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The Canadian Forest Service describes the current Mountain Pine Beetle epidemic as the largest known insect infestation in North American History. There is little doubt that the American Forest Service and the National Parks Service are having a terrible time battling the ever-growing outbreak as well. It spreads from Alaska through New Mexico - anywere that there are conifer forests.
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The Mountain Pine Beetle is not much different than any other 4-stage insect as far as the development cycle goes.
The major difference is WHERE the metamorphosis takes place: UNDER the bark of the pine tree where it is impossible to affect the cycle.
The adult female bores into the tree, tunnelling under the bark and creating characteristic sawdust dropping to the base of the tree. When she has reached the inside, she lays her eggs (usually about 75) in a vertical egg gallery. |
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When the eggs hatch and metomorphosize into larvae, the tiny "worms" begin eating and crawling their way away from the egg nursery, leaving trails under the bark. They spend the winter in this state and transform in the spring into pupae in June and July and then into adults during the "flying season" usually mid-July - mid-August depending on the tree species and the altitude.
Logde Pole Pines in Colorado usually become infested in early July to early August; Ponderosa Pine from Aug 1 to end August. (these are estimates only) Beetles shun sunlight and heat, so prefer to stay in shady, cool parts of the forest. They start to migrate when the temperature reaches about 70 degrees but stays below 90 degrees. They do not fly in the rain.
If the temperature drops adequately during the winter months
(-30 ° F for 5 days) they will succumb to freezing. However, with climate change, warm winters, and drought that weakens the trees, the beetles have been surviving the winter freeze to become reproducing adults.
They live to bore, reproduce, fly and bore again into the next targeted tree.
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Mountain Pine Beetles are attracted to larger trees, weaker trees, and trees that have been damaged. They are invited by the scent of tree sap. If a tree is damaged by construction such as house-building or cutting of lumber trees, beetles seek it out. When roots are damaged or the bark is skinned, the tree emits sap to seal the wound. It's that process that attracts the beetles. We've seen a tree damaged by contruction in July look like a machine gun shot beetles into it in August.
This proclivity of pine beetles to attack damaged trees has led some lumbermen to actually damage "target" trees on purpose in areas scheduled to be cut down. The aim is to attract the beetles to trees that will be logged and properly handled, and keep them away from the green parts of the forest that will be left. This might be an option on some properties where small stands in clearings can be sacrificed to save the denser areas outside the clearing.
When the beetle bores into the tree, the tree fights back with the same wound-healing pitch it uses for other assaults.
Below is a picture of a tree with mountain pine beetle bore holes surrounded by the pitch the tree has produced in an effort to expell, trap, or kill the beetle. |
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The tree above is a Lodge Pole Pine in the Estes Park area of Colorado. The "pitch tubes" indicate that this tree was attacked last season by Mountain Pine Beetles. The tree has produced copious pitch in an effort to fight the beetle attack. Probably the best course of action is to cut the tree down and take precautions against allowing the beetles inside to escape to new target trees. (see "After Pine Beetle Infestation" ) |
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 Although pitch tubes are evident, the top of this particular tree had not begun to rust. Therein lies one of the hardest parts of deciding which trees are doomed and which have successfully fought back.
A rather large pitch tube with no entrance hole often indicates that the tree has been successful in fending off the beetles. It was a healthy enough tree to produce enough pitch to thwart the beetle efforts. Further investigation is needed to find out of the tree is actually harboring beetles.
By using a draw knife to take a portion of bark off of the tree around the pitch tube, you can discern whether the phloem tissue (the part of the tree's plumbing system that transports the trees photosynthesized food downward from the leaves or upward to the growing buds) or the wood (the part that carries water and salts upward from the roots) has any signs of bluing - a sure sign of beetle victory.
Upon scraping the bark, the tree above was not showing obvious signs of blue stain but was sacrificed anyway on the chance of its harboring beetles. It was in an area that was in need of thinning and not an important ornamental tree.
To complicate matters, according to Colorado State University Extension Service ( http://www.ext.colostate.edu/pubs/insect/05528.html ) under dry conditions (experienced in the West in recent years), trees may NOT produce pitch tubes when infested, therefore healthy trees are not as obvious. Time may need to be spent looking for sawdust around a tree’s circumference and at the base of the tree.
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