CSI for EAB
How to detect and diagnose infestations of Emerald Ash Borer
A new, exotic insect pest poses a severe threat to the ash trees that populate our natural
and urban forests. This pest is called the Emerald Ash Borer, Agrillus plenipennis - a.k.a.
the EAB. This pest is native to Asia and likely arrived in this country in packing material.
It was first detected in Michigan and now has spread to more than sixteen counties in the
Great Lakes State. Since the first detection in 2002, it has been found in Ohio, Canada,
and Maryland. The Maryland detection is the first case that confirmed the suspicions and
fears of many, namely, that the EAB can be transported and relocated with infested
nursery stock. In this case, trees infested with EAB were shipped in April of 2003 to a
nursery in the Maryland suburbs near Washington, DC. During the spring and summer
beetles moved from Michigan trees to ones grown in the nursery. Some of these trees
have been shipped and installed in several locations in the greater Washington, DC area.
Officials from the Maryland Department of Agriculture are conducting search and
destroy missions in an attempt to impede or stop the spread of the beetle.
It is unlikely that the situation in Maryland is an isolated event. The EAB is spreading throughout the North Central region and ash trees may have been moved from this region to other areas of the country and installed in landscapes with a complement of undetected beetles. One of the most important roles arborists will play in the next several years will be as sleuths for incipient infestations of these devastating pests. This service will be invaluable as state and federal officials attempt to delimit the distribution and reduce the spread of this devastating pest.
To wage the war against EAB, arborists will need know and recognize their enemy. How will you distinguish EAB from other agents that kill ashes? Compacted soils, drought, nutrient deficiency, restricted root zones, elevated soil temperatures all can result in gross symptoms similar to those observed when trees are under attack by pests. These include discoloration of the foliage, dieback in the canopy, and the production of water sprouts. Some pathogens of ash such as anthracnose, verticillium wilt and the phytoplasma that causes ash yellows will cause dieback and leaf discoloration. However, these abiotic stressors and pathogenic organisms do not leave behind the telltale signs characteristic of wood boring insects.
Symptoms are the injuries caused by a pest and the plant’s response to these injuries. Signs are the pest itself and its products such as wax, webbing, or shed skins. The following guides will help you to learn the symptoms and signs associated with EAB and other pests associated with ash. Like Crime Scene Investigators your assignment will be to use these symptoms and signs to identify the perpetrator.
Clues 1, 2, and 3 – Examine the victim
When you arrive at the crime-scene look for three symptoms – dieback in the canopy, discolored leaves, and water sprouts. These easy to spot symptoms are non-specific symptoms that could indicate infestation by borers. Although they usually accompany borer infestations they cannot be used alone to rule out pathogens or abiotic stress factors. However, the following symptoms and signs can.
Clue 4 – Read the frass (Figure 1)
Carefully examine the ground beneath the tree and the surface of the bark. If you see course or fine sawdust-like material on the ground or bark as in (Figure 1), the news may be good. Sawdust-like material produced by wood boring caterpillars as they feed is frass, a euphemistic term for insect feces. In the most parts of the country, clearwing borer larvae are the most important native insect pests boring in ash trees in the urban forest. Two species are common - the banded ash clearwing, Podosesia aureocincta and the lilac / ash borer, P. syringae. The lilac / ash borer attacks several genera of trees in the olive and ash families including Fraxinus (ash), Syringa (lilac), and Ligustrum (privet). The banded ash clearwing is believed to attack only ash. It is not unusual for ashes in landscape plantings to contain both species simultaneously. As larvae of clearwing borers feed, especially in the later instars, they expel frass from their galleries. Some other native beetle borers of lesser importance will also expel frass as they feed. This frass can accumulate at the base of the tree or sometimes on the tree bark near the galleries of the borers.
The presence of frass is a good clue that EAB is not the perp. EAB does not expel frass. Larvae of this beetle borer pack their frass in the gallery inside the tree as they feed beneath the bark.
Clue 5 – Examine the exit wounds (Figure 1)
After feeding beneath the bark as larvae, borers must leave the tree to mate and complete their life cycle. This means that they leave a wound or exit hole behind as they emerge from the tree. The exit holes of clearwing borers such as banded ash and lilac / ash borer will be round or oval as in (Figure 1). Sometimes this emergence hole may be obscured by a crack or irregularity in the bark but when the hole is visible, it will be round or oval. The same is true for most other species of beetle borers that attack ash trees in North America. This includes several roundheaded beetle borers such as the banded ash borer (not to be confused with the banded ash clearwing), redheaded ash borer, and ash privet borer that attack ash. They also leave round or oval exit holes upon emergence from the tree. In contrast, the Emerald Ash Borer is a flatheaded borer and close kin to our native bronze birch borer and two-lined chestnut borers. The exit hole of the EAB will have one side noticeably flattened and it will resemble the letter D (Figure 1). The flat side of the D may be up, down, or sideways depending on how the beetle larvae was oriented when it completed its development.
Clue 6 - Skeletons tell tales(Figure 1)
As insects develop and grow they leave behind a shed skin – an exoskeleton – at each molt. Adults of the banded ash clearwing and lilac / ash borer emerge from exoskeletons – pupal cases - that protrude from the exit hole and are clearly visible on the bark of the tree (Figure 1). These pupal cases are a clear sign confirming the presence of clearwing borers. Pupal cases of the lilac / ash borer are seen in spring and summer when these pests complete their development. Banded ash clearwing borer pupal cases are seen in the late summer and autumn when adults of this pest emerge.
Larvae of the EAB complete development and pupate inside their gallery beneath the surface of the tree. Pupal cases of EAB will not be observed protruding from exit holes.
Closing in on the perp - Trails in the wood (Figure 2)
By now you have several clues to help with the identification of the pest. If you have the ability to proceed further with the investigation, that is to remove the bark of an infested tree, you will gain more evidence and, perhaps, if you are lucky, capture the perpetrator. There are several clues to help with a definitive identification. First, examine the shape of the galleries beneath the bark in the cambium, phloem, and sapwood. If galleries tend to be straight or wind only slightly, and are concave in shape, then they are likely to be created by clearwing borer or roundheaded borer larvae (Figure 2). Small linear galleries that radiate out from a straight central gallery may be bark beetles or shot hole borers (Figure 2). If the galleries are noticeably serpentine, that is S-shaped, and rather shallow with fine frass packed inside, then the perp is likely to be the EAB (Figure 2). Trails are not foolproof. Significant variation exists in the shape of galleries of many groups of borers. When you find the trail follow it. Remove the bark in both directions. If the trail narrows, you have found the borer entry site and the hunt is over. Reverse course and follow the trail in the opposite direction. You may be rewarded by capturing the perpetrator. If you find a larva, then the following lineup can be used to separate clearwing borers from EAB.
Identifying the perp – A line-up of two (Figure 3)
The Maryland Department of Natural Resources Forest Service wants to know if this service is helpful for you or your company, please send comments and/or suggestions to Maryland DNR.
Updated August 11, 2005