Epizootiology of a Nuclear Polyhedrosis Virus (Baculoviridae) in European Spruce Sawfly (Gilpinia hercyniae): Spread of Disease from Small Epicentres in Comparison with Spread of Baculovirus Diseases in Other Hosts

The patterns of spatial growth of small epicentres of nuclear polyhedrosis virus disease of Gilpinia hercyniae (GHNPV) were studied in Welsh spruce forests. Very little spread of virus occurred in the first sawfly generation infected but thereafter it was considerable. Three phases were recognized. The pattern of primary spread followed an indented curve of rapidly diminishing disease incidence with distance from the epicentre. Logarithmic conversion of the units of distance and disease incidence transformed the curves to straight lines. The gradient of primary dispersal (b = -1.98 ± 0.16) was similar for spread in different forests and in different years. Epicentral flattening of the primary dispersal curve preceded the development of a wave-like pattern which characterized the secondary dispersal phase. Following this, the wave form was lost and the pattern of disease became less coherent. This was called the interference phase. The tripartite developmental sequence agreed well, both in form and scale, with figures published by Bird & Burk (1961) on the spread of a GHNPV disease in G. hercyniae in Canada. A very similar sequence is detectable in data published by Young (1974) on the spread of a non-occluded baculovirus disease of Oryctes rhinoceros (ORBV) in one of the Tonga Islands. A short-term study on the spread of Malacosoma disstria nuclear polyhedrosis virus (MDNPV) in Canada (Stairs 1965) indicated the development of the same pattern of primary dispersal. Disease spread further in O. rhinoceros and M. disstria than in G. hercyniae, the estimated gradients of primary dispersa: being -0.25 and -0.37 for ORBV and MDNPV diseases, respectively.