Damage to the trunk of an elm tree from an infestation of Emerald ash borer. The pest now threatens Elm trees in much of Pennsylvania.

Damage to the trunk of an elm tree from an infestation of Emerald ash borer. The pest now threatens Elm trees in much of Pennsylvania.

by William Hengst

Over the past century, North America’s forests have become silent battlefields, pitting trees against foreign invaders or pathogens. Witness the demise of the grandiose American chestnut and, later, the loss of our most elegant urban shade tree – the American elm.

Now a new exotic pest – emerald ash borer (or EAB) Agrilus planipennis – threatens to extinguish all ash trees in the United States because they have no natural resistance to this unwelcome guest. EAB attacks only ash, and all species of ash.

First discovered in 2002 near Detroit, Mich., and Windsor, Ont., in what turned out to be ground zero, EAB multiplied exponentially, infesting and eventually killing nearly every ash tree there.

According to Wikipedia, “In a typical scenario, each year the EAB population multiplies by a factor of 50. At year 9 of an infestation, the EAB population originating from one female beetle will have reached nearly one-trillion insects.”

The culprit is a dark metallic-green beetle. The adult – one-half-inch long and one-eighth-inch wide – could fit easily on a penny.

Following its discovery in Michigan and Canada, EAB soon spread to Ohio in 2003; Indiana in 2004; Chicago and northern Illinois and Maryland in 2006; western Pennsylvania and West Virginia in 2007; Wisconsin, Missouri and Virginia in 2008; Minnesota, New York and Kentucky in 2009; Iowa, and Tennessee in 2010; Connecticut, Kansas and Massachusetts in 2012; New Hampshire, North Carolina, Georgia and Colorado in 2013; and New Jersey in 2014 – for a total of 22 states.

According to a summary in Wikipedia, it is estimated that “it has killed at least 50 to 100 million ash trees so far, and threatens to kill most of the 7.5 billion ash trees throughout North America.” In the wake of this destruction, “there are many millions of dead standing trees in Canada and the United States.”

Early on, quarantines prohibiting the movement of ash wood (cut firewood, green lumber, logs, stumps, roots, branches, and nursery stock) from one state to another were imposed by the U.S. Department of Agriculture (USDA).

“Immense efforts were put into place in Michigan and northwest Ohio, and in Prince Georges County, Maryland, to survey, contain, and eradicate EAB,” wrote Noel Schneeberger, Forest Health Program Leader for USDA’s Northeastern Area Forest Service, in an email communication. “All of these efforts failed, largely because EAB can very easily be moved by people unintentionally to new areas.”

Upon its detection in western Pennsylvania in 2007, the state imposed an internal quarantine to prevent moving ash wood between counties. However the quarantine was lifted in 2011. The reasoning, according to a Penn State Extension bulletin in 2011: “Because of the beetle’s aggressive movement across Pennsylvania, the in-state quarantine – initially intended to slow the pest’s spread is now unnecessary.” The federal quarantine still remains in effect.

EAB actually arrived in Michigan at least eight years before detection, most likely in its larval stage, embedded in ash wood-packing materials shipped from Asia. Because the beetle had never been seen before in this country, it took scientists several months to identify and give it a name.

The larvae kill the tree, but both larvae and beetle spread EAB

The lifespan of the adult beetle is short – a couple of months in summer – during which it feeds on the ash leaves and mates and after which the females lay 70 to 100 tiny white eggs on the bark of the tree. (Other borer insects typically lay only a couple of eggs—the ash borer lays many.)

A week or so later, the eggs hatch into larvae, which chew their way through the outer bark to the sapwood (or cambial layer) where they spend the entire summer and fall out of sight, eating and growing, creating serpentine tunnels in the sapwood, which cut off the flow of nutrients and water to the tree’s upper reaches.

The larvae spend winter in the tree. In early spring they morph into pupae, then into beetles. In late spring and summer, the new generation of adults emerge by boring D-shaped holes (one-eighth-inch across) in the bark, and begin the cycle all over again.

If more ash trees are nearby, they may feed on their leaves and mate, or they may fly further on their own – as much as a mile – to attack another host. The beetle senses an ash tree by its smell, especially that of a stressed ash tree, according to Dr. Donald Eggen, Forest Health Manager at the Pennsylvania Bureau of Forestry.

The decline of an infested tree is rapid, starting with progressive dieback of the branches. Mature ash usually are dead within three to four years after detection of an initial infestation, saplings and smaller trees in one to two years.

When external symptoms show, it may be too late

The external symptoms, however, may not be visible for several years while the larvae are eating away inside, because the dieback first manifests itself far up in the crown of the tree, then moves downward. Actual detection of EAB may not happen until the exit holes appear on the trunk.

Ash are undergoing an overall decline in this country, caused by native pests and diseases, which are brought about by environmental stresses, such as droughts. “Ash yellows” is the most widespread and is spread by a native leafhopper insect. It usually takes much longer for “ash yellows” to kill a tree, but its symptoms – leaf and branch dieback – mirror the early symptoms of EAB. As well, EAB attacks stressed trees first before healthy ones, which further complicates an early diagnosis.

“EAB has a suite of other symptoms, such as epicormic shoots” (growing up from the base of the trunk), “woodpecker damages, and bark splits that can be used in the diagnostics,” wrote Dr. Houping Liu, Forest Entomologist at the Pennsylvania Bureau of Forestry, in an email communication. When an unusual number of woodpeckers are active stripping the bark and drilling in to eat the larvae, it’s too late to save the tree.

EAB in Pennsylvania

County by county EAB infestation map of Pennsylvania.

County by county EAB infestation map of Pennsylvania.

Since detection in 2007 in Butler County near Pittsburgh, EAB marched clear across Pennsylvania in five years and has been detected in 46 of the state’s 67 counties. In 2012 it was detected in Warrington Township, Bucks County and in Horsham Township, Montgomery County in 2013.

A landscape contractor discovered the Warrington outbreak at a condominium complex there. This infestation was estimated to be several years old.

What makes the Warrington discovery so ominous is the location – at least 80 miles east of the nearest detection in Pennsylvania at that time – which confirms how EAB can “hitch a ride,” usually in its larval stage inside freshly-cut ash logs or other ash materials.

“The bug has not disappeared,” wrote Scott Guiser, Environmental Educator with Penn State’s Extension Service, on his post in April 2013. “There were many ash trees infested at this site and no doubt the adults that emerged from those trees flew off to mate and infest other trees.”

“The EAB insect belongs to the family of metallic, wood-boring beetles, which are usually strong fliers,” said Christopher Tipping, Professor of Entomology at Delaware Valley College, in a telephone interview. “Who’s to say that in high winds the beetle might travel five miles instead of one mile?”

Sure enough, in August 2013 an EAB beetle was found in Horsham Township inside a purple panel trap, set out by the Pennsylvania Bureau of Forestry to monitor the spread of EAB.

Next: “What homeowners and communities can do to protect their ash trees”

William Hengst, a Mt. Airy resident, is a longtime member of Friends of the Wissahickon.