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UBC Theses and Dissertations

Development of a DNA probe for detection of nuclear polyhedrosis virus in the forest tent caterpillar Malacosoma disstria Hbn Kukan, Barbara


Many species of forest Lepidoptera show eight to twelve year population cycles. Viral disease could explain characteristics of population fluctuations in forest tent caterpillars (FTC) Malacosoma disstria Hbn. Until now, virus detection relied on microscopic exam ination for polyhedra of virus in smears of caterpillars or other potentially contaminated material on stained microscope slides. The main objective of this thesis was to develop an efficient detection system for nuclear polyhedrosis virus (NPV) of FTC in the form of a NPV DNA probe. Pure DNA was isolated from virions of the western tent caterpillar, digested with BamHI, and a 2,000 bp fragment which hybridized to the polyhedrin gene of Autographa californica NPV cloned into pGEM then purified for use as a probe. Samples of FTC from a peak density population in the vicinity of Prince George B.C. were tested for presence of virus using both detection systems, the DNA hybridization to a NPV DNA probe, and microscopic examination for polyhedra. Results showed variation between the two techniques but no consistent trend toward one technique showing more or fewer infected individuals. Large sample sizes are necessary to overcome this variation. Because it is much easier to process large sample sizes with the DNA probe its benefits are evident. Distribution of virus in the Prince George population of tent caterpillars was examined. Samples of different life stages of FTC adults, larvae, pupae, egg masses as well as FTC parasitoids were tested with the DNA probe. Some individuals of all stages and parasitoids gave positive responses to the DNA probe with adults and parasitoids showing a particularly high frequency of infection. Egg mass and parasitoid contamination could play a role in transmission of the virus. Transmission of virus was tested by feeding caterpillars virus and testing adults which survived and emerged for the presence of NPV. A high proportion of adults gave positive results. A higher proportion of smaller caterpillars were positive for NPV in six of eight comparisons of large and small caterpillars from the same populations. This suggests that the virus might reduce growth of the infected caterpillar or that smaller caterpillars die before they reach large size which reduces the proportion of infected individuals among large caterpillars. A comparison of large egg masses (many eggs) and small egg masses (few eggs) showed that fewer small egg masses were contaminated with NPV. The cause of this variation is unclear, but this relationship could be crucial to the interpretation of changes in egg mass size in field populations and requires further study.

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