MOUNTAIN PINE BEETLE
Invasion of Boreal Forests
by David Langor, Adrianne Rice, and Daryl Williams

The mountain pine beetle (MPB) is a tiny insect with a lot of clout. These 4-6 mm-long beetles seem clumsy and harmless as they walk around in the palm of your hand; however, their power and impact stems from their immense numbers. The extensive over-mature pine forests of BC provides an ideal breeding ground that produces trillions of beetles every year. Currently over 8 million ha of lodgepole pine forests have been infested in BC, and over the last 7-8 years the MPB has been steadily expanding its impact and range in Alberta. The recent dramatic increase in MPB populations have been attributed to a combination of several successive, warm winters that aid beetle survival, and a copious abundance of over-mature lodgepole pine forests that are highly susceptible to beetle attack.
Adult beetles fly well and are very efficient at dispersing while locating suitable hosts. Once a host is located (late July and August), female beetles produce chemical attractants that call in other beetles to mass attack the tree. The MPB is allied with several species of fungi that they inoculate into the trees. Once the fungi become established they spread quickly to block the water and food transportation system in the tree, thereby helping to overcome tree defenses. These fungi stain the outer wood (sapwood) a blue-green colour, and are thus called blue-stain fungi. After the beetles lay eggs, hatched larvae feed in the phloem, further destroying the conducting tissue of the tree. Larvae overwinter, continue feeding and development the following spring, and a new generation of adults emerge in late July and August. At high elevations and latitudes, development may take longer than one year.
The MPB is rapidly expanding its range. Although it is native to Alberta, previous outbreaks in the 1940s and 1977-1985 were restricted to the southwest. Since the mid-1980s, the MPB has spread at least two degrees latitude northward in Alberta. Populations have persisted in the Wilmore Wilderness area for the past 8 years, and dramatically increased from 2004-2006. As well, the MPB has invaded forests east of the Rocky Mountains in northeastern BC. In 2006, beetles penetrated into northwestern Alberta at least 250 km, resulting in widespread and successful colonization of pine in the western boreal forest. There is great concern that the MPB may continue to move eastward in the boreal, ultimately resulting in colonization of jack pine. If beetle populations became established in jack pine, this would establish a potential conduit for invasion of eastern Canada and the southeastern US.
The MPB has a broad diet of pine hosts. Although lodgepole pine is its most common host, the MPB attacks and kills many native and introduced pine species within its range. Jack pine, which ranges across the boreal region from Alberta to the east coast, is taxonomically and chemically similar to lodgepole pine. In Alberta, lodgepole and jack pines readily hybridize, creating a large “hybrid zone” over much of the north central and northwestern part of the province. In 2006, the MPB successfully colonized hybrids in the Grand Prairie and Peace River areas, as far east as Fox Creek. Therefore, the risk of MPB continuing to disperse eastward, with prevailing winds, into jack pine forests is high. MPB can successfully reproduce in cut logs of jack pine, but this success cannot be extrapolated to natural stands of healthy, living trees. It is unknown whether MPB can or will naturally colonize and breed in living jack pine. The answer to this question is critical to enable appropriate management planning in boreal forests. While it cannot be answered directly without risking the introduction of MPB into susceptible stands, it can be predicted using a series of lab and field experiments. Over the last two years, staff at the CFS Northern Forestry Centre in Edmonton has been involved in research to assess the risk of invasion of jack pine by the MPB. This work is funded by the provinces of Alberta, Saskatchewan, Manitoba and Ontario, the federal Mountain Pine Beetle Initiative, and the US Forest Service.
Inoculations on live trees have shown that jack pine is at least as susceptible to the three species of MPB-associated, blue-stain fungi as is lodgepole pine. We also found that fungi are adapted to the colder, boreal temperatures, although the three species perform differently depending on temperature profiles, virulence (the speed of fungal spread in the tree), and spore production.
As beetles and larvae are restricted to the phloem layer of the bark, the thickness of this layer is critical to beetle success and population increases. Our work shows that jack and hybrid pines have much thinner phloem (0.8-1.5 mm) than mature lodgepole pine (2.0-3.5 mm), suggesting that jack pine and hybrids may be less suitable hosts for MPB. However, laboratory breeding work showed that MPB does at least as well (in terms of fecundity and survival) in jack pine bolts as in lodgepole pine bolts of similar thin phloem thickness (1.0-1.3 mm), but not as well as beetles in thick-phloem lodgepole pine. Furthermore, beetles emerging from thin-phloem hosts are significantly smaller than those from wild populations in lodgepole pine, and this has implications for beetle fecundity and survival. Based on this work, there is no obvious biological barrier to MPB invasion of hybrids and jack pine.
In 2006, the large penetration of MPB into northwestern Alberta resulted in successful colonization of hybrid pines, corroborating our predictions. This unexpected, large eastward dispersal also provided us with an opportunity to commence investigating the success of MPB in hybrids in the wild. This preliminary work revealed several interesting facts: 
1) MPB successfully attacked and bred in hybrids, as our lab work predicted.
2) Most brood developed to fourth larval instars, pupae, and adults before the onset of winter, compared to the more normal situation of overwintering in first, second, and third larval instars. As larger larval instars have better cold tolerance, we expect populations in northwestern Alberta to have very good overwintering survival. Furthermore, we expect an unusually early start to the dispersal period. This may result in quite a different life cycle for MPB in the boreal compared to portions of the range further west and south. The implications of this scenario need to be explored.
3) Mortality of MPB in hybrids preceding winter was quite low. It is particularly interesting that there was virtually no parasitism. This will aid rapid increase in MPB populations.
Clearly the invasion of the boreal forest by MPB is well underway. The possible spread of MPB to jack pine has serious economic, social, and environmental implications for Canada. Alberta is now the front line for battling the MPB and preventing (or slowing) eastward invasion. Our continuing research is critical to understand how the MPB is adapting to the boreal environment and a novel host in order to predict future trends and provide the best advice to forest managers to enable appropriate proactive planning.

David Langor is Research Scientist - Insect Management & Biodiversity with the Canadian Forest Service and can be reached at 780-435-7330.