North America’s enormous conifer forests are being decimated by tiny beetles. About the size of a pencil eraser, bark beetles are native pests that have gotten carried away in recent years with help from climate change. They’ve killed 46 million acres of forest in the Western U.S. alone since 2000, and the U.S. Forest Service has estimated they cause an average of 100,000 trees to fall every day.
Beetle outbreaks vary from year to year, but warmer weather can help them survive winter and expand their range over time. That sets the stage for a variety of ecological and economic trouble, including swaths of dead trees that provide fuel for wildfires, especially during severe droughts.
People try to contain the damage by thinning forests and spraying synthetic insecticides, but those solutions can pose new problems. Since bark beetles are natural pests running amok with human help, what if we could balance things out just by helping other members of their ecosystem catch up?
That’s what Richard Hofstetter wants to do. An entomologist at Northern Arizona University, he has spent 17 years trying to protect American forests from bark beetles. He’s made news in recent years with some creative strategies, like blasting the bugs with Rush Limbaugh, Guns N’ Roses, Queen and even their own calls. But now Hofstetter is working on a better idea: He has identified a strain of forest fungus that naturally battles pine beetles from within. Certain fungi have evolved to prey on specific beetle species, and Hofstetter hopes to coax them to not only do our dirty work for us, but to make it less dirty.
“It’s a naturally occurring fungus, so we’re not introducing anything exotic or anything new,” Hofstetter tells MNN. “The strains we’re testing are found around the United States. Some are from around where I work, and others are found in Montana. They all come from areas with bark beetle infestations.”
A fungus among us
The fungus he’s testing is Beauveria bassiana, a common insect pathogen found around the world. When its spores come into contact with a susceptible insect, they trigger a condition called “white muscadine disease” that can spread quickly through a population. B. bassiana is already widely used to control crop pests on farms, but using it to protect forests from beetles would be a new frontier.
“Each strain can be very specific or quite general in how it affects insects,” Hofstetter says. “The fungus we’re studying is very specific to bark beetles. It proceeds into the ground or a tree, and when the insect rubs against the fungus or its spores, it penetrates the exoskeleton of the insect, where it grows.”
From there, the fungus spreads inside the insect’s body, producing toxins and draining nutrients until the host eventually dies. The fungus then grows out through the exoskeleton again, covering the dead insect with a white, downy mold that releases millions of new spores into the environment.
Hofstetter’s strain of B. bassiana has a high success rate against mountain pine beetles, one of the most destructive bark beetles in the U.S. West. Not only does it usually kill them in one or two days, but it poses little danger to other wildlife. Hofstetter has found the fungus may kill one non-target insect species, the clerid beetle, yet that’s still an improvement over many broad-spectrum insecticides, which often harm an array of non-target insects along with larger animals like birds. And B. bassiana also provides a different perk that’s beyond the scope of most synthetic insecticides: adaptability.
“Another advantage of using the fungus is it can actually adapt,” Hofstetter says. “The fungus is much better at adapting to the bark beetle, and gets better at killing that species over time. It might start out 50 percent effective, then we test it later and it’s 90 percent.”
How can that happen? “I think it’s due to the variation in the spores,” he adds. “Spores that are effective against the beetles are likely to produce more spores that are effective. So it’s natural selection; it’s kind of a feedback loop. Spores that work make more spores that work.”
While music and talk radio didn’t seem to faze bark beetles in Hofstetter’s forestry lab, he was able to affect them with recordings of beetle calls. Playing an aggression call made beetles flee the speaker as if avoiding another beetle, and the sounds could even disrupt mating or inspire one beetle to kill another.
“We observed and recorded beetles mating two or three times,” Hofstetter said in a 2010 press release about the research. “Then we’d play the beetle sounds that we manipulated and watch in horror as the male beetle would tear the female apart. This is not normal behavior in the natural world.”
Hofstetter followed up the lab tests by taking audio devices into the field last year, but he couldn’t get statistically relevant data because there were too few local beetle outbreaks at the time. He says he still plans to study that strategy, but he also has another idea for using noise against bark beetles.
“We’re looking at how sound affects the fungi. Some fungi slow their growth when you play sounds for them, and some actually increase their growth,” he says. “Beauveria can increase their growth rate toward the sound of a bark beetle. It might be a strategy of this fungal pathogen to find the insect, which has never been proposed before. So that’s kind of exciting.”
Even without extra sound, B. bassiana is killing 90 percent of pine beetles in the lab. But since it’s native to the same forests as bark beetles, why isn’t it already limiting their spread in the wild?
“I think it actually can have some impact on bark beetles in a natural setting when densities get really high,” Hofstetter says. The beetles may have ways of protecting themselves — pine beetles are already known to carry a different kind of fungus that disables a tree’s natural defenses, for example, and some beetles have antibacterial properties in their mouths to fend off infection. Beauveria might be able to counteract such hurdles, though, if it can keep up with the beetles’ abundance.
“Our goal is to augment this fungus by getting more of the spores out there,” he says. “It’s like a trap — we lure beetles into the tree and let them leave, but with spores to infect other members of the population. We want to produce a product that can increase the abundance of this natural fungus.”
Hofstetter is working with Cliff Bradley of Montana BioAgriculture to produce pure spores of the fungus, which he can then mix in water and spray onto beetle-infested wood. It’s working like magic in the lab, and this summer he’ll see if he can replicate that success in an actual forest.
Sprucing things up
The pace of pine beetle attacks has slowed in recent years, but that isn’t necessarily a sign things are improving. After more than a decade of feasting on pine forests — along with major droughts that have weakened trees’ ability to muster a biological defense — pine beetles may be starting to exhaust their food supply. “I think the mountain pine beetles are running out of trees, for the most part,” University of Idaho geographer Jeffrey Hicke told the National Center for Atmospheric Research in 2013.
Pine beetles haven’t thrown in the towel, though, and neither has the beetle-boosting heat and aridity that enabled their explosion. Hofstetter hopes his strain of B. bassiana can eventually help pine forests recover, but he’s also investigating how the fungus might be able to help other tree species, some of which may have yet to see the worst of their own bark-beetle epidemics.
Spruce beetles can be infected by B. bassiana, and given their recent pace of destruction in parts of western North America, Hofstetter calls them a good candidate for testing. “The spruce beetle has been an issue just like the pine beetle,” he says. “It’s one of the species at higher elevations, and it’s definitely becoming a greater issue. It’s one of the species we’re going to test this fungal pathogen on.”
Hofstetter has been testing 20 strains of B. bassiana on logs in the lab, and over the next few months he’ll be spraying the spores on pine trees in the Centennial Forest near Flagstaff. If he can replicate even a fraction of the fungus’s indoor potency — he says 50 percent effectiveness is “quite possible” — it could mark a turning point in our ability to offset the effects of climate change on forests.
“I hope to have an answer at the end of the summer,” he says. “The lab is just different from the field. There could be situations in the forest where rain reduces the effectiveness, or sunlight kills spores on a tree, so that’s something we have to think about. A lot can happen outside that wouldn’t happen inside.”