Shade tree borers
Orders: Coleoptera, Lepidoptera, Hymenoptera
Description and life history
Many different insects are classified as wood borers, including certain species of Coleoptera, Lepidoptera, and Hymenoptera. These insects develop by tunnelling into the branches, trunks, or roots of trees and shrubs. Some of these wood-boring pests can significantly damage healthy trees, while others are secondary pests of wounded trees.
- Shade tree borers are insects, typically beetles or moths, that develop within trees and shrubs.
- Most of these wood boring insects attack unhealthy trees stressed from drought, injury, or disease. However, some of the more aggressive borers, such as emerald ash borer, pose a major threat to healthy trees.
- Some signs of a wood borer infestation include the presence of holes in bark, galleries underneath bark, oozing wounds on the bark of trunks and large branches, and canopy thinning.
- The first line of defense is maintaining healthy trees through proper irrigation, fertilizing, and pruning practices. Insecticides are also available to help manage these pests once an infestation is present.
Larva of flatheaded appletree borer. Note the elongated body and enlarged thorax. Image credit: David Cappaert, Bugwood.org
Adult of locust borer. Note the very long antennae, which is a key feature of longhorn beetles. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Adult of poplar-and-willow borer. Note the elongated snout and multi-colored body. Image credit: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org
Adult of peachtree borer. Note the clear wings. Image credit: Joseph Berger, Bugwood.org
Adult of pigeon tremex. Female horntails have an ovipositor protruding from the rear of the abdomen. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Exit hole produced by emerald ash borer. Note that the hole is D-shaped. Image credit: Kenneth R. Law, USDA APHIS PPQ, Bugwood.org
Ash tree with bark removed to expose galleries produced by emerald ash borer. Image credit: Edward Czerwinski, Ontario Ministry of Natural Resources, Bugwood.org
Pupal skin of lilac borer protruding from an exit hole. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Description, life history, and host range continued…
Coleoptera (families: Buprestidae, Cerambycidae, and Curculionidae)
Buprestidae: Buprestid beetles are known as metallic wood borers or flatheaded borers, due to the coloration of beetles and the morphology of larvae, respectively. Some examples include emerald ash borer (Agrilus planipennis), gambel oak borer (Agrilus quercicola), and the flatheaded appletree borer (Chrysobothris femorata). Adults of this family have elongated, oval-shaped bodies that measure 6-64 mm (1/4-2 ½ inches) long and are often metallic green, bronze, blue, or other bright colors. The larvae of buprestid beetles often have dark, chewing mouthparts and an enlarged thorax with a hardened plate on the top. The thorax is noticeably broader than the head and abdominal segments. Adults lay eggs in bark crevices and feed on foliage and after hatching, the larvae tunnel beneath bark, creating meandering mines that can disrupt the host’s nutrient transport. The emergence of adults leaves behind D-shaped exit holes in bark. These beetles often take one year or longer to complete their development, which occurs entirely beneath the bark of a host tree. Buprestid beetles develop in coniferous trees such as ponderosa pine (Pinus ponderosa) and Douglas fir (Pseudotsuga menziesii), or certain deciduous trees such as ash (Fraxinus spp.) cottonwood (Poplus spp.), and aspen (Poplus spp.), depending on the pest species.
Cerambycidae: Cerambycid beetles are commonly referred to as longhorned beetles or roundheaded borers. Examples include the locust borer (Megacyllene robiniae) and poplar borer (Saperda calcarata). Adults measure up to 2.5 cm (1 inch) long and have long antennae, often longer than the body of the insect. These beetles have patterns of bright colors. The larvae have rounded, elongated pale bodies with dark jaws. Unlike the metallic wood borers, longhorned beetles chew oval-shaped exit holes in bark. Typically, these beetles require one or more years to complete their life cycle. These beetles have a wide host range that includes maple (Acer spp.), poplar (Populus spp.), willow (Salix spp.), sycamore (Platanus spp.), ash (Fraxinus spp.), elm (Ulmus spp.), and birch (Betula spp.), among others.
Curculionidae: This family includes bark beetles and weevils. While there are many species occurring in Colorado, only the poplar-and-willow borer (Cryptorhynchus lapathi) is considered a significant wood boring pest. The adult weevils are a mix of black and pink or pale white and measure about 1 cm (~2/5 inch) long. The larvae are white and legless, measuring about 6.5 mm (1/4 inch) when fully mature. Adults tend to lay eggs underneath bark near wounds, buds, or lenticels. After hatching, larvae begin feeding on the cambium and bore deeper in the heartwood to pupate. While one year is needed to complete development, adults can live for up to three years which can accelerate the pest’s population growth. The poplar-and-willow borer can attack willow, alder (Alnus spp.), birch, and poplar.
