Naturally-occuring nuclear polyhedrosis virus for the biological control of douglas-fir tussock moth, Orgyia pseudotsugata, McDunnough, (Lepidoptera : Lymantriidae)
Date
1995
Authors
Laitinen, Ann Marie
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Abstract
Studies were conducted to investigate the interaction of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough) (Lepidoptera: Lymantriidae), and its naturally-occurring nuclear polyhedrosis virus. Egg masses were collected from 42 locations in the Kamloops Forest Region of British Columbia in the fall of 1991 and 1992. Regression equations for the number of eggs per egg mass against the weight of the egg mass with and without the cocoon, area and volume of the egg mass, showed that weight without the cocoon is the most closely correlated with the number of eggs. The best estimate of the number of eggs per egg mass can be obtained from the weight without the cocoon, although all regression equations can be used but they are less accurate. The regression equations were created for each age of the outbreak (age 0, age 1 and age 2) and for each collection year (1991 and 1992). Since the regression equations were significantly different, except for weight without the cocoon in 1991, the area and volume of the egg mass in 1992 and the volume of the egg mass for age 1 when the two collection years were compared, they cannot be combined. The regression equations do not pass through the origin.
The incidence of NPV infection was higher in 1992 than in 1991, and higher in age 1 of the outbreak than age 2 of the outbreak. It was hoped that a threshold level of NPV infection could be identified and that would allow forest managers to treat only those sites not close to natural collapse with NPV. However a trend of increasing or decreasing NPV infection that would allow such a prediction has not been established in this study. To ensure that the incidence of viral infection is not raised artificially in the laboratory, tussock moth egg masses should be stored dry in paper bags for diapause development conditions.
Late instar, virus infected larvae can be used for extraction of viral DNA and genotypic variation studies. When Southern blot analysis of viral DNA was completed, polymorphisms were identified when digested with Pst I, Sal I, Bgl II and Hind III restriction enzymes. Five genotypes were obtained when viral DNA was digested with Pst I and Sal I enzymes. Three of the Pst I genotypes are unique and two are only found in the Kamloops geographic region.
When viral DNA extractions were completed, a coincident infection with a cytoplasmic polyhedrosis virus was noted. The CPV viral RNA is resistant to digestion with RNAase A at high ionic strengths, but digests under low ionic strength. The pattern of the RNA segments resembles those previously published and is within the reported size range for the DFTM sock cytoplasmic polyhedrosis virus. The viral extracts do not hybridize to chemiluminescent labelled Douglas-fir tussock moth viral DNA, when Southern blotted.