I am worried about the big tree in my front yard. It may be as old as the house itself, a 1955 edition. It seems like a healthy Baby Boomer. However, I’m told that it’s an ash tree, and as such, it may be doomed in a few more years when a certain green borer gets here.
When and if it dies, it will join millions of others that we’ve lost here in the U.S., to one cause or another: pests, disease, drought, climate change, fires — or a combination of factors.
We are losing an extraordinary amount of trees in the U.S. and in North America. No one knows the exact total, but it’s in the millions. And apparently, we’re not alone.
“It’s a global phenomenon, even bigger than just our nation,” says Richard Cobb, Ph.D., who teaches forest health (insects and pathogens), dendrology and ecology at California Polytechnic State University San Luis Obispo. “One of the problems is that it has no one single cause. There are many different reasons why we are losing trees. Many of them are linked to changes in the environment and some to the invasion of exotic pests from overseas.”
We are losing canopy cover, according to David J. Nowak, Ph.D., a senior scientist at the U.S. Department of Agriculture’s Forest Service Northern Research Station in Syracuse, New York. But he agrees that there is no single overarching cause. “There are a whole bunch of reasons for the loss,” says Nowak. “Trees get old and they die, creating gaps in the canopy. So do big storms, insects and diseases, development and other human choices.”
Some have blamed climate change for the loss. “Climate change is certainly playing a role in how bad these pest outbreaks are,” says Cobb.
“In many of these tree mortality events, we are seeing native insects or native pathogens that are emerging and becoming a more important cause of tree death because of changes in the environment.”
“In California the problem is drought; in other parts of the country, it’s mainly because of changes in temperature that directly affects the rates that insects develop or whether or not they survive the winter,” Cobb says. Nowak agrees that higher temperatures may be a contributing factor, especially when it comes to the overwintering question.
The role of drought
As is often true in nature, causality is a multifaceted, interlinked thing. Trees are dying due to a number of factors playing off of each other. The massive die-off in California’s pine forest is a prime example.
One-hundred-two million of the state’s pine trees have been killed, mainly in the southern Sierra Nevada. The dead trees outnumber the living ones and create a serious fire risk.
California’s recent five-year drought is the culprit, or should we say co-culprit, because drought alone didn’t kill those trees. Nor did an exotic insect.
Rather, it was a combination of drought and a native beetle that’s always present, the western pine bark beetle. Under normal conditions, this beetle is kept in check by the trees themselves, who exude sticky sap or “pitch” to drown them.
But the thirsty trees can’t exude enough pitch. Without this natural defense mechanism to curb their numbers, the beetle population had its own baby boom.
Another example is clear across the country in Massachusetts. Drought over the last few years there has brought about a big resurgence of the gypsy moth. Since the 1980s, a larvae-destroying fungus called Entomophaga maimaga has helped keep their numbers in check. But it needs moisture to grow.
As a result, “it’s been the craziest four years I’ve ever seen in my life,” says Bill Joseph, a certified Massachusetts arborist at Lynch Plant Healthcare in Sudbury. “In the last two years, it’s just been grotesque. The fuzzy caterpillars were everywhere, all over people’s houses, cars and lawns. We constantly got calls from people saying, ‘Come kill these things. Their hairs are giving my kids a rash.’” The gypsy moth is an invasive, non-native species that was brought over from Europe in 1868 in the hope they’d be useful for silk production. The specimens escaped, and their descendants have been spreading around the U.S. ever since. Its hungry, hungry caterpillars defoliate the leaves of oaks, aspen, apple, sweetgum, paper birch, poplar and other species.
To nuke the larvae, “we spray,” says Joseph. “We could do an injection, but spraying is a more economical approach, especially if multiple trees are on a property. We use a low-toxicity EPA-approved pesticide that’s not supposed to harm honeybees.”
Gypsy moth isn’t the only pest he deals with. “Something I’ve just seen in the last few years, cryptomeria scale (another invasive from Asia), is affecting fir trees in landscapes. And another one, magnolia scale, is on almost every magnolia tree.”
The word “exotic” sounds sexy, but in this context all it means is something our flora didn’t co-evolve with and therefore has no natural defenses against. Global commerce assures that exotic pests like gypsy moth coming from other lands are always going to be a problem.
Such is the case with a fatal tree disease called fusarium dieback that’s spreading throughout Southern California at an alarming rate. It’s transmitted by two invasive exotic Asian pests, the fungus-farming polyphagous shot hole borer and the Kuroshio shot hole borer, two closely related beetles first discovered in Los Angeles in 2012. Fusarium is the name of the fungus that feeds the larvae and kills the trees.
It threatens over 300 different trees, including agricultural species such as olives and avocados. Unlike most invasive pests that attack only weakened or stressed trees, this one goes after healthy ones, too.
The Forest Service says this single insect could kill as many as 27 million trees — roughly 38 percent of all the trees in the region. “If people think this isn’t apocalyptic, they’re wrong,” says Ann Hope, western states account representative at microinjection insecticide maker J.J. Mauget, Arcadia, California.