Lepidoptera (families: Sesiidae, Pyralidae, Cossidae)
Sessidae: Moths in the family Sesiidae have transparent wings and are commonly referred to as the clearwing borers. The moths resemble bees or wasps to deter predators but do not sting. They are often yellow, orange, red, black, dark blue, or a combination of colors, depending on species. These pests can injure healthy plants, and examples of clearwing borers include the lilac borer (Podosesia syringae), peachtree borer (Synanthedon exitiosa), and currant borer (Synanthedon tipuliformis). The larvae are pale with dark jaws, measure 2.5-3.8 cm (1-1 ½ inches) long, and resemble the larvae of roundheaded borers but have five pairs of prolegs (short leg-like structures) on the underside of the abdomen. Adults lay eggs on the bark surface and larvae tend to concentrate their feeding in areas underneath bark. Mature larvae excavate pupation chambers just underneath a thin layer of bark and adults pull the pupal case through the exit hole when emerging. The pupal case often remains protruding from the exit site. Clearwing borers require one year to complete their development, and collectively have a very wide host range that includes sycamore, olive (Olea spp.), lilac (Syringa spp.), oak (Quercus spp.), ash, alder, birch, and stone fruit trees (Prunus spp.).
Pyralidae: There are many moth species belonging to the family Pyralidae, but only a few of these are wood boring pests. Examples include Dioryctria ponderosae, commonly referred to as the pinyon pitch mass borer, and the Zimmerman pine moth (Dioryctria zimmermani). The adults of these species measure roughly 15 mm (3/5 inch) long and are usually grey with white zig-zag markings on the forewings. They have five pairs of prolegs on the underside of the abdomen and vary in color. Larvae of the Zimmerman pine moth first begin tunneling around the tips of branches and within shoots. Later in the season, larvae concentrate their feeding at the base of branches and are often found under masses of pitch. Mature larvae remain in the tunnel to pupate and emerge in late July or August. Female moths mate and lay eggs near earlier wounds. After hatching, the young larvae feed briefly before overwintering in a protective cocoon until the following spring. These moths often have a one-year life cycle, though some species may require two years to complete their development. Collectively, these pests attack pines and other coniferous trees in the Rocky Mountain Region.
Cossidae: Moths and larvae in the family Cossidae can be very large. One species, the carpenterworm (Prionoxystus robiniae), can develop in a variety of deciduous trees, but is most often associated with cottonwood and poplar. Adults of this species have a wingspan of about 7.6 cm (3 inches) and have black and grey forewings that provide excellent camouflage when resting on a tree trunk. Caterpillars can grow up to 7.6 cm (3 inches) long and are green or white with a dark brown head capsule. Like the larvae of other moths and butterflies, carpenterworm larvae have abdominal prolegs. The moths lay eggs in groups of three to six on the outer surface of bark near existing gallery entrances or other wound sites on the host. Larvae begin tunnelling into wood immediately after hatching and form small rectangular entrance holes that increase in size over time, expelling sawdust from entrance holes as they grow. This species requires three to four years to complete its development and has a very wide host range that includes oak, elm, maple, willow, cottonwood, black locust, boxelder, sycamore, and ash. Poplars are the preferred host in the Rocky Mountain Region, while green ash and elms are preferred in the Great Plains.
Hymenoptera (family: Siricidae)
Wasps in this family do not sting and develop in the trunks and branches of dead or dying trees. Species in this family are often referred to as horntails and many attack recently cut or killed trees, while other species such as the pigeon tremex (Tremex columba) attack deciduous trees. Adults are moderately large, measuring about 37.5 mm (1 ½ inches), and have black or brown cylindrical bodies with rust, orange, or yellow markings. These wasps have broad heads, small eyes, and do not have the constricted waist observed on other wasp species. Female horntails have a long ovipositor protruding from the end of the body and can introduce white rot fungi (Amylostereum spp.) into trunks when laying eggs, which can accelerate death of the host tree. Upon exiting the tree, horntails leave circular exit holes in bark about the diameter of a pencil. The larvae are white and legless with a dark spine at the end of the abdomen. Horntails can take several years to complete their development and most frequently attack coniferous trees, especially pines, but some species can also attack maple, ash, and elm.
