Hardwood trees provide a great many benefits to property owners. Unfortunately, in many scenarios, the positive effects of shade, wildlife enhancement, songbird habitat, color in summer and fall, framing, texture, a sense of place, provision of a ceiling plane and winter color of bark go underappreciated.

As tree care providers, your duty is not only to inform your customers of these benefits, but to keep them coming – and make yourself look good in the process. One of the important steps in doing so is controlling insects such as borers.

Common borer species

Emerald ash borer (Agrilus planipennis): The emerald ash borer (EAB) is certainly one of the most talked about pests in the past 10 years. EAB is an exotic, .5-inch-long, metallic-green beetle from Asia that has already destroyed over 30 million ash trees in the upper Midwest since its introduction in 2002. Nearly all ash species are susceptible to EAB. An early sign of EAB infestation is the appearance of weak and dying stems and branches in the crown of the tree. Closer inspection will reveal 1/8-inch D-shaped holes on the trunk where adult borers have exited, and zigzag tunnels packed with frass (insect excrement and sawdust) under the bark. Later symptoms may include water sprouts and suckers around the trunk, split or loose bark and increased woodpecker activity. While most borer species are only attracted to weak or dying trees, EAB will attack young and old, healthy and stressed trees. Emerald ash borers generally have one generation per year, but may require two years to complete their life cycle in cooler regions.

Emerald ash borer adult. Photo: Barry Lyons, Canada Forest Service

Redheaded ash borer (Neoclytus acuminatus): Ash is subject to attack from several borer species. This roundheaded borer attacks several species of shade trees, but causes the most serious damage to green ash. The adult is a long-horned beetle that is .5 to 1 inch long and reddish brown to black with transverse white or yellow stripes on the wing covers. These beetles are attracted to weakened trees where they deposit eggs in cracks in the bark. The newly hatched larvae initially feed under the bark and later tunnel into the sapwood. The redheaded ash borer has a one-year life cycle.

Asian longhorned beetles (Anoplophora glabripennis): A recent introduction to the United States, Asian longhorned beetles are quickly becoming a serious pest. Most likely arriving via various cargo shipments from China, the beetles were first discovered in New York and Chicago. Maples have been the most commonly infested tree species, but many others have been damaged as well, including elm, locust, poplar, willow and horsechestnut. Unfortunately, at this time, biological and chemical treatment strategies have not been developed to control them. Because they are not a native species to North America, they have few natural predators. Control efforts have been limited to quarantines, established around areas of known infestations. Additionally, the Animal and Plant Health Inspection Service (APHIS) has established regulations on solid wood packing materials from China.

Lilac/ash borers (Podosesia spp.): Adults of both of these species are day-flying, clear-winged moths that resemble wasps. Lilac/ash borers spend the winter as larvae in the heartwood and sapwood of infested trees and shrubs. In spring, they transform into pupae, and eventually emerge as moths with a wing span of about 1.5 inches. After mating in June and July, females deposit their eggs in cracks and crevices in the bark. The newly hatched caterpillars bore into the tree trunk or lower scaffold limbs. On some trees, a sawdust-like material can be found around holes chewed in the bark. When a tree is repeatedly infested, the bark swells and cracks, causing the limb or trunk to become weakened at the feeding area. If the summer rains are inadequate, or the customer fails to water properly, it’s not uncommon for terminal shoots of infested plants to wilt. There is one generation per year.

Cottonwood borer adult. Photo: David L. Keith, UNL

Cottonwood borer (Plectrodera scalator): Cottonwood borers infest the trunks of cottonwood and willow trees. Adults of this long-horned beetle are 1 to 1 3/8 inch long and black with numerous white patches and transverse stripes. Beetles emerge in late spring and early summer and feed on the tender new shoots of young trees. They deposit eggs in openings chewed into the bark at the bases of trees below the soil line. The larvae burrow into the bases and roots of trees, pushing out frass at the entry points. Cottonwood borers generally have a two-year life cycle.

Poplar borer (Saperda calcarata): This borer attacks cottonwood, poplar and willow trees. The adults are approximately 1 inch long and are dark gray with small orange spots on the wing covers. They emerge in summer and lay eggs in slits cut in bark, usually near the middle portion of trees. The larvae, which are white and about 1.25 inch long, bore into the heartwood. They take about three years to mature. Damage appears as swollen areas on trunks and larger branches. Holes where larval excrement is pushed out and where adults have emerged are also signs of an infestation.

