Inside an Infestation: The Dance of the Pine Beetle

Inside an Infestation: The Dance of the PineBeetle

28 August 2006

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    Photo courtesy of the National Forest Service. By Paul Driscoll

As the news of yet another forest fire season descends like smoke over our western Montana towns, I’ve taken some time to reflect on just what is burning out there. And why.

A wildfire pretty much burns whatever is in its path: crop or weed, shrub or tree, mobile home or McMansion. But throughout much of the Intermountain West, forest fires consume a disproportionate quantity of pinus contorta — the ubiquitous lodgepole pine.

Lodgepole pine is a natural monoculture and the dominant species over much of the mid and upper elevations all along the Rockies. Lodgepole is both vulnerable to wildfire and reliant upon it for procreation. In a macabre variant of symbiosis, the tree releases its seed even as it is consumed by the heat of fire.

Lodgepole and fire go way back. It is a tree comfortable with fire. Yes, lodgpole has been burned before but still invites the old flame over to spend the night. Should fire be unavailable or otherwise preoccupied, lodgepole turns to another old friend to heighten the allure: the mountain pine bark beetle.

Held in the palm of my hand, Dendroctonus ponderosae in its adult beetle form is a diminutive thing not much bigger than a mouse turd. Yet the pine beetle, as it is known around here, is the perpetrator of the rust-red forests that stretch from Arizona and New Mexico through Alberta and British Columbia. As a free-flying adult, the beetle is rather hard to find since it spends only a few days of its life outside the tree. These hot, dry, breezy days in late July and early August seem the preferred time to step out of one’s tree and maybe hook-up with a beetle of the opposite sex.

The next time you are out in forest and field, take a walk through a lodgpole stand infested by pine beetle. It won’t be hard to find. The rust-red trees are usually dead from a previous year’s infestation and form a mosaic within the forest. The earliest killed trees are often older, more mature specimens. Entrenched infestations kill with less discrimination. Look for chips around the bases caused by woodpeckers and other birds working the bark for grubs and larvae. But the true sign of an active infestation is the tell-tale pitch blebs on the trunks of surrounding, seemingly healthy trees. In late summer, you might see sawdust from freshly fertilized adult female beetles as they bore into the bark. That the word ponderosa is in the Latin name for the pine beetle is no mistake. A pine beetle is fully capable of boring through even the thick bark of a mature ponderosa. Look for tiny little holes in the pitch bleb, known as tubes. The pitch bleb is the tree’s defense against the boring beetle. Sometimes you can find a beetle drowned in the gluey stuff. But in drought years, and under heavy attack, a tree cannot bleed sufficient sap to fend off the invaders. Besides, in its own secret way, the tree may be asking for it.

Silviculturists speculate that older, drought-distressed trees in particular offer an invitational signal to the beetle. For their part, adult female beetles issue a signal of their own, called the aggregating pheromone. Beetles in the general vicinity know which trees to gang-attack by the pheromones put out by these scouts. The frenzy is then on and the tree is descended upon by the beetles, bored into, and eggs deposited in the rich, pithy underbark. Unlike infestations of the exotic spotted knapweed, say, the pine bark beetle is a native species. Its interplay with lodgepole pine has been going on, well, just about as long as lodgepole’s interplay with fire. This mountain love triangle certainly predates our own feeble interventions. We might just as well say that the hills are infested with logdgepole pine.

In the fight against pine beetle outbreaks, scientists have cleverly synthesized the aggregating pheromone. The flying beetles can be attracted to a sacrificial tree baited with the scent. As the love fest congregates, an insecticide is sprayed over the sacrificial tree. The timing must be precise, though. This is why property owners, foresters, and entomologists are always on the lookout for flying pine beetles. Small green boxes resembling milk cartons are often baited with the pheromone to attract male beetles. When beetles start showing up in the test traps it’s time to spray. The strategy spares the expense and problems of broadcast spraying. Scenic evidence suggests that it also probably doesn’t work that well, particularly in drought induced infestations. Sometimes sacrifice trees are cut down and burned.

Studies have shown that the beetle prefers larger trees with a thick layer of phloem — the nutrient conducting layer under the bark. The thicker layer in more mature trees provides plenty of food for the larval and pupae phases while protecting against temperature extremes. But almost any tree will do during an epidemic or a drought. The aforementioned woodpeckers are a natural predator of the beetles during larval and pupal phases, and may affect the number of adults that emerge for the late summer tryst. Predatory hornets may take a few grubs. Other beetle grubs may compete and even devour the pine beetle larvae. But the best natural control of pine beetle seems to be extended, bitter cold in midwinter and, well, a good wildfire in late summer.

