Can spruce beetle (Dendroctonus rufipennis Kirky) pheromone trap catches or stand conditions predict Engelmann spruce (Picea engelmannii Parry ex Engelm.) tree mortality in Colorado?

Bark beetles (Coleoptera: Curculionidae: Scolytinae) can cause extensive tree mortality in forests dominated by their hosts. Among these, the spruce beetle (Dendroctonus rufipennis) is one of the most important beetles in western North America causing Engelmann spruce (Picea engelmannii) tree mortality. Although pheromone traps with attractants are commonly used to monitor spruce beetle populations, the relationship between the numbers of beetles caught in pheromone traps and subsequent tree mortality has not been investigated adequately. We used pheromone traps to catch spruce beetles in plots throughout the insect flight period, quantified subsequent tree mortality, and modelled spruce tree mortality as a function of spruce beetle trap catches and stand conditions. The number of beetles caught was not different between years. It was also positively associated with tree mortality, as was the amount of available host. The year of sampling was significant in all models as a result of different mortality levels between years. We conclude that, although the models had good fit, the difference in mortality between the years with a similar beetle catch negates reliable estimates of tree mortality across years. Managers and forest health specialists will be better served by continued monitoring of spruce beetle populations with pheromone traps and the use of stand variables to identify susceptible stands.