By Joe Rankin, Forests for Maine’s Future Writer
Sometimes the name of an organism carries weighty meaning. For instance, that of the southern pine beetle, Dendroctonus frontalis. The first name, that of a genus of bark beetles, means tree murderer.
That genus has a lot of members, some of which are already present in Maine. The southern pine beetle isn’t. But that may change. The beetle is moving north as the world’s climate changes.
And that has a lot of people very, very worried.
Because the southern pine beetle is like the Genghis Khan of bark beetles. It is a cousin to the notorious mountain pine beetle, Dendroctonus ponderosae, which since 1996 has ravaged hundreds of millions of acres of lodgepole and ponderosa pine forests of the intermountain west.
“It’s something we’re concerned about and have been surveying for it for three years now. It’s definitely on our radar,” said Allison Kanoti, Maine’s acting state entomologist. So far it hasn’t been found here, or at least hasn’t been found in the pheromone-laced traps the Maine Forest Service has deployed.
The southern pine beetle, as its name suggests, is endemic to the vast pine forests in the southeastern U.S., from the mid-Atlantic states west to Oklahoma and Texas. In the south it has caused timber losses in the hundreds of millions of dollars over the past few decades.
Over the last few years southern pine beetle has been documented in New Jersey, where it threatens the iconic Pine Barrens, and on Long Island, N.Y. and in Connecticut. The beetle is normally killed by cold winter temperatures, limiting its range and its depredations.
But scientists say that as the climate warms it’s allowing the beetles to expand their range. How far and how fast that expansion will occur, and what types of damage will result, is still unknown.
Writing in the journal Nature Climate Change, a team of researchers predicts “a plausible new threat” from the beetle “to vast areas of pine forest in eastern North America by 2050 and into subarctic Canada after 2080 under continued climate change.”
“It is a very big deal . . . it’s definitely a considerable forest pest,” said Kevin Dodds, a forest entomologist with the U.S. Forest Service who has studied the southern pine beetle and other bark beetles for 20 years and was one of the authors of the Nature Climate Change article.
The southern pine beetle is a tiny little thing. Adults are a fraction of an inch long. But they can do some serious damage. In the southern U.S. the beetle is normally present in pine forests at fairly low populations. Under those conditions it normally attacks only damaged or stressed pines.
But the beetle’s populations go through cycles and during periodic outbreaks its numbers mushroom and it can spread rapidly, killing pines both healthy and unhealthy in vast numbers.
Adult female beetles are the ones who seek out new hosts. When they find a vulnerable pine they burrow into the tree and produce chemical secretions called pheromones that call other beetles, male and female to the tree. They are helped by the smell of resin, which the pine produces in an effort to protect itself. Scientists call this beetle mob a “mass attack.” And during a mass attack the tree’s resin production capability is compromised and the pine overwhelmed. It may still be alive, but it’s a dead tree standing.
The beetles burrow into the pine’s inner bark, creating twisting and overlapping “galleries” or tunnels where they lay their eggs. After the eggs hatch the larvae tunnel toward the outer bark. When they emerge they open their wings and fly off to another tree. Experts say they can fly up to two miles. Further if aided by wind. Which, let it be said, does not bode well. Neither does the fact that the beetle can produce multiple generations in a year.SPB attach site with adult stuck in resin, photo courtesy of Kevin Dodds
In the south the beetle attacks every kind of hard pine it encounters, according to a 2018 article in the Journal of Forestry, on which Dodds was the lead author. Loblolly and shortleaf pines, particularly, but it has also attacked Norway spruce, red spruce, and eastern hemlock. It has attacked pitch pine in the mid Atlantic states and red, Scots and white pines. It is, apparently, a fairly indiscriminate killer.
Dodds noted in an interview that, though the beetle has been observed in white pines on Long Island, its reproduction in that species there has not been documented. It has reproduced in white pines farther south, however, which is not good news.
White pine, being a “soft” pine as opposed to the hard pines of the south, might be a less-than-ideal host, we just don’t know. Unlike some other pests, it’s a difficult organism to work with in the lab, said Dodds.
