Mountain Pine Beetle and Their Hosts
What does mountain pine beetle look like?
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Mountain pine beetle life cycle |
Mountain pine beetle usually only breed one generation each year, but because of the warmer weather, two generations in some areas have been seen. Usually mountain pine beetle emerge out from hosts and spread out in July or August. After adults found suitable hosts, they started to mate and lay eggs under bark of trees. Larvae hatched out around September, they usually survive from winter as the third stage or fourth stage larvae. In next spring, larvae will become pupae and adults will emerge from pupae in June or July. After adults mature enough to fly out, a new spread will start. But mountain pine beetle life cycle could be influenced by temperature to a big degree, the specific time of each life stages could be quite different in different years.
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Mountain pine beetle in Alberta
The first outbreak of mountain pine beetles in record of Alberta happened during 1940 to 1943 in Banff (Powell, 1966). But the cold weather and artificial intervention stopped the spread successfully. However, from the beginning of 1970, an infestation of mountain pine beetle has been found in the north-west corner of Alberta. From 2006, a long distance disperse by wind carried more mountain pine beetles arrived in the similar area in Alberta, a spread out has been detected in Alberta since then. However, more researches have predicted that mountain pine beetles are showing the tendency to move far more east in Canada. (sources from Alberta environment and parks).
Hosts of mountain pine beetle
Mountain pine beetle's host range is quite wide. Although the original hosts in western Canada are lodgepole pine, ponderosa pine and western white pine only, actually the other north American pines and some exotic species can be infected and killed. In Alberta, pine represents approximately 41% of the coniferous forests, most of pines are lodgepole pines and jack pines. However both of species had been infected by mountain pine beetles, even there are a big proportion of lodgepole pines in Alberta. The potential hosts of mountain pine beetles seem to make a larger range than before.
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Changes on lodgepole pines after mountain pine beetle attack
Mountain pine beetles only attack the phloem part of lodgepole pine trees, but they usually bring in several kinds of fungus, called "blue stain fungi". Those fungi can attack xylem of pine trees which damage transpiration very quick. Because of losing water, needles turn into less bright and dull green after being attacked by mountain pine beetle within the first year. From second year, needles change to red (Fig 7 and Fig 8), during this time the trees are dead already. But those foliage will keep changing to gray and falling down until the bark peeling off. These progress could be up to 10 years which means the killed trees could stand up to 10 years without other abiotic disturbance coming in (such as fire, windstorm, etc.).
HOWEVER, mountain pine beetles do NOT attack every single lodgepole pine:
As a kind of bark beetles, mountain pine beetles usually hide under bark of lodgepole pine to survive the cold temperature in winter. The bark also helps beetles to escape from predators and prositoids. The thicker bark could protect beetles better. So mountain pine beetles usually attack larger size of trees. That preferences is one of big reasons which mortality of lodgepole pines shows a huge range (0%-100%) in different environmental situation. In Fig 6 and Fig 7, mortality of lodgepole pines (in red circles) is very different in different elevation, different mixed-wood stands with different basal area of lodgepole pines. The moisture of mountain slopes could drive the morality of pine trees quite different as well. That big range of mortality of lodgepole pines brought more variabilities in post-MPB stands (post mountain pine beetle stands).
Post-MPB stands
Lodgepole pine is a shade-intolerance species, which means their seedlings need certain area of open canopy to grow up. However, given to most of dead trees standing in forestry, those shade-tolerance species showed more advantages in post-MPB stands, such as Douglas-fir, subalpine fir and white spruce. On the other hand, losing big proportion of mature lodgepole pines made seed bank decrease dramatically. Without fire coming in to stands, those seeds cannot generate and finally lose chance to open after couple years. In this case, those residual lodgepole pines in post-MPB stands, as the only seed resources for future lodgepole pine species, become way more important. Understanding their living conditions and current healthy status will be very helpful for future forest management and forestry ecology.