What are spruce beetles?
Spruce beetles (Dedroctonus rufipennis) are small bark beetles that have a geographical range that spans from Alaska to Newfoundland and as far south as New Mexico1. Bark beetles get their name because they reproduce in the inner bark of trees where they also feed on tree tissue. In the southern Rocky Mountains, spruce beetles, which are no larger than a grain of rice (Fig. 1), generally infect the Engelmann spruce tree in high elevation alpine forests. Although spruce beetles are a natural component of alpine forest ecosystems, under certain circumstances they can cause an outbreak that leads to significant tree mortality. High alpine forests are important habitats for wildlife, including the snowshoe hare and three-toed woodpecker, and provide clean air and water that contributes to a good quality of life for surrounding communities1.
Do spruce beetle outbreaks originate from a single location?
Understanding the origins of spruce beetle outbreaks can better equip forest managers with information to successfully contain deadly outbreaks. Recently, a group of scientists from the University of Colorado conducted a study using satellite data that aimed to determine whether spruce beetle outbreaks had a single origin or arose in multiple locations. They used satellite imagery to assess outbreaks of spruce beetles in the Southern Rocky Mountain Ecoregion, which includes New Mexico, Colorado, and Wyoming, from 2000-2014. First author Sarah Hart and her colleagues determined that approximately 29% of the spruce-fir tree zone in this region was impacted by spruce beetle outbreaks during the period from 2000 to 2014. Their findings further suggested that spruce beetle outbreaks originated from multiple distinct locations, which indicates that the cause of these outbreaks is regional in scale and not the result of a localized factor.
Does drought initiate spruce beetle outbreaks?
The authors were also interested in whether drought events helped to initiate spruce beetle outbreaks. During normal conditions, spruce trees allocate a portion of their energy to producing chemical compounds that deter beetles. Under these circumstances, spruce beetles generally only infest unhealthy trees that have less defensive compounds or downed trees. When water is limited, however, otherwise healthy trees have less energy reserves to devote to producing defensive chemicals and can therefore be more susceptible to spruce beetle infestation. This is especially true when spruce beetles are in high concentrations, since they use hormones that attract other spruce beetles to coordinate attacks against trees. Normally spruce beetles prefer to infest large spruce trees with a diameter greater than 16 inches, but during outbreaks trees with diameters as small as 3 inches can be targeted by beetles1.
By using the long-term satellite data on spruce beetle infestations and comparing it to data on summer, winter, and multi-year drought events, Sarah Hart and her colleagues were able to determine that droughts do indeed increase the likelihood of spruce beetle outbreaks. Of particular interest is the association between winter drought and beetle outbreaks, because it suggests that snowmelt water is a critical resource for trees to be able to sustain their defenses against beetles during dry, warm summers. Beetle infestation generally occurs during the summer months and having an adequate supply of snowmelt water may give trees a head start to produce defensive compounds to guard against spruce beetles.
What does the future hold for high altitude forests?
Spruce beetle outbreaks have been previously tied to increasing temperatures as a result of global climate change2. The current research findings by Sarah Hart and her colleagues highlight the importance of also considering the impacts of drought on these outbreaks. Global climate change will impact precipitation patterns and is likely to lead to increased drought events in many locations. In the southern and mountain ecoregions of the U.S., research has shown that drought severity increased from 1984-20113. By understanding the regional causes of spruce beetle outbreaks, forest managers will have better information to help them take preventative measures to help prevent wide scale damage. Currently, managers use a combination of strategies including removing downed spruce trees and creating tree stands that are composed of a variety of species to increase resilence1. Other strategies include preventative spraying of high value trees and using bait trees to contain and eliminate spruce beetle outbreaks1. Bait trees are treated with a hormone that attracts emerging adult beetles, and once the tree becomes infested, it is removed and destroyed in an effort to decrease the spruce beetle population1. As our understanding of the drivers and dynamics of spruce beetle outbreaks improves, better proactive management strategies will be developed to help minimize their negative impacts in the future.
Hart, S.J., Veblen, T.T., Schneider, D., Molotch, N.P. 2017. Summer and winter drought drive the initiation and spread of spruce beetle outbreak. Ecology 98: 2698-2707.
2Bentz, B.J.J., Regniere, J, Fettig, C.J., Hanson, E.M., Hayes, J.L., Hicke, J.A., Kelsey, R.G., Negron, J.F., Seybold, S.J. 2010. Climate change and bark beetles of the Western United States and Canada: direct and indirect effects. BioScience 60: 602-613.
3Dennison, P.E., Brewer, S.C., Arnold, J.D., Mortiz, M.A. 2014. Large wildfire trends in the western United States, 1984-2011. Geophysical Research Letters 41: 2928-2933.