The invasive beetle continues its destructive path across the country

Cambial feeding galleries of EAB larvae.

One of the most destructive pests of our time is the emerald ash borer (EAB). Many entomologists, foresters and arborists put it in the same category as gypsy moths and the mountain pine beetle. Many insect species, including the lilac/ash borer, the banded ash borer and the red headed ash borer, act much like a cheetah, running down a sick or elderly wildebeest. When we see this in action on the Nature channel, a golden-voiced narrator explains that this is the natural order of things, that Mother Nature allows for a thinning of the herd to keep the remaining animals strong and fit to survive, and in so doing provides food for others. Unfortunately, emerald ash borers do not follow this pattern. They attack all species of ash in all stages of development, and do not distinguish between healthy and stressed trees.

EAB origins

The emerald ash borer, Agrilus planipennis Fairmaire, is a nonnative exotic beetle that was discovered near Detroit, Mich., in the summer of 2002. The beetles feed on ash foliage, but cause little significant damage. The larvae feed on the cambium tissues, disrupting the tree’s ability to transport water and nutrients. EAB most likely arrived in the U.S. on solid wood packing material carried in cargo ships or airplanes originating in its native Asia.

Most recently

The areas of infestation by EAB are widespread and expanding outward from the Northeast and Great Lake states. According to http://www.emeraldashborer.info, EAB was confirmed in Ohio in 2003; northern Indiana in 2004; northern Illinois and Maryland in 2006; western Pennsylvania and West Virginia in 2007; Wisconsin, Missouri and Virginia in the summer of 2008; Minnesota, New York and Kentucky in the spring of 2009; eastern Iowa in the spring of 2010; Tennessee in the summer of 2010; Connecticut, Kansas and Massachusetts in the summer of 2012; New Hampshire in the spring of 2013; North Carolina and Georgia in the summer of 2013; and Colorado and western Iowa in the fall of 2013. It’s also established in Windsor, Ont.

D-shaped exit holes of adult emerald ash borers.
Photos by John C. Fech, UNL, unless otherwise noted.

Since its discovery in 2002, EAB has killed tens of millions of ash trees in southeastern Michigan alone, with tens of millions more lost in Colorado, Connecticut, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Massachusetts, Maryland, Minnesota, Missouri, New Hampshire, New York, North Carolina, Ohio, Ontario, Pennsylvania, Tennessee, Quebec, Virginia, West Virginia and Wisconsin. In response, various regulatory agencies and the USDA have enforced quarantines (Michigan, Connecticut, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Minnesota, Missouri, Ohio, New Hampshire, New York, North Carolina, Ontario, Pennsylvania, Tennessee, Virginia, West Virginia, Wisconsin and Environment Canada) and fines to prevent potentially infested ash trees, logs or hardwood firewood from moving out of areas where EAB occurs. As a result, EAB efforts have cost municipalities, property owners, nursery operators and forest products industries tens of millions of dollars.

Making control decisions

Due to the aggressive nature of this pest, control decisions should be considered carefully. If several trees exist on a property, a hierarchy of importance should be established for their protection. If an ash tree is in a strategic location, or is a memorial or historic specimen, control efforts are generally more justified than for random plantings that serve little purpose other than to photosynthesize and cool the air. In this sense, a priority system can serve as a helpful guide for focusing control efforts.

For example, if two framing white ash trees serve as welcoming elements for an upscale residential subdivision called Ash Hollow, a higher level of emphasis can be placed on preventing or controlling EAB than in a remote part of a golf course where dozens of ash trees populate the rough, some of which may pose a bit of a safety issue for golfers and workers by blocking important views between tees and greens.

Control strategies

For many pest species, cultural practices are helpful in reducing the severity of infestations. For example, many crabapple cultivars have been developed to be genetically resistant to apple scab disease. Planting these trees in a landscape greatly reduces the need for fungicides, which saves money for the client and creates less potential pesticide exposure problems for tree care workers.

Unfortunately, similar control strategies have not yet been developed for EAB. However, programs that encourage diversity in the landscape can have a positive impact. In Nebraska, for example, to promote species diversity, a program called ReTree Nebraska (http://www.retreenebraska.unl.edu) selected a group of preferred tree species that perform well in Nebraska but aren’t widely planted. ReTree Nebraska’s 10 for 2010 helped individuals choose the right tree for their landscape while improving the species diversity and vitality of Nebraska’s community forests.

Another helpful approach for limiting the expansion of EAB infestations, or at least slowing their spread, involves restricting the movement of firewood from infested to noninfested locations. Many exotic and native forest pests, including the emerald ash borer and Asian long-horned beetle (Anoplophora glabripennis), have the potential to disperse from one area to another on nursery stock and in firewood.

Emerald ash borer damage.
Photo by James A. Kalisch, UNL.

Insecticide options

A variety of treatment options are available for controlling and/or slowing the spread of EAB. These include bark and foliage sprays, soil treatments and trunk injections.

Bark and foliage sprays – Bark sprays protect the trunk and branches from newly hatched EAB larvae. Foliage sprays control adult beetles feeding on leaves and moving about in the canopy. Insecticides labeled for use as bark and foliage sprays include bifenthrin, beta cyfluthrin, dinotefuran, imidacloprid and permethrin. These treatments are applied in late spring or early summer prior to adult emergence. A second application may be required in midsummer.

Soil treatments – Soil treatments can be applied as a drench or by injecting into the soil. They’re absorbed by the roots and translocated throughout the tree. Soil treatments typically require 60 days or longer to be distributed throughout the tree. Insecticides labeled for use as soil treatments include dinotefuran and imidacloprid. Soil treatments are applied in the fall or early to mid-spring.

Trunk injections – These involve the injection of insecticides into the lower trunk of the tree. This approach requires puncturing or drilling the holes, which can cause internal damage to the trunk. Insecticides labeled for use as trunk injections include acephate, azadirachtin, dicrotophos, emamectin benzoate and imidacloprid. Trunk injections are usually performed in late spring to early summer before EAB eggs hatch.

Regardless of the actions taken to control or slow the spread of EAB, efforts should also be undertaken to increase plant diversity in the landscape. Ideally, a diverse landscape should include many different tree species, both conifer and broadleaf trees, with no more than 10 percent of the trees made up of the same species. Considering the aggressiveness of EAB, it’s wise to consider alternatives to ash when establishing new plantings.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.