Emerald Ash Borer: Biology and Integrated Pest Management in North Dakota
(E1634, Reviewed June 2023)This publication summarizes the threat of invasive metallic wood-boring beetle, emerald ash borer, to North Dakota's ash trees. It's identification, biology, damage, and pest management strategies including cultural, plant resistance, biological control and chemical control are discussed. If you suspect that your ash tree is infested with emerald ash borer, it also tells you what to do.
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The Emerald ash borer (EAB), Agrilus planipennis, is an invasive, metallic, wood-boring beetle (Coleoptera: Buprestidae) that is a major cause of ash tree decline and mortality in the Midwest. This highly destructive pest attacks only species of ash (Fraxinus spp.).
Emerald ash borer was introduced to North America accidentally in the mid-1990s and was detected first in southeastern Michigan in 2002. Unlike native borer insects, which typically only attack trees already in decline, EAB attacks both stressed and healthy trees. Emerald ash borer has been responsible for killing hundreds of millions of ash trees since its arrival in the U.S.
Although EAB has not been detected in North Dakota, a find was observed in Moorhead, Clay County, Minnesota (across the Red River from Fargo) in February 2023. North Dakota residents living close to Clay County should routinely inspect their ash trees for any symptoms of EAB infestation.
Distribution
Emerald ash borer is native to Asia, where it can be found on several species of ash and is not considered a pest. Emerald ash borer most likely was transported to North America as larvae or pupae embedded in ash pallets, crates or packing material transported by cargo ships. As of April 2023, EAB has spread to 36 states and five Canadian provinces (Figure 1). Information on EAB’s most current distribution can be found at www.emeraldashborer.info
Figure 1. New county detections of emerald ash borer as of March 3, 2023 (USDA APHIS PPQ) (Esri, HERE, Garmin, FAO, NOAA, USGS, EPA, Esri, USGS).
Identification
Adults are recognized as metallic, wood-boring beetles (Family Buprestidae) by their short saw-toothed antennae, blunt head and elongate yet compact body with metallic coloration.
The adult EAB is distinguished from other North Dakota Buprestidae by its size (about ½ inch or 13 millimeters [mm]), overall metallic green with coppery reflections on the pronotum (shieldlike body segment behind the head), and the bright metallic red of the upper surface of the abdomen (Figures 2 and 3).
Figure 2. Size of emerald ash borer adult compared with a penny
Figure 3. Emerald ash borer adult (D. Cappaert, Bugwood.org)
The elytra (hard front wing covers) and membranous hind wings must be spread apart completely to view the dorsal surface of the abdomen. The abdomen projects beyond the elytra as a blunt-ended spine.
Eggs are oval to round, less than 0.039 of an inch (1 mm) in diameter, and although white when laid, they rapidly turn red-orange (Figure 4). Because eggs are laid in bark crevices, they are not readily observed.
Figure 4. Emerald ash borer egg
Larvae create characteristic serpentine tunnels beneath the bark of their host ash trees (Figure 5). Tunnels curve at near right angles so that the tunnel length, as measured in a straight line from start to end point, is less than half of the actual total tunnel distance through the wood.
Figure 5. Serpentine tunnel created by emerald ash borer larva
EAB larvae (Figure 6) are recognized by their enlarged and flattened pronotum, elongate body shape with abdominal segments one to seven trapezoidal, abdominal segment eight bell-shaped, and the last abdominal segment round with two spines (urogomphi, Figure 7).
Figure 6. Emerald ash borer larva
Figure 7. Comparison of prepupa of emerald ash borer (left) and red-headed ash borer (right). Circle shows two spines (urogomphi) on emerald ash borer larva
Overwintering larvae excavate a deeper chamber at the end of their tunnel and take on a compact appearance, with body segments somewhat telescoped together. At this point, they are called prepupae. A comparison of common wood-boring ash insect larvae is provided in Table 1.
Table 1. Larvae of ash-boring insects in North Dakota: comparison with emerald ash borer.
|
Character |
Emerald ash borer |
Red-headed ash borer |
Carpenterworm |
Ash/lilac borer |
Ash bark beetles |
|
|
Larva |
Prepupa |
Larva |
Larva |
Larva |
Larva |
|
|
Length |
0.71 inch |
0.51 inch |
0.39 inch |
3 inches |
0.59 inch |
0.12 inch |
|
Shape |
thin and wormlike |
thick and wormlike |
thick and wormlike |
caterpillarlike |
caterpillarlike |
grublike |
|
Thoracic legs |
absent |
absent |
present |
present |
present |
absent |
|
Prolegs |
absent |
absent |
absent |
present |
present |
absent |
|
Urogomphi |
present |
present |
absent |
absent |
absent |
absent |
Pupae have the characteristic shape of the adult beetle, with short, serrate antennae and blunt spine at the tip of the last abdominal segment (Figure 8). Newly formed pupae are white.