Larvae of buprestid beetles can excavate extensive mines through plant tissues, which disrupts nutrient and can lead to thinning of the canopy. Adult buprestids tend to leave D-shaped exit holes in bark after exiting the host tree. Larvae of longhorn beetles often expel coarse and stringy sawdust from openings in the bark, and adults leave oval-shaped exit holes after emerging from the host tree. The poplar-and-willow borer tends to leave entrance holes of larvae plugged with coarse frass (excrement), appearing as irregular holes on bark. Stems infested with poplar-and-willow borer larvae can become deformed with gall-like swellings and dead bark. Infested host trees are more susceptible to attack by other wood-boring insects and plant pathogens.
Clearwing borers produce large gouging wounds that are often concentrated in the upper root crown, though extensive tunneling can also occur within the lower trunk. After pupating, adults often pull the pupal case through the exit hole which remains in place after the moth has emerged. Feeding of pyralid moths produces large gouging wounds under bark on trunks or larger branches that are characterized by large amounts of oozing pitch. Extensive feeding of the carpenterworm can weaken branches and make them prone to breakage during high winds, which is considered a major hazard in urban areas. Infestations can also cause girdling and dieback of branches.
Trees infested with horntails can wilt and foliage may change from dark green to light green, yellow, and eventually red. Egg laying behaviors may also produce resin beads or drippings on infested trees. Horntails tend to target unhealthy trees.
Many of these wood boring insects attack unhealthy or stressed trees. Therefore, maintaining healthy trees is the first management recommendation, which entails selecting trees and shrubs that are well adapted to the site conditions, eliminating damaged or overshaded limbs, and adequate irrigation. However, pest populations can build when there are abundant weakened host trees in an area. In such instances, protecting healthy neighboring trees with insecticides may be warranted.
There are several approaches to managing wood boring insects with insecticides, including preventative bark sprays, soil-applied systemic insecticides, trunk sprayed systemic insecticides, and trunk injected insecticides. Optimally timing the application for maximum efficiency is necessary since larvae are protected after entering the host plant.
Spraying bark with preventative insecticides can kill emerging adults and newly hatched larvae before they bore into the host plant. This method is the easiest and most cost-effective way to control most wood borers. To be effective, the insecticide needs to have a long residual activity spanning several weeks, and the insecticide should be applied when the adult stage is first present. The timing of adult activity depends on the specific pest. A summary of the more common wood borers and their period of adult activity is available in the full factsheet.
Soil applications of systemic insecticides can be effective against flatheaded borers, but are not effective against roundheaded borers, clearwing borers, carpenterworm, or pyralid moths. A soil drench is often used to apply insecticides to soil so it can be taken up through the roots and transported throughout the plant. Applying systemic insecticides to the soil is also discouraged on flowering plants attractive to pollinators since the insecticide can accumulate in nectar. It is recommended that the drench solution be applied when the insect begins laying eggs since the plant can take up to three weeks to uptake the insecticide.
Systemic insecticides can also be applied as trunk sprays for controlling wood boring beetles. Sprays are most effective when applied around the time of first expected adult emergence and egg laying, or for a month or two after this period. However, trunk sprays require expertise and should only be made by licensed professionals.
Trunk injections with systemic insecticides are used to control emerald ash borer in Colorado. This application method requires considerable skill and specialized equipment to avoid wounding the plant. Trunk injections are more expensive than trunk sprays but can control most wood borers reliably over a two- or three-year period. This method may be desired in sites with conditions that prevent the use of sprayed or soil-applied insecticides.
Cranshaw, W. (n.d.). Sirex Horntails. Bugwood Wiki – Colorado Insects of Interest. Available https://wiki.bugwood.org/HPIPM:Sirex_Horntails#:~:text=Male%20”Sirex%20noctilio”,that%20presently%20occur%20in%20Colorado.
PNW Handbook. (n.d.). Willow (Salix)-Poplar-and-willow-borer. Pacific Northwest Pest Management Handbook. Available https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/willow-salix-poplar-willow-borer#:~:text=Pest%20description%20and%20damage%20The,bark%20of%20trunk%20and%20branches.
UCANR. 2013. Clearwing Moths. University of California – Integrated Pest Management. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn7477.html#:~:text=Clearwing%20larvae%20are%201%20to,the%20base%20of%20the%20tree
UCANR. 2010. Carpenterworm. University of California – Integrated Pest Management. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn74105.html
USDA. 2011. Flatheaded Wood Borers (Metallic Wood Borers). United States Department of Agriculture – Forest Service. sAvailable https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5320268.pdf