Borer management requires a comprehensive approach, proper timing of control measures and persistence. The most important step is a thorough inspection. Begin by focusing your attention on the bark of the tree; look at the fissures, the small cracks, the flat surfaces and in some species, the exfoliating tissues. Any abnormality should be noted and compared to the shape and size of exit holes of known tree borers.
Know the different borers are in your area. Pests usually infest the lower parts of the tree, in the trunk or lower scaffold limbs, so a close inspection of the lower 15 feet or so should suffice.
Redheaded ash borer (Neoclytus acuminatus)—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. They 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. This borer has a one-year life cycle.
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 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, a sawdust-like excrement, at the entry points. They 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 grey 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 and emerge continuously during the summer. All stages of the life cycle are present throughout the growing season, making the timing of control actions difficult. Pine sawyer beetles 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 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 begin to yellow in midsummer, changing 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. Good indicators of bronze birch borer activity are the appearance of D-shaped exit holes on the trunk and larger branches as adult borers begin to emerge from the tree.
Emerald ash borer (Agrilus planipennis)—The emerald ash borer (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. An early sign of EAB infestation is the appearance of weak and dying stems and branches in the crown of the tree. Closer inspections reveal 1/8 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 watersprouts and suckers around the trunk, split or loose bark and increased woodpecker activity. EAB will attack all trees-young and old, healthy and stressed. They generally have one generation per year, but may require two years to complete their life cycle in cooler regions.
Lilac and ash borers (Podosesia spp.)—Adults 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 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. There is one generation per year.
Pine moths (Dioryctria spp.)—Pine moth caterpillars cause significant damage to Scotch, Austrian and red pines. The first obvious symptoms are the presence of large resin or pitch masses on the tree trunks where the larvae are feeding beneath the bark. Pitch masses are usually golf ball-sized and located at branch whorls where branches join the main trunk. Fresh pitch masses where the larvae are active will be white, soft and shiny. Pitch masses from earlier generations will be hard, gray and dull. In mid to late summer, females lay their eggs on tree trunks, usually near wounds. The larvae hatch from the eggs by late August and crawl under loose bark scales or into wounds to spend the winter. In spring, the larvae enter the bark and feed for the next few months. Full-grown larvae are about .75 inch long and have a brown head with pink to greenish bodies covered with numerous tiny dark spots. In late summer, the larvae pupate at the end of the larval tunnel and two weeks later emerge as moths to start the cycle over.
Carpenterworm (Prionoxystus robiniae)—Cottonwood and ash are the preferred hosts, but this insect will attack many shade and fruit trees and 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 drectly 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.
Injection offers the most immediate method of control available. It is an invasive technique, and a small amount of damage occurs when a hole is drilled or needle-like apparatus enters the bark. In some cases where borer damage has rendered the cambium nonfunctional, insecticide efficacy can be reduced. Whenever possible, select the technique and product system that produces the smallest hole. Insecticides labeled for tree injection include acephate, dicrotophos, emamectin benzoate and imidacloprid.
Liquid applications of a residual insecticide applied to the lower 15 feet of the trunk bark offer good borer control. However, timing for trunk sprays is crucial; they must be applied before the eggs hatch and borers enter the tree. Also, 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. Liquid formulations containing bifenthrin, carbaryl, chlorantraniliprole, chlorpyrifos (nursery only), cyfluthrin, dinotefuran, endosulfan and permethrin should be applied to the bark to the point of runoff from ground level up to, and including, the bases of the lower branches.
Certain products are available for soil application. 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. 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. Chlothianidin, imidacloprid and thiamethoxam can be applied as a drench around the base of the tree. These materials are absorbed by the roots and move systemically through the conductive vessels of the tree or shrub. This approach can be effective for borers already in the tree, but feeding by borers may have destroyed the conductive vessels, limiting product uptake.
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.