Five groups of wood boring beetles infest trees and shrubs in North Carolina: roundheaded wood borers, flatheaded wood borers, weevil grubs, ambrosia beetles and bark beetles. Many common wood borers are the roundheaded and flatheaded borers. The azalea stem borer, carpenterworm, locust borer, southern pine beetle, granulate ambrosia beetle and the whitepine weevil are discussed in more detail in Ornamental and Turf Insect Notes on trees. It should be noted that many of these borers are secondary invaders of trees that are dying or dead. Borers are particularly damaging to newly transplanted trees and those weakened by various causes such as drought, sunscald of the bark, mechanical damage, ice storms, and fire damage. Diseases, frost damage, pesticide injury or defoliation by leaf feeding insects can weaken trees. Filling, excavating or compacting soil beneath trees can hurt their health. Installation of utility pipes and lines may also damage roots.
Roundheaded Wood Borers
Roundheaded borers commonly burrow under the bark and into the heartwood, chewing round holes. The burrow entrance usually is packed with coarse, excelsior-like frass. An accumulation of these wood fibers, or frass, often is observed around the base of the tree. Occasionally there is a discharge of sap from the tunnel opening which wets and discolors the bark of the tree below it. The adults are called longhorned beetles because of their long antennae. Body length varies from 1/4 to almost 3 inches. The larvae have no noticeable legs and are white or yellowish and are usually slender. Beetles emerge from infested trees from late spring to early fall. After mating, females seek egg laying sites, often under bark scales, in crevices or in wounds of trees under stress or in poor health. Some of the longhorned beetles chew elliptical niches in the bark to lay eggs. After hatching, the larvae of some species may feed beneath the bark in the cambium region for a time before entering the wood. Other species do not bore into the wood but remain under the bark. Life cycles of the different species vary from several months to 2 or 3 years. We have several roundheaded borers that attack trees and shrubs in North Carolina including the azalea stem borer, dogwood twig borer, locust borer, raspberry cane borer, redheaded ash borer, southern pine sawyer, twig girdler, and the twig pruner.
Adults of the flatheaded borers are called metalic woodboring beetles because they are often beautifully colored and metallic at least on some parts of the body. They are boat-shaped and 1/2 to 1 inch long. These borers are sometimes destructive to newly transplanted shade and ornamental trees, particularly trees under stress. The larvae, called flatheaded borers, are 1/4 to 2 inches long, white or yellowish, legless, and have a wide, flattened segment (the thorax) just behind the head. Metallic woodboring beetles emerge from most trees in the early spring and summer and lay eggs around cracks and wounds. After hatching, the larvae bore in and tunnel in the cambium region beneath the bark and before boring into the heartwood. Their elliptical tunnels are long and winding and are packed with fine borings arranged in concentric layers so that arc-like bands appear when the galleries are exposed. Most species complete their life cycle in a year, but some require 2 to 3 years. In hardwoods, dark, dead areas of bark, often with sap exuding, result from their burrowing. Tunnels made by the larvae do not have outside exit holes, although the sawdust-like frass may be visible in cracks in the bark or where the bark sloughs from the trees. The flatheaded appletree borer and the sculptured pine borer are two of the more familiar flatheaded borers in North Carolina. The emerald ash borer has been only recently found here.
Relatively few weevils bore into woody plants. In general, weevils have a distinctive snout that protrudes from the head and bears mandibles at the tip. Immature weevils are usually fat, legless grubs with brown heads. Overwintering weevil adults become active around mid-March. Wood boring weevils overwinter as adults that may be active all winter (but in reproductive diapause) in forest litter or as larvae inside the host plant. Adults feed on the stems where the bark is still green but not in the needles. Cypress weevil larvae were reported to feed in the wood around the crown or base of the Leyland cypress. After emerging in spring, white pine weevils feed on succulent bark of white pines 7 to 10 inches below the top most bud of the terminal stem. Females move to the very tip of the leader to make more holes in which small, pearly white, translucent eggs are inserted. Larvae hatch 1 or 2 weeks later. These grubs often work downward as they feed on the cambium, completely girdling the leader. By late May, the larvae girdle the stem and the expanded candles wilt, forming a characteristic "Shepard's crook." By mid to late June, the larvae often burrow past last year's whorl of branches thereby killing the entire top set of branches of the tree. One or more of the top branches may grow upward to replace the leader, which results in a crooked or multiple trunk tree. As they mature, the grubs burrow into the pith and form a pupal cell although some grubs form chip cocoons just under the bark. About 2 weeks later the grubs pupate. Then after 12more days the new adult molts from the pupal skin. Development takes 7 or 8 weeks. The new adults eventually chew out through the bark of the dead leader and feed for a short time on the new buds or young growth. The new weevils feed on smaller branches of host trees, but usually doesn't cause significant damage although sometimes the adult feeding is severe enough to kill entire shoots. They then seek an overwintering site even though the weather may be warm.
Ambrosia beetles are dark, and small to tiny. Most ambrosia beetles attack stressed or dead trees, but a few attack healthy plants! Ambrosia beetles tend to bore through the bark and right into the wood. After females bore into a twig, they form a small chamber in which they lay a loose cluster of eggs. They introduce a fungus, Fusarium solani, into the burrow that produces a white fungal "ambrosia" on which the tiny grubs feed. Black twig borers attack twigs or branches and bore in to the pith (or if the twig is large, bore into the wood about half to one and one half inches). Small twigs (< 5⁄16 inch) are usually infested by only one beetle and such twigs usually die. Branches up to 7⁄8 inch may be infested by as many as 20 females. Such branches may die or may form a canker around the infested area. Wilting symptoms usually appear just weeks after the first attack. The grubs apparently feed on the pith as well. Development takes 28-30 days from egg to adult during the growing season. In winter, development is much slower. Females begin laying eggs as the weather warms in spring and continue until the weather cools in autumn. Grubs are most numerous during the growing season. Pupation and mating takes place in the infested twigs. They overwinters as beetles inside the stems.The heaviest beetle flight coincides about the time that dogwoods are flowering. Camphor shot borers attack common nursery plants in at least 20 plant families. Sweetgum appears to be a preferred host. Camphor shot borer attacks have been associated with plant stressors such as herbicide injury, poorly drained soil, trunk and branch damage, and inadequately managed container production. Plants infested with Camphor shot borers may display symptoms including leaf wilting, branch dieback, and plant death. "Sawdust" collects at the base of heavily infested plants. Granulate ambrosia beetles emerge in early spring and attack thin barked, deciduous trees. Strings of boring dust that may protrude from the bark as tiny sticks or tooth picks. These strands may reach 1.5 inches if wind and rain don't break them off. The beetles are present most of the year, but are most active in March to early April. They remain active at low levels through the summer and into the fall.
Bark beetles usually attack weakened and lightening damaged trees. On pines, if the tree has enough vigor, a pitch tube forms at the entrance hole. Bark beetles and their grubs feed on phloem tissue just under the bark. Each pair excavates an