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An unusual feeding behavior of the western spruce budworm (Choristoneuraoccidentalis Freeman) reduced height growth and deformed young western larch (Larixoccidentalis Nutt.). On western larch, budworm larvae did not confine their feeding to foliage, their usual diet on other conifers; they also fed on and severed stems of current-year terminal and lateral shoots. A 5-year study in young larch stands showed that: (1) amount and severity of budworm damage increased annually, (2) all trees were damaged sometime during the study, (3) severance of the terminal shoot was the most serious type of budworm damage to larch, (4) severances reduced net height growth at least one-fourth and resulted in forked, bushy-topped trees, and (5) specific forks in the tree did not persist more than 5 years because of the strong apical dominance characteristic of larch.

Unseasonal temperatures, as low as −6°C, were recorded in western Montana in mid-June 1969, a period when western spruce budworm (Choristoneura occidentalis Freeman) larvae were actively feeding on newly developing foliage of coniferous host trees. Studies in progress at that time provided circumstantial evidence that the freeze reduced: (1) budworm populations on Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), western larch (Larix occidentalis Nutt.), and ponderosa pine (Pinus ponderosa Laws.) over 90%; and (2) budworm damage to young larch 54–71%.