Aerial surveys of six Montana National Forests in 1975 found a 22.1% increase in the area of visible defoliation caused by the western spruce budworm. It is estimated that 2,278,804 acres of Douglas-fir forests are now suffering at least 25% defoliation. This is an increase of 503,706 acres over the 1,775,098 acres defoliated in 1974. Egg mass surveys were made in infested areas to predict the degree of defoliation in 1976. Based on this survey, the following mean percent defoliation is predicted for each National Forest: Deerlodge, 56%; Helena, 57%; Gallatin, 29%; Beaverhead, 37%; Flathead, 5%; Lolo, 24%.
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