Last summer we took a drive to Granby, just outside of Rocky Mountain National Park. While I had heard about the Mountain Pine Beetle for years, I was unprepared for the extent of the devastation. Entire mountainsides were covered in dead and dying pines, eerily resembling New England’s beautiful red fall foliage. But rather than deciduous maples and other hardwoods, these were conifers, largely ponderosas. They wouldn’t be turning green again come spring.
Many of us who live along the Front Range of the Rockies have ponderosa or other pines on our property. They’re well adapted to our climate and soils, and very resilient. But in spite of their suitability for our area, there are two major problems that pines can encounter. I discussed mistletoe last December. The other major cause of mortality is the mountain pine beetle (MPB).
Not all beetles that chew on trees are harmful. Most wait until the tree is dead, then move in to dismantle it, assisting in the decay process that returns nutrients to the soil. However, some beetles aren’t so polite, and attack still-living trees. Two bark beetles, the Ips Beetle and the MPB, are major pests of western forests, with the MPB being the most destructive.
Trees most at risk include ponderosa, lodgepole, Scotch and limber pines, although bristlecones and pinyon pines are also susceptible. As is typical with insect pests, plants under stress are most likely to be targeted. The first line of defense is to grow healthy trees. But while a tree or two in a home landscape is fairly easy to keep watered and mulched, it’s a totally different matter if you live on acreage covered with forest. There, the trees must depend on natural rainfall for irrigation. The extreme drought years centered around 2003 have much to do with the current destruction of our forests. (So does overcrowding due to fire suppression, but that’s a matter for another day.)
Actually, it isn’t the beetles that kill the tree. Rather, they are carriers of a blue fungus that clogs the conductive tissue under the bark. Unable to move food and water between the roots and the needles, the trees turn brown and die.
Once a tree has been attacked by MPBs and infected with the fungus, there is nothing practical that can be done it save it. Thus, prevention is essential. And the key to prevention is understanding the beetles’ life cycle.
Adult MPBs tunnel into new, unaffected trees in late summer. Once under the bark, they mate, then chew out vertical galleries in which to lay their eggs. The eggs hatch into larvae, which then move horizontally, producing a characteristic pattern of tunnels.
By the time summer returns, the larvae have grown enough to pupate. The adults begin to emerge late June, with the bulk of the population leaving their trees between late July and mid-August. These adults are capable of flying as far as two miles, but most just stick around looking for the next tree to invade.
If you have an infected tree, it is essential to remove it and properly dispose of the wood before the adult beetles emerge to spread their destruction. (Once the beetles have exited the bark, there is no longer any urgency in removing the tree, at least until it dies.)
If you have a valuable tree on your property that you wish to protect, the best time to spray is in April and May. It’s best to hire a professional tree service to do the spraying, as achieving complete coverage of a full-sized pine tree is difficult without specialized equipment. Preventative spraying will protect the tree through one entire season; it must be repeated every spring.
To learn more about MPBs: how to identify them, how to tell if your tree is infected, what to spray, and how to safely dispose of infected wood, I recommend reading Colorado State University’s excellent fact sheet on this topic.
Photo credits: Adult MPB byErich G. Vallery; larvae by Scott Tunnock, both from the USDA Forest Service, Bugwood.org.