Anyone who’s burned much lodgpole for fuel knows the blue-tinged fiber of standing dead wood. You see it in the outer rim of cross-cuts and along the length of split wood, standing out like a blueberry stain on a cream colored backdrop. You see, an added price of a pine beetle infestation is that the little buggers just won’t wipe their feet before busting into the underbark. The pine beetle is the prime vector of the aptly-named bluestain fungi, which spreads from the underbark into the tree. So, in addition to laying eggs that hatch into voracious larvae, the beetle also infects the tree. The combination is deadly. A tree may not look dead by the following midsummer when the next generation of adult beetle emerges, but it’s a goner. The tree becomes fodder for the next wild fire. Which may be part of the larger strategy at work here.

A lodgepole pine forest mottled with beetle kill is an offering to fire. Lodgepole needs fire to kickstart the next generation. What does a forest have to do? Throw itself upon a fire? Well, almost. Following a brief experiment with the so-called “let-burn” strategy in the 1980s, fire suppression is enjoying something of a rehabilitation these days. The let-burn approach to forest management was consigned to the ash heap after the big fire year of 1988 when much of Yellowstone Park went poof. In spite of suppression efforts, the year 2000 got a bit out of control and this summer is getting surly, too. Timber extraction on federal land still awaits a social uptick, though, in spite of administrative support for more salvage and fire control cutting. Many private timber holders whose forests border public land often log off beetle-killed lumber while it still retains some value. Although the U.S. Forest Service sometimes allows cutters to cross onto its property and harvest beetle-killed trees, the logs cannot be commercially marketed except as firewood. So, you can put another beetle-killed log on the fire, but don’t even think about putting a little fire onto the beetle-killed logs.

The general district north of Butte in southwest Montana is called The Lowlands, probably because of its comparatively pleasant elevation as the Continental Divide meanders through at about 6,200 feet. It is an ideal place for lodgepole pine. Consequently, today it is under a devastating attack by the mountain pine bark beetle. I grew up at play in these forests and fields of the Lord during the late ‘60s and early ‘70s. The trees then seemed healthy, but that probably had less to do with fire and pine beetles than it did with the copper mines in Butte.

Underground mining, as it was practiced out West a century ago, had a huge appetite for lumber. Underground mines required incredible quantities of timber to support drifts and shafts, to mount rail track, and for charcoal to fire-process the ores. With mine shafts extending to 5,000 feet, the equivalent of an entire national forest today lies buried under the Butte mining district. That timber came off of nearby forests first. Most of the lodgepole, spruce, and fir from the surrounding portions of the Deerlodge National Forest was cut by 1900 or so. The outlying forests couldn’t have lasted much longer. By World War I, most of the Clark Fork River drainage above Missoula was scalped.

Little historical data survives of what those forests looked like ahead of the axe. What is known is that those early logging operators cleared the land by horse, oxen, and by hand. It was clearcutting on a scale not seen today. Every useable tree was harvested. Slash was broadcast burned. Then the lumberjacks casually moved on to the next big stand of virgin timber. But lodgepole likes it rough, and the second generation forest grew in strong and healthy. Only a bit of crumbling evidence of the era remains upon the closest inspection today: stumpage, of course; a rotten wagon tongue; a section of flume; perhaps a bit of two-track road seemingly leading nowhere. Until the last decade or so, the forest succession itself was the strongest evidence from that acquisition era on the Deerlodge Forest.

An old Scandinavian saying goes that a tree’s life is like that of a man’s. At 25 years, flexible, but lacking strength. By age 70, though, both are interesting to look at and strong. By age 100, each is almost worthless. Early day Norwegians did a lot of the forest work and knew what they were talking about. Lodgepole pine is just not meant to live much beyond the century mark and the fact is a lot of trees are old and decrepit.

I spent much of the first half of my life under the rich canopy of healthy lodgepole forest. It isn’t easy to see the stands of fire-blackened trees that dominate the north portion of the Elkhorn Mountains outside of Helena. It’s also hard to watch the forests turning red along the borders of Elk Park on my commute to Butte. Clearcuts aren’t eye candy, either. A lot of what I see is evidence of hard choices, natural and otherwise. What I don’t see is that tiny little pine bark beetle that I know is out there, cruising the lodgepole canopy looking for a date.


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