brumata, and its nuclear polyhedrosis virus. The sa me patterns, categorised by the same universal parameters, are observed. Th us, it is suggested that the strength of infection and the relative dispers al rates of the host and pathogen are influential in determining the patter ns of host outbreaks observed in this insect.