FOCUS ON FIRE
IN THE FOREST
FUNCTION OF FIRE
A GRASP ON SMOKE
BEYOND THE BLAZES
Warmer temperatures grow fungi beneficial to mountain pine beetles
By Cary Shimek
Mountain pine beetles don’t look like much. They are typically a generic dark brown in color, and the largest measure only about 8 millimeters. These bark beetles are just one variety among 7,500 species worldwide and 500 species in the United States.
Innocuous as they seem, these little critters are rapacious tree killers — America’s No. 1 forest pest — destroying more trees in North America than fires, other insects and diseases combined.
And a new discovery by Diana Six, a UM entomologist and pathologist, and her colleague Barbara Bentz, a U.S. Forest Service entomologist from Utah, suggests future pine beetle infestations may worsen over time.
Six says when a mountain pine beetle bores into a tree, it releases a pheromone that attracts more of its fellows for a mass invasion. When enough beetles are attracted to kill the tree, the insects release another pheromone that repels any newcomers, sending them off to nearby trees.
This allows the beetles to kill the tree, yet avoid competition among their young for food. Once the tree is dead, the beetles make tunnels under the bark called galleries, lay eggs and infect the tree with fungi they carry. The larvae then hatch and subsist on the tree and fungi.
“As you can see, they are quite complex and fascinating,” says Six, who specializes in insects and their interactions with fungi.
She says the fungi carried by pine beetles is essential for their survival. In fact, they have evolved little organs called mycangia to carry the stuff about.
“They use the fungi for nutrition,” Six says. “They get nutrients from the fungi that they can’t get from trees. This allows them to use a food source — trees — that few other creatures can use. They basically bring along their own nutritional supplement.”
In fact, pine beetles inhabiting a tree with no fungi produce no young and die off. The fungi also get something from this symbiotic relationship, Six says, since the beetles provide a mass transit system to spread it among trees.
It turns out the bugs generally carry two varieties of fungi. Ophiostoma montium (OM for short) is better for the beetles because it provides more nutrition, so they don’t have to feed as much on wood. The other, Ophiostoma clavigerum (OC), is not as nutritious and requires them to feed more extensively on trees.
“So we’ve been calling OM ‘the good fungi’ and OC ‘the bad fungi,’” she said, “but OC is still pretty good for beetles. Maybe we should call it the ‘not-quite-as-good fungi?’”
Other studies have indicated that “the good” OM fungi may help increase bark beetle populations more rapidly, and Six and Bentz originally set out to answer the question of whether OM facilitates pine beetle outbreaks. They are still working on that question. But, as happens so often in science, their path to discovery took a detour that lead to an unexpected breakthrough.
“We found evidence that temperature plays a key role in determining the relative abundance of the two types of fungi,” Six says. “OM is warm-tolerant, while OC is cool-tolerant.”
To conduct the research, Six collected beetles at several sites in Montana and Idaho. They used pheromone traps — a series of funnels vaguely resembling a tree that channels the insects into collection chambers. The traps are baited with a synthetic version of the beetles’ “summoning” pheromone.
“Basically we use their own chemistry against them,” Six says. “One trap can collect more than 1,000 beetles.”
Once the bugs are captured, Six transports them back to her lab and laboriously dissects them under a microscope to determine whether they carry the OM or OC fungus. She dissected thousands of beetles from several sites over several years. She also grew the two species of fungi in the lab to discover the temperatures in which they grow best.
“What we found in Montana is a complete shift from one fungus to the other over a season,” she says. “The beetles carry one fungus — OC — early in the season and carry the other fungus — OM — late in the season. In Idaho sites we saw a shift but a different one — from OC early on to OM and then back to OC late in the season.”
Why the shifts? Researchers learned cool-tolerant OC grows and sporulates best at temperatures under about 78 degrees Fahrenheit. Between 78 and 89 degrees both fungus species do well. But when maximum temperatures exceed 89 degrees, only OM can grow.
“The growth experiments we did in the lab showed OM grows very well at (89 degrees), but OC won’t grow at all,” Six says. “The temperature tolerances we saw in the lab turned out to be near-perfect correlations to what we found in the field at every site. The beetles would switch from one fungus to the other depending on the temperature.”
She says beetles only can pick up and carry fungi that are sporulating at the time they leave the tree. If it’s too hot, OC doesn’t spread its spores and doesn’t get transported.
She says this may be the first time anyone has found that the composition of a land-based symbiotic relationship is determined by temperature. However, temperature-driven shifts of symbiotic algae associated with corals have been observed in the ocean.
This is all important, Six says, because climate change appears to be creating a warmer West that may promote an increased prevalence of OM fungi with pine beetles. The more nutritious OM could support faster-growing beetle populations with higher survival, spawning larger and more frequent mountain pine beetle outbreaks. And if temperatures continue to rise as expected in coming decades, it’s conceivable that OC, which is not as beneficial for the beetles, could drop out of the system.
A warming climate also could cause more drought, which stresses trees. Six says trees stressed by drought are easy for beetles to kill, “which is why during drought you see a lot of beetle activity on the landscape.”
She says, “British Columbia is now experiencing its biggest outbreak in history, and much of it is temperature and drought driven. Eighty percent of their trees are predicted to be killed by mountain pine beetles. So this is really relevant to the Pacific Northwest.”
The fungal temperature discovery also offers a rationale for how a host creature can have more than one symbiont.
“Why don’t the two symbionts compete without one eventually losing?” Six asks. “Well, this shows that if you have two different fungi with very different growth requirements, this can allow each to dominate at different times, keeping both in the system.”
Working at an office and lab in UM’s new BioResearch Building, Six keeps busy with 23 separate entomology and mycology projects. Some involve beetles, but she works with a variety of other insects as well. To offer an example, she and a graduate student recently learned why ants prosper in areas infested by spotted knapweed. It seems knapweed produces a little node full of nutrients that attracts ants. The ants pick up the seeds, eat the node and then discard the seed. So the native ants are spreading the seeds of an exotic weed while getting a huge food reward in the process. Basically, the two species have teamed up to form a new mutualism, Six says.
But the discovery involving fungal temperatures makes the mountain pine beetle her insect of the moment.
“It’s exciting,” she says. “When I figured out the temperature connection, I couldn’t sleep for three days.”
For more information, e-mail Six at firstname.lastname@example.org.