FOREST HEALTH: Eating Themselves Out of House and Home: Mountain Pine Beetle Attack Spruce!
by Brian Aukema, Robert Hodgkinson, Dezene Huber, and Staffan Lindgren

The current outbreak of mountain pine beetle (MPB) in BC, and now Alberta, has attracted immense media attention as it has rolled through more than nine million hectares of mature lodgepole and ponderosa pine. Mountain pine beetles breed in almost all pines, raising concern that the insect may expand into the jack pine of the boreal forest that stretches across Canada to the Maritimes. Behind the eastern front, most of the insect’s typical food supplies have been exhausted. For example, in many parts of the central interior of BC, the insect, facing starvation, is increasingly attacking young lodgepole pine and, to a much lesser degree, mature spruce.

For mountain pine beetles to successfully reproduce in trees, two things must occur. First, there must be enough insects to successfully overwhelm the tree’s defensive capacities. Mountain pine beetles use chemicals known as pheromones to attract their mates to trees en masse. This mass attack, in concert with fungi vectored by the beetles, is able to kill mature trees in outbreak conditions. Second, the subcortical tissue of the trees, once gained, must be suitable for brood to develop. For example, while stands of small-diameter juvenile lodgepole pine killed by mountain pine beetle may pose serious challenges for the future timber supply, attacking such trees is actually a dead end for the beetle as a reproductive strategy. In young pine with diameters less than approximately 17 cm, the phloem, which the insects eat, is simply too thin for larvae to mature to adults.

Over the past two years we have noted a number of successful colonizations of mature interior hybrid spruce by MPB. In truth, attacks by MPB on spruce have been documented several times in the past one hundred years, stretching back to records from the father of forest entomology, A.D. Hopkins, in 1921. Such attacks often occur when the insects are abundant, such as during outbreak conditions. As a rule, however, the attacks are not very successful. Spruce trees are rarely killed, and even if the trees are colonized, the broods rarely develop to adults. Thus, we have been surprised to find that brood production in spruce, at least in the wake of the current outbreak, may be sufficient in some instances to result in a new generation of beetles emerging to attack new trees!

We are conducting research to address four potential hypotheses for why MPB are reproducing in the occasional spruce. First, this so-called epiphenomenon may be simply due to the unprecedented population pressure given the magnitude of the current outbreak. Second, we may be witnessing an exceedingly rare, but not impossible, host switch. Species of bark beetles in the genus dendroctonus exhibit many different host preferences. Some attack all pines, others only one, while others attack only spruce or Douglas-fir, or larches. A likely step in the speciation of tree-killing bark beetles that specialize on certain hosts is the formation of “host races” or strains that become reproductively isolated in different tree species. Third, differences in the population genetics or characteristics of MPB between locations may be facilitating the phenomenon. Perhaps there are always a few insects that are able to colonize spruce, but it has taken until the current outbreak to concentrate sufficient numbers in one location. Finally, the phenomenon simply may be due to site-specific characteristics of these individual spruce trees, and not the insects at all. There may be something unique about the trees’ vigour, physiology, and beetle susceptibility that have made them desired candidates versus other nearby spruce.

We are currently studying colonization behaviour, reproduction potential, and fungal transmission by MPB in spruce using a combination of laboratory and field experiments. Our research, based at the University of Northern British Columbia (UNBC), is aided by graduate and undergraduate students at the institution and collaborators at the Canadian Forest Service Pacific Forestry Centre in Victoria, the BC Ministry of Forests, and UBC. To date we have drawn only one conclusion after reading Hopkins and others’ initial notes in this subject area: the bugs never read the scientific literature!

Brian Aukema (baukema@nrcan.gc.ca) is a research scientist in forest entomology with the Canadian Forest Service, Natural Resources Canada, based at UNBC as an assistant adjunct professor in the Ecosystem Science and Management program. Robert Hodgkinson (Robert.Hodgkinson@gov.bc.ca) is forest entomologist for the Northern Interior Forest Region for the BC Ministry of Forests and Range in Prince George. Dezene Huber (huber@unbc.ca) is an assistant professor and the Canada Research Chair in Forest Entomology and Chemical Ecology at UNBC. Staffan Lindgren (lindgren@unbc.ca) is a professor of forest entomology in the Ecosystem Science and Management Program at UNBC.