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Characterization of nucleopolyhedroviruses infecting western (Malacosoma californicum pluviale) and forest tent caterpillars (Malacosoma disstria) in British Columbia, and detection of sublethal infection in field populations of forest tent caterpillars Cooper, Dawn M.

Abstract

Viral diseases are an important feature of fluctuating lepidopteran populations. Epizootics of baculovirus disease can kill large numbers of hosts, making them attractive agents or biological control. The drive to improve virus efficacy by genetic manipulation has focused attention on the behaviour of baculoviruses in nature. I examined the genetic diversity present in nucleopolyherovirus (NPV) populations infecting tent caterpillars in British Columbia. Natural baculovirus isolates are genetically diverse in many different lepidopteran systems. I used REN analysis to survey genetic diversity in six different NPV populations infecting the western tent caterpillar, Malacosoma californicum pluviale (Dyar). In total, 14 genetic variants were found among the six populations sampled. Genetic variation was structured at the population level. Bioassays detected subtle differences in pathogenecity associated with different NPV populations, suggesting that local genetic variation in NPV could influence local host dynamics. The use of baculoviruses as biopesticides has emphasized a need to determine host range for a number of different viruses. I characterized the NPVs infecting two closely related host species, the western tent caterpillar, Malacosoma californicum pluviale (Dyar), and the forest tent caterpillar, Malacosoma disstria (Hiibner), and determined the cross infective potential of each virus. M.c. pluviale NPV (McplNPV) and M.-disstria NPV (MadiNPV) were genetically distinct. Cross infection bioassays demonstrated that 3rd instar M.c. pluviale larvae were not susceptible MadiNPV. In the reciprocal cross infection, only a small proportion of M. disstria larvae was susceptible to McplNPV. The majority of mortalities were the result of an unexpected MadiNPV infection. PCR failed to detect MadiNPV contamination in the McplNPV stock. Laboratory contamination and contamination of egg mass surfaces were unlikely. I suggest the virus was present as either a latent or sublethal infection that was vertically transmitted from females to larvae within the eggs themselves. These findings suggest that NPV may influence host dynamics. First, genetic variability may be associated with life history traits important for virus survival and may therefore influence local host dynamics. Second, the detection of a latent or sublethal infections provides a mechanism for both enhancing virus dispersal and maintaining virus during low host densities.

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