Genotypic variation among Douglas-fir tussock moth nucleopolyhedrovirus (OpNPV) isolates in the western United States

Abstract

Periodic outbreaks of the Douglas-fir tussock moth (Orgyia pseudotsugata) in forests of western North America generally end with a sudden collapse due primarily to an epizootic caused by a nucleopolyhedrovirus (NPV) that occurs naturally within O. pseudotsugata populations. We genotypically characterized NPV populations from Washington State, Oregon, Idaho, New Mexico and California for the first time. Of 159 infected tussock moth samples, restriction fragment length polymorphism (RFLP) analysis showed that 125 (78.6%) contained single nucleopolyhedrovirus (OpSNPV), 28 (17.6%) contained multiple nucleopolyhedrovirus (OpMNPV), and six (3.8%) contained both OpSNPV and OpMNPV. In comparison, our previous studies in the southern interior of British Columbia showed that all 298 samples examined were infected with OpMNPV, and none were infected with OpSNPV. More than half of the Washington OpSNPV samples shared the same genotype, but most OpSNPV genotypes were rare or unique: across the five states, 31 of the 43 different OpSNPV genotypes were each only found in a single sample. In contrast, only four different OpMNPV genotypes were found, and 29 of the 34 OpMNPV samples shared the same genotype, designated genotype AA. This strain of OpMNPV has been developed, registered and used in both Canada and the United States to control outbreaks of the Douglas-fir tussock moth. It is also the most common genotype in southern British Columbia. The estimated degree of genetic divergence ranged from 0% to 4.19% for the various OpSNPV genotypes and from 0% to 3.16% for the OpMNPV genotypes (based on number of shared bands). This is the first description of the genotypic diversity in a population of OpSNPV, and the first genotypic characterization of NPVs infecting O. pseudotsugata in the USA.