Figure 8. Emerald ash borer pupa
As the beetle within develops, the pupa takes on the adult form. When the adult emerges, the pupal exuvia (shed skin) remains in the pupal chamber. By contrast, two other common ash-boring insects, ash/lilac borer (Podosesia syringae) and carpenterworm (Prionoxystus robiniae), have the pupal skin partially or mostly extruded from the adult exit hole. Buprestids of the genus Agrilus, such as EAB, leave D-shaped emergence holes (Figure 9).
Figure 9. D-shaped emergence hole of emerald ash borer
Life Cycle
The EAB is a holometabolous insect, meaning it undergoes complete metamorphosis. The EAB has four life stages: egg, larva, pupa and adult. The life cycle of the EAB is completed in about one to two years. In northern states, such as North Dakota, larvae are expected to take two years to mature and complete development due to the colder climate and shorter growing season.
Females can lay 60 to 90 eggs in their lifetime. They deposit individual eggs on the bark surface or in bark cracks and crevices from mid-May through July. Eggs hatch one to two weeks later.
Larvae bore into the tree by chewing into the inner bark and cambium, creating serpentine (S-shaped) galleries. Larvae feed during the summer, usually from late June through October. These galleries increase in size as the larvae grow and feed. Larval galleries have been found in trunks and branches measuring as small as 1 inch in diameter.
When EAB larval densities are high, the tree’s water and nutrient flow can be interrupted, causing crown dieback and death during a two- to five-year period, depending on the tree’s size and relative health.
Larvae overwinter in a small gallery and pupate in early spring (April or May) of the following year. Based on the degree day (DD) accumulations, adults first emerge at 450 to 550 DD (using a base temperature of 50 F or 10 C), which in North Dakota is from mid-June through mid-July. Peak emergence is at 900 to 1,100 DD, from mid- to late July. Research indicates that winter temperatures colder than minus 30 F will cause about 98 percent mortality of EAB larvae. However, larvae found on the south side of trees often were not dead due to heat from sun in winter. In areas with harsh winter climate, the spread of EAB and time it takes to kill a tree is expected to be longer.
Adults live for only three to six weeks and feed on foliage for one to two weeks prior to mating. Foliar feeding by EAB adults causes little damage to the ash tree (Figure 10). They mate and repeat the life cycle.
Figure 10. Emerald ash borer adult feeding on an ash leaf
Potential Hosts
Emerald ash borer attacks and kills all of the North American ash species in native woodlands, shelterbelts and urban forests.
In North Dakota, the most common ash species is green ash (Fraxinus pennsylvanica, Figure 11). Black ash (F. nigra, Figure 12), Manchurian ash (F. mandshurica) and white ash (F. americana) are uncommon in urban areas; however, these species also are susceptible to EAB, although Manchurian ash does have some level of resistance to this pest.
Figure 11. Green ash tree
Figure 12. Black ash tree
Note that EAB will infest healthy and unhealthy ash trees, large and small (down to 1 inch or 2.54 centimeters [cm] in diameter), and all of the named cultivars. Mountain-ash (Sorbus spp.) is not susceptible to EAB because it is not a true ash tree.
Damage
The vast majority of damage comes from feeding by the larvae in the phloem tissue, just under the bark. EAB larval galleries (Figure 13) also can extend into the sapwood. This feeding behavior damages the vascular system of the tree, blocking water and nutrient transport.
Figure 13. Larval galleries of emerald ash borer
Visible symptoms of damage are dieback of the tree crown (Figure 14) and excessive sprouting (epicormic branches) along the main stem of the tree (Figure 15). However, these symptoms are unlikely to be seen during the first year of infestation. Instead, dieback probably will be observed only in trees that have been infested for three years or longer.
Figure 14. Dieback of tree crown
Figure 15. Epicormic branching from tree attacked by emerald ash borer
Keep in mind that environmental stress and diseases, such as ash yellows, commonly are responsible for symptoms similar to EAB infestation. Woodpeckers are attracted to EAB-infested trees, and excessive pecking damage by woodpeckers may be another visible symptom of EAB presence (Figure 16).
Figure 16. Woodpecker damage on emerald ash borer-infested tree
The potential economic cost incurred by EAB is enormous. The most basic expenses will be those of tree removal and replacement. Finding a qualified contractor who holds a North Dakota contractor’s license and has liability insurance for tree removal is important.
Beyond removal and replacement, the loss of benefits provided by ash trees will be enormous and expensive. Shelterbelts protect farmsteads, fields and livestock from harsh, drying winds in the summer and they capture snow in the winter, acting as living snow fences along highways and around many communities. In urban EAB-infested areas where ash trees have been destroyed, the loss of shade/protection from ash trees has resulted in increased costs associated with summer air conditioning and lawn watering, and home heating in the winter.
INTEGRATED PEST MANAGEMENT of Emerald Ash Borer
Monitoring and Surveying
Several methods are being used in survey efforts for EAB. Large purple prism traps (Figure 17) are used in government-sponsored survey programs for EAB and are very visible to the public in parks and recreation areas.