Bronze birch borer adult. Photo: David L. Keith, UNL

Bronze birch borer (Agrilus anxius): Serious pests of birch, adult borers are slender, metallic-coppery beetles about 3/8 inch long. Larvae feed underneath the bark and are white, segmented, legless grubs with an enlarged area behind the head. They are about .5 inch long when mature. When a tree initially becomes infested, the foliage on some branches in the upper crown begins to yellow in midsummer, progressing to browned or dead leaves as the season progresses. This results in the death of smaller branches in the upper crown. Over time, large branches begin to die back and, eventually, the entire tree may die. After repeated feeding activity, ridges begin to appear on the bark of the trunk and larger branches as the cambium tissue is damaged. A good indicator of bronze birch borer activity is the appearance of D-shaped exit holes on the trunk and larger branches as adult borers begin to emerge from the tree.

Carpenterworm (Prionoxystus robiniae): Cottonwood and ash are the preferred hosts, but this insect will attack many other shade and fruit trees, as well as shrubs. Adults are active from June through July. Females deposit their eggs on the bark of trees, usually on the lower trunk. After hatching from the eggs, young carpenterworms tunnel directly into the inner bark and later bore into the heartwood. Heavily infested trees are structurally weakened and may be broken during high winds. The caterpillars require more than one year to complete their feeding and may be up to 2 inches long at maturity.

Inspection

The first step in controlling borers is determining whether they are present. This may sound obvious, but if I had a nickel for each time a newly hired landscaper or tree care provider thought their customers’ trees were infested with borers but were mistaken, I’d have a tidy sum to use for my next vacation. It’s easy to understand why this diagnosis is arrived at by inexperienced tree care workers.

a. It’s a quick and easy conclusion.

b. The remedy will provide instant cash for the company, which usually makes the boss happy.

c. The customer doesn’t have the knowledge or capacity to refute it in the vast majority of cases, so it’s like pressing the easy button.

Instead of jumping to conclusions, use the descriptions above to make sure that borers are indeed a primary cause of injury to the tree.

Unless a legitimate need has been established, it’s not only poor arboriculture to treat a tree for borers; it’s downright dishonest, bordering on fraudulent. Instead of earning money based on how much product you can spray on a tree, develop regular inspection protocols to identify problems as they arise. In this way, each inspection becomes a valuable tree service that can be invoiced for. The invoice should be a combination of a set of notes from the current inspection, as well as problems that have the initial signs of becoming an issue. For example, the current inspection fee would be noted, followed by the observations of existing tree pests such as aphids and apple scab. The future problems section would include items such as the onset of decay or other developing problems.

The exit holes created by the emerald ash borer are quite distinctive. Photos by James Kalisch and John Fech, UNL

Control

1. Keep trees healthy. Communicate with clients that good tree care practices – mulching, separating trees from turf, examination of existing irrigation systems, compaction relief, soil testing and pruning of diseased or dead wood – are preventative steps that will reduce their costs in the long run. With the exception of emerald ash borer, these are pests that tend to select for weak or struggling trees in a landscape.

2. Trunk spray insecticide applications.

Pros — Liquid applications of a residual insecticide applied to the lower 15 feet of the trunk bark offer good borer control without injury to the tree.

Cons — Timing for trunk sprays is crucial; they must be applied before the eggs hatch and borers enter the tree. Additionally, close attention must be paid to the length of residual for these products so the tree is not left unprotected during extended or multiple egg-laying periods.

The exit holes created by the redheaded ash borer are quite distinctive. Photos by James Kalisch and John Fech, UNL

3. Soil-applied insecticide applications.

Pros — The advantage of this method is that it doesn’t cause any injury to the tree. In addition, application on days when light rain is falling is still permissible, as these products should be watered in for maximum effectiveness.

Cons — The downside to soil application is that they are not a good fit for properties with a slope, as there is a greater tendency for runoff and uneven infiltration. Likewise, when the rootzone is compromised by concrete or limited in some way, lower uptake is likely.

Exit hole and cambial damage due to carpenterworms. Photo: James Kalisch, UNL

4. Trunk-injected insecticides.

Pros — In some cases, the property owner may have failed to notice there is a problem until significant damage is already present. Injection offers the most immediate method of control available.

Cons — Injection is an invasive technique; a small amount of damage occurs when a hole is drilled or needle-like apparatus enters the bark. Holes in bark can lead to pathogen invasion and decay. This approach can be effective for borers already in the tree, but feeding by borers may have destroyed the conductive vessels, limiting product uptake.