Then there’s the notorious EAB, the emerald ash borer. Since it was discovered around the Detroit/ Windsor, Ontario, area in 2002, it’s killed untoldmillions of ash trees through 30 U.S. states and two Canadian provinces and keeps expanding its range in every compass direction.
In September of 2013, it made its first known incursion into the mountain West, turning up in Boulder, Colorado. Since then, it’s added New Jersey, Arkansas, Louisiana, Texas and Alabama to its North American tour roster, with the latest sighting taking place last year in South Carolina.
EAB is native to Asia, where it’s kept in check by parasitic wasps that target its larval and egg stages. But on this continent it has no natural predators, which is why its spread has been so relentless.
According to a 2016 story in The Guardian, the USDA approved four species of these parasitic wasps for release. It will take years, however, for them to build up in enough numbers to make a difference Meanwhile, people in the not-yet-affected states wait and worry. In Montana, “one of our greatest concerns right now is the continued spread of the EAB,” says John Norieka, a consulting arborist certified with the International Society of Arboriculture and employed with Bozeman Tree, Lawn & Landscape Care in Bozeman, Montana. He’s also certified by the National Association of Landscape Professionals as a landscape technician and is an ISA-qualified tree risk assessor.
“We don’t have this pest here yet, but upwards of 50 percent of the large mature trees in our downtown area are green ash. The city is taking steps to prepare now. We’re hopeful it doesn’t come, but as we’ve watched it spread across the country, we’re definitely worried.”
Dave Entwistle is worried about the iridescent green beetle, too. He’s a commercial arborist and commercial business developer at Lakewood, Colorado-based Mountain High Tree, Lawn & Landscape, which recently became part of SavATree, a national provider of tree, shrub and lawn care based in Bedford Hills, New York.
“The EAB was found around Boulder in 2013 and has moved into Longmont but hasn’t made it down into the Denver metro area yet. We’re waiting for someone to bring firewood in or for a high wind to move it along. We anticipate finding it here this year for sure.”
Once it arrives, it’s going to find plenty of ash trees to feast upon. “We have a ton of them,” says Entwistle. “On public land, there are about 300,000 or 400,000. On private properties, its logarithmic how many there are. A lot of Purple Ash, in the White Ash family, were planted in the late 80s, early 90s for fall color. When it finally does hit Denver it’s going to be a big deal.”
Fortunately, there are treatments for EAB. Systemic trunk injections or soil drenches with dinotefuran or imidacloprid will kill the larvae. Specifics can be found at www.emeraldashborer.info. On that site, there’s a link to a fact sheet entitled “Insecticide Options for Protecting Ash Trees.”
It seems like now would be the time to prophylactically treat Denver’s ash trees before they’re infested. Even when they catch the bug, if caught early enough, many can be saved by treatment.
You don’t want to start too early, though, because the treatments are only effective for about two years. Without it, a tree will be dead in two years.
“It’s a fine line to walk,” says Entwistle. “We’re telling people, if you have a key tree that you can’t afford to lose, then yeah, go ahead and treat it. But we’re not trying to scare people into spending money, insisting that they should treat their trees now.
We are telling them to have an estimate on hand so when it finally does come, they can pick up the phone and call us.”
Those are just a few examples. There are many more pests and diseases killing trees: Asian longhorned beetle, hemlock wooly adelgid, thousand cankers disease, sudden oak death and many more. In Montana, Norieka is battling another invasive, the European elm scale which, present in enough numbers, can kill a tree.
Nowak is about to publish a five-year study on loss of canopy cover in our urban forests. “We know we’ve lost canopy from 2009 to 2014. Will that continue? Who knows? It depends on what humans do.”
He says if we want more tree cover in our cities then we must do something differently. Trees naturally regenerate all the time; if we didn’t mow, we’d have lots more of them.
There’s an advantage to having mowed landscape and grass. He’s not against that. But he says if we want more trees, we need to create more space where natural regeneration can occur unabated … and to keep planting them.
What can we do to save trees?
We can’t stop international commerce from bringing in exotic pests, nor control the weather. But in terms of protecting trees in the landscapes under our care, there’s much landscape professionals can do, and we don’t have to be arborists to do it.
We can look for signs like discolored leaves, premature leaf drop, dripping sap, small holes in trunks and especially, dieback in crowns.
“There are distinctive patterns of dieback,” says Entwistle. “If it’s drought stress you will see top-down outside-in dieback; if it’s too much water, you’ll see inside-out, bottom-up dieback. Different pests show different patterns of dieback.”
Pests and diseases aren’t the only threats to trees. Some of them come from ill-advised landscape practices. The sidebar on page 16 illustrates some things we should avoid doing.
Cleaning our tools between jobs can slow the movement of pathogens and pests from landscape to landscape. We can learn what invasive pests or diseases may be headed to our neck of the woods, and if we see something, say something.
It boils down to simply this: care and be aware. The trees would thank you if they could speak. Your clients certainly will.
DID YOU KNOW?
The gypsy moth made its North American debut in 1868 after Europeans brought them in the hope they’d be useful for silk production.