Figure 17. Purple prism trap used for surveying for emerald ash borer
Prism traps are pre-coated with an insect-trapping adhesive. Lures are attached to the trap and are effective in the field for 20 days as an attractant. The chemical component of the lure has gotten better in recent years but still has room for improvement. Prism traps are hung over sturdy branches in the mid to lower canopy of ash trees of at least 8 inches (20 cm) in diameter before EAB emergence is expected.
In addition to the prism traps, girdled “trap trees” also have been used for locating EAB, but this method is time-consuming and, therefore, expensive. Other traps that have been developed included green funnel and prism traps, but these are not widely used in the U.S.
Simple visual monitoring by the general public is also critical because the trapping materials and techniques have not been perfected yet. If you suspect that EAB is in your ash trees, contact one of the organizations listed at the end of this publication.
Biological Control
Biological control involves the use of natural enemies (predators or parasitoids) to control insect pests naturally. Biological control primarily is being targeted at EAB in forests. The EAB has no known predators other than woodpeckers that occasionally feed on larvae and kill about 30 to 50 percent of large EAB larvae.
In 2007, three species of hymenopteran parasitoids (wasps) from China were released for biocontrol of EAB by the U.S. Department of Agriculture (USDA) Forest Service and USDA’s Animal Plant Health Inspection Service (APHIS). Two introduced parasitoids, Spathius agrili (Braconidae) and Tetrastichus planipennisi (Eupelmidae, Figure 18), attack EAB larvae. The other introduced parasitoid, Oobius agrili (Encyrtidae), parasitizes EAB eggs. In 2015, an external larval parasitoid, S. galinae (Braconidae), was released from Russia.
Figure 18. Eulophid wasp, Tetrastichus planipennisi, of emerald ash borer
Two of the released parasitoids, O. agrili and T. planipennisi, are established and are being widely released. However, S. agrili remains unconfirmed in northern areas. Spathius galinae does reproduce in northern areas.
In addition, researchers are surveying extensively for indigenous natural enemies of EAB. A native wasp, Atanycolus sp. (Braconidae, Figure 19), has been found parasitizing EAB larvae in Michigan. Another native solitary wasp, Cerceris fumipennis (Crabronidae), captures buprestid beetles, including EAB, as prey and provisions its ground nest with the beetle prey as food for its developing young. This solitary wasp also is used as a biosurveillance tool for the detection and survey of EAB populations.
Figure 19. Braconid wasp, Atanycolus cappearti, of emerald ash borer
Parasitism of EAB eggs by O. agrili was observed to be near 22 percent. For T. planipennisi, larval parasitism was at 20 percent in young ash trees and 50 to 80 percent in ash saplings, and it is spreading rapidly in all EAB-infested parts of North America. Researchers are optimistic that a combination of introduced and native parasitoids will help reduce EAB densities below a tolerance threshold so that ash trees can survive in the forest ecosystem.
Trunk-injected Systemic Insecticides
Trunk injections are available for use by tree professionals and homeowners (most products), (Figure 20 and 21).
Figure 20. Trunk injection for control of emerald ash borer
Figure 21. Mauget capsule injection for control of emerald ash borer
Trunk-injected insecticides are used frequently in situations in which soil treatments are not practical due to saturated soil conditions, porous sandy soils or other sensitive environments.
The recommended treatment timing is when the ash leaves are starting to emerge but before EAB eggs hatch, usually mid-May to mid-June. Although trunk injections are absorbed quicker than soil treatments, they still require three to four weeks to translocate throughout the tree.
Trunk injections should be performed when temperatures are not hot (greater than 90 F or 37 C) and soil conditions are not dry. Morning is typically the best application time.
One of the negatives of using this technique is that it wounds the tree, which can affect the tree’s long-term health. However, researchers found that a single injection of emamectin benzoate applied mid-May or early June provided excellent (greater than 99 percent) control of EAB for at least two years, even under high pest pressures.
Imidacloprid trunk injections resulted in less insect mortality and varying degrees of EAB control. Overall, trunk-injected emamectin benzoate provided the highest level of EAB control when compared with other insecticide products and application techniques.
References
Herms, D.A. and D.G. McCullough. 2014. Emerald ash borer invasion of North America: history, biology, ecology, impacts, and management. Annu. Rev. Entomol. 59: 13-30.
Herms, D.A., D.G. McCullough, D.R. Smitley, C.S. Sadof, R.C. Williamson and P.L. Nixon. 2009. Insecticide options for protecting ash trees from emerald ash borer. North Central IPM Center Bulletin. 12 pp.
Hahn, J., D.A. Herms and D.G. McCullough. 2011. Frequently asked questions regarding potential side effects of systemic insecticides used to control emerald ash borer. University of Minnesota Extension, Michigan State University and The Ohio State University Extension, 4 pp. www.emeraldashborer.info
Cover photograph – Emerald ash borer adult (L. Bauer, USDA Forest Service Northern Research Station, Bugwood.org)
This publication was also authored by James S. Walker, Graduate Student, NDSU, 2013.
Partial funding for this publication is made available through support from the USDA Forest Service State and Private Forestry Program.
