Whether native or introduced, hardwood borers are inner feeders. As such, they are also largely invisible … that is, until their activity has been going on for at least a season or two. Leaf feeders are different in that the actual insects or at least the damage they cause is readily apparent as the tree is inspected. Evidence of borer injury is usually best seen when a dead or dying tree is removed from the landscape and the bark is peeled back. Once the inner tissues are exposed, galleries of the larval stages are visible, clear evidence of cambium destruction.
In many cases, damage from boring insects can be lethal to the tree. Not to totally discount the harm caused by leaf feeders, but damage to the conductive vessels by wood-boring insects is rarely without consequences. In addition to the damage to the tree’s transport system, the trunk and scaffold limbs are usually injured during feeding operations, which can put a tree at risk of structural failure. Dealing effectively with hardwood borers requires an inclusive approach, proper timing of control measures and perseverance.
Most hardwood borers are opportunists. Weakened or stressed trees tend to be the target of their feeding activity. Borers have well-developed acoustic and chemical receptors and hone in on changes in plant chemistry, sounds of collapsing cells, and breaks in water columns. In some cases they inhabit softened cambium and sapwood tissues, such as those that have been injured from cankers or decay organisms. When this occurs, borers often develop more rapidly than if they had established themselves in healthy wood tissues.
Preventive borer management should focus on steps to minimize stressful growing conditions. Several factors are involved: location, soils and care provided.
Location: Trees growing on sites with adequate room for root expansion, away from impervious hardscape materials, are less likely to become infested with borers than those adjacent to a street or house foundation.
Soils: Trees planted in soils with appropriate amounts of organic matter, humus, porosity, nutrients and suitable pH levels are healthier and have a greater capacity to resist borer infestation.
Tree care: Mulch, water and nutrients are at the heart of good plant health care. Too little or too much of any of these elements can weaken a tree’s defense system and cause it to be more vulnerable to borer infestation. Trees that are normally adapted for low nutrient conditions can become stressed when overfertilized, resulting in high nutrient content and weak growth, which make trees attractive to borers and conducive to future structural problems.
In 2012 and 2013, much of the nation was experiencing severe drought conditions. High temperatures, increased concentrations of sugars and other nutrients in plant tissues, and sound emissions from collapsing cells associated with the drought increased the susceptibility of many trees to borer invasion. Because many borer species have multiple year life cycles, some of these infestations will continue to be problematic for arborists in 2015 and beyond.
Triage for borers
Although borer infestations are hard to see, if you look closely there are observable symptoms. The first is an overall lackluster appearance, which is also characteristic of other maladies such as girdled roots, overwatering and heat stress. Some borers cause dieback of individual branches, while others produce raised or “rumpled” bark. In the later stages of an infestation, frass is often found on the bark or soil under the tree. Another indicator of borer activity is the presence of round or D-shaped holes in the bark, which are made by an adult borer as it exits the tree after completing its life cycle.
Regular inspection is a crucial step in evaluating your clients’ trees. While inspecting a landscape, always be on the lookout for other existing or potential problems.
Borer control involves prevention, inspection, sanitation (removal of infested tree parts) and insecticide application. The main goal of these four steps is to maintain water and nutrient movement throughout the tree by limiting borer damage to the tree’s vascular system.
Borer control products can be applied as canopy or trunk sprays, soil applications around the base of the infested tree, or injections into the tree trunk. Site conditions and the species of borer present will dictate the best option.
For trunk sprays, liquid formulations containing bifenthrin, carbaryl, chlorantraniliprole, cyfluthrin, dinotefuran and permethrin can be applied to the bark to the point of runoff from ground level up to and including the bases of lower branches.
Systemic soil treatments containing chlorantraniliprole, clothianidin, imidacloprid or thiamethoxam are applied as a drench around the base of a tree. Once absorbed by the roots they move systemically through the conductive vessels of the tree. This approach can be effective, but feeding by borers may have destroyed the tree’s conductive vessels, thus limiting product uptake.
A third approach for borer control involves injecting the insecticide directly into the inner tissues of the tree trunk. Insecticides labeled for tree injection include acephate, dicrotophos, emamectin benzoate and imidacloprid.
Common borer species
Redheaded ash borer (Neoclytus acuminatus): This round-headed 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 0.5 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. They have a one-year life cycle.
Cottonwood borer (Plectrodera scalator): Cottonwood borers infest the trunks of cottonwood and willow trees. Adults of this longhorned beetle are 1 to 1.375 inch long with black patches and transverse white stripes. Beetles emerge in late spring and early summer and feed on 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 inches 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.
Pine sawyers (Monochamus spp.): This group of long-horned beetles is relatively common, with several species included in the group. Adults are mottled gray and brown. Sawyers get their name from the noisy, sawlike sound that feeding larvae make as they gnaw away at the wood, producing coarse fragments that they often pack into their galleries or push outside. Adults emerge continuously during the summer, and all stages of the life cycle are present throughout the growing season. This makes the timing of control actions difficult. As feeding beetles chew into the bark of both healthy and weakened trees, they transmit the immature stages of pine wood nematodes, which cause the pine wilt disease that has destroyed plantings of Scotch and other pines in the Midwest and southern Plains states.
Bronze birch borer (Agrilus anxius): Adult borers are slender, metallic coppery beetles about 0.375 inch long. Larvae — white, segmented, legless grubs with an enlarged area behind the head — exist underneath the bark. They are about 0.5 inch long when mature. When a tree initially becomes infested, the foliage on some branches in the upper crown begins to yellow in midsummer, which progresses to browned or dead leaves. 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 D-shaped exit holes on the trunk and larger
Emerald ash borer (Agrilus planipennis): The emerald ash borer (EAB) is an exotic, 0.5-inch-long metallic green beetle from Asia that has destroyed over 30 million ash trees in the upper Midwest since its introduction in 2002, and as of last month had been confirmed in 24 states. An early sign of EAB infestation is the appearance of weak and dying stems and branches in the crown of the tree. Closer inspections will reveal 0.125-inch D-shaped holes on the trunk where adult borers have exited and zigzag tunnels packed with frass 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 a year, but may require two years to complete their life cycle in cooler regions.
Lilac and ash borers (Podosesia spp.): Adults of both of these species are day-flying, clear-winged moths that resemble wasps. Ash/lilac borers spend the winter as larvae in the heartwood and sapwood of infested trees and shrubs. In spring they change 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 the entrance holes. 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 tree is not watered properly, it’s not uncommon for terminal shoots of infested plants to wilt. There is one generation per year.
COVER PHOTO: JAMES KALISCH, UNL.