by William Hengst
— The second in a series on the emerald ash borer, a destructive ash tree parasite
It is estimated that there are 300 million ash trees in Pennsylvania, all of which are at risk due to the spread of the emerald ash borer (EAB), a tiny Asian insect that began destroying ash trees in Pennsylvania in 2007.
“It is essentially everywhere in Pennsylvania, though we have not officially detected it in 21 counties,” wrote Dr. Donald Eggen, forest health manager at the Pennsylvania Bureau of Forestry, in an email. “Our efforts in 2013 found 16 new counties, which means EAB has spread and is building up in many locations.”
“It is now considered the most destructive forest pest ever seen in North America,” wrote Dr. Deborah McCullough, a professor in the departments of entomology and forestry at Michigan State University who has studied and co-authored numerous articles on EAB since its detection in Michigan. “We literally cannot keep up with it.”
It is likely to become more of a problem in Pennsylvania’s urban areas than in less populated places, simply because cities and towns typically have a higher proportion of ash. During the 1960s, ash trees were often planted to replace elms killed off by Dutch-elm disease.
Over the past decade, scientists have learned much about EAB and methods to protect ash trees, which are more cost effective than tree removal or doing nothing.
Know where your ash trees are
The first step is to do an inventory, easier for homeowners than municipalities with many more trees. The purpose: to identify trees to be saved, such as high-value ash – mature specimens, trees in prominent locations, trees of historic importance – also trees that may become a public hazard. Next, a plan or strategy is needed, with cost estimates and a timetable for tree protection, removals and replacements.
Tree companies with professional arborists are well-qualified to undertake the inventory and make recommendations.
The most effective EAB treatment is systemic (meaning absorbed by the tree) insecticides, injected into the trunk of the ash tree or the surrounding soil. These injections spread through the vascular system of the tree and are ingested by the EAB larvae as they feed. Although the treatments have proven effective in many cases, heavily infested trees are not likely to benefit, as their vascular systems are too badly destroyed to deliver the insecticide to the larvae. (Source: www.chicagobotanic.org)
In other words, insecticides need to be applied while the tree is still relatively healthy. If the tree has lost more than 50 percent of its canopy, it is unlikely the treatments will save it.
There are a number of products on the market for treatment of individual trees, which vary in cost and re-treatment requirements – ranging from one to three years. Costs for trunk injections average ten dollars a trunk-diameter inch, or $120 for a 12-inch diameter tree. Soil applications are less expensive.
“Homeowners can buy systemic insecticides to self-treat their healthy ash trees to prevent infestation, but they have to do this every year,” wrote Dr. Eggen in an email communication. “I recommend that they start treating if their county is infested or a neighboring county is infested.”
The general rule-of-thumb is to start treatments if the tree is within 15 miles of a known infestation.
“Once an area becomes heavily infested with EAB, they will have to switch to Tree-Age,” Dr. Eggen said. “The best insecticide, Tree-Age is a restricted use pesticide and must be applied by a certified pesticide applicator. It only has to be applied every three years.”
Tree removal and replacement
Because dead trees can become a public safety hazard and therefore a liability, removal of ash will become necessary at some point, which will be far more expensive than the cost of insecticide treatments.
Replanting is also very important, if the urban tree canopy is to be maintained. Planting for diversity, rather than a monoculture, is essential.
Inventories, insecticide treatments, and selected tree removal are all steps communities and homeowners can take to manage their ash trees.
The Pennsylvania Bureau of Forestry continues to track the spread of EAB in the state by undertaking visual surveys near where infestations have been detected, and by deploying purple panel traps. (The color attracts the beetle as well as two oil lures which mimic the smell of a stressed ash tree.) However, “these traps are not good at detecting low populations of EAB,” according to Dr. Eggen.
“In Asia, EAB does attack ash trees, but the trees are resistant or more tolerant to EAB so it does not kill the tree, largely because of various species of parasitic stingless wasps, or parisitoids, which destroy the eggs or larvae,” wrote Dr. Eggen.
Several of the Asian wasp species were imported from forests in China, bred in a laboratory in the United States, and approved by the U.S. Department of Agriculture for release in 2011 in eight states, including five sites in Pennsylvania. The Pennsylvania Bureau of Forestry is still collecting data on the results.
“The bio-control effort is a long-term program,” wrote Dr. Eggen. “It will take additional species of parasitoids to have an impact on EAB populations.”