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Forecasting western spruce budworm defoliation in mixed conifer stands by counting hibernating larvae

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  • An attempt was made in January 1971 to relate the numbers of hibernating western spruce budworm larvae on a square foot of bark surface with subsequent shoot damage on Douglas-fir and grand fir in northern Idaho. Twenty-six plots sampled in January were also sampled in April to determine if larval mortality due to breaking dormancy in midwinter occurred. An average of 3.9 and 1.8 larvae per square foot of bark surface occurred on Douglas-fir and grand fir logs respectively in January. In April, the average was 7.7 on Douglas-fir and 3.0 on grand fir. In January there was a significant difference between numbers of larvae on Douglas-fir and grand fir logs, but no difference in April. There was no significant difference in numbers of larvae on Douglas-fir logs between January and April, but there was a significantly higher number of larvae on grand fir logs in April. A regression analysis was made to obtain the correlation between larvae per square foot of bark surface in January and resultant foliage damage (sampled in September). There was no correlation on grand fir plots, and only a weak correlation on Douglas-fir plots.
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