Infection of young western larch by larch dwarf mistletoe in Northern Idaho and Western Montana

Six-hundred dwarf mistletoe-infected western larch (Larix occidentalis) ranging in age from 10-20 yr were sampled to determine the approximate age and height at which they were initially infected by larch dwarf mistletoe (Arceuthobium laricis). The age of each observable mistletoe infection was determined for each sapling, and the age and height of the trees when they were initially infected were estimated based on the age of the oldest mistletoe infection. Few of the young larch sampled were infected before they reached 7 yr old or 1.5 m in height. Most of the young larch, however, were infected before they were 14 yr old or 4 m in height. Initial age and height of infection were significantly influenced by the infection intensity of the overstory larch growing within 12 m of the infected saplings. Because parasitism by larch dwarf mistletoe can be extremely damaging to western larch, infected overstory larch should be removed or killed before nearby larch regeneration reaches 7 yr old or 1 m in height when minimizing infection of the regeneration is a management objective.