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

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Abstract
The patterns of spatial growth of small epicenters of nuclear polyhedrosis virus disease of G. hercyniae (GHNPV) were studied in Welsh [UK] spruce forests [Picea abies and P. sitchensis]. Very little spread of virus occurred in the 1st 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 epicenter. 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.