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.
Abstract
The patterns of spatial growth of small epicentres of nuclear polyhedrosis virus disease of Gilpinia hercyniae (Htg.) (GHNPV) were studied in spruce forests in Wales. Very little virus spread occurred in the 1st infected sawfly generation, but thereafter it was considerable. Three phases were distinguished. 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 plus or minus 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 that characterised the secondary dispersal phase. Following this, the wave form was lost, and the pattern of disease became less coherent. This 3rd phase is referred to as the interference phase. The tripartite development sequence agreed well, both in terms of form and scale, with information on the spread of GHNPV disease in G. hercyniae in Canada obtained by F.T. Bird & J.M. Burk [RAE/A 50, p. 24-5]. A very similar sequence was detected in published information on the spread of a non-occluded baculovirus of Oryctes rhinoceros (L.) in Tonga. The same pattern of primary dispersal was evident in studies in Canada on the spread of the nuclear polyhedrosis virus affecting Malacosoma disstria Hb. [RAE/A 55, p. 829]. The diseases spread further in O. rhinoceros and M. disstria than in G. hercyniae, the estimated gradients of primary dispersal being -0.25 and -0.37, respectively.