Other pines of the north, however, are likely to be more vulnerable. In New York and New Jersey, notes Dodds, pitch pine has proven “a really good host” and though the beetle hasn’t gotten into jack pines yet, Dodds said jack pine is a lot like pitch pine and could be susceptible. Red pines, as well as lesser-known pine species such as lacebark pines and Japanese black pine have also been attacked. And, again, the beetle preys on Norway spruce as well.
Maine has a lot of pines. White pine being the iconic symbol of the state and pine lumber a major forest SPB attack sites, photo courtesy of Kevin Doddsproduct. That’s something to think about.
“One of the difficulties is we have is that this insect is new in this area” of the country, said Kanoti, the Maine state entomologist. “And we don’t really know how much they will attack things like our white pines. Right now it’s usually cold enough in the winter to keep it at bay or low populations for some time.”
If and when it does appear it will likely be in warmer coastal areas, probably in York County, she added. U.S. Department of Agriculture Hardiness Zone 5 and warmer “is where we’re really concerned,” she said. “and where we expect to see tree damage first would be southern York County along the coast where we have hard pines.”
In Maine white and red pines may be linchpins of the economy, but pitch and jack pines also contribute ecologically and economically, in an indirect way. Pitch pine barrens like the Kennebunk Plains and the Waterboro Barrens in York County are already increasingly rare ecosystems, according to the Maine Natural Areas Program. They are considered particularly vulnerable to southern pine beetle.
But Kanoti notes that pitch and jack pines also occur elsewhere and help give the Maine coast its trademark rugged look. “Hard pines are important natural communities and are important aesthetically as part of the Maine scenery,” she said.
Many of the pitch pine barrens in the northeast, including Maine, are very vulnerable, said Dodds. The beetle prefers dense stands rather than isolated trees. Many of the pitch pine barrens of the northeast are overgrown. They depend on fire to thin themselves, remove competing vegetation and create the conditions for pine regrowth. Humans have been on a fire suppression kick for a long time and many pitch pine ecosystems are seriously out of whack.
There aren’t too many stands of pure white pine in Maine. Usually the tree is spotty on the landscape, which may protect it and reduce the beetle’s spread. Or not. “Pine-sparse hardwood forests in the northern United States may present a barrier to continued SPB dispersal into the red and jack pine forests further north,” noted the authors of the Nature Climate Change article. “However, SPB has already traversed large pine-sparse regions in the eastern United States, and forests further north feature a higher, more continuous density of pine.”
Plus, Kanoti notes, there’s always the danger that someone coming to Maine for vacation will bring a carload of infected firewood with them and allow the beetle to leapfrog uninfected areas further south.
Another concern: the health of Maine’s white pines. Recent droughts and the presence for the past decade of a fungus that causes premature needle loss have stressed the trees. “Weakened trees are less likely to be able to respond to stresses,” said Kanoti. “It’s like when your immune system is compromised you’re more vulnerable.”
Red pines stands in New England and New York are usually present on ridge tops and rocky outcrops, note the authors of the Journal of Forestry article. That’s a place usually associated with poor soils. “The presence of outbreak populations of SPB in natural red pine stands could threaten the persistence of these forest types in the region and result in the wholesale loss of these stands.”
Still, the outlook is not all doom and gloom. Scientists say there are some tried and true methods for slowing the beetle’s spread and reducing the damage, mainly thinning of vulnerable, but not-yet-infected, stands.
That has proven to work in the South, according to the Journal of Forestry article’s authors. Yet they note that it has not been widely done in the northeast. The cost of thinning and lack of markets for low-value wood could make it uneconomical, they write. Plus, “public perception of forest thinning in these forest types would also have to be addressed . . . early attempts at small-scale thinning operations on Long Island were met with significant opposition.”
And, in case you are hoping for a white knight in the form of a natural predator, well, you’re probably going to be disappointed. Certain mites and other insects do prey on the beetle, particularly the checkered or clerid beetle. But they haven’t been shown to tamp down outbreaks, said Dodds. And there’s no indication that southern pine beetle will encounter new, more powerful predators or parasites as it journeys north.
Joe Rankin writes on forestry, nature and sustainability for magazines, websites and other outlets. He lives in New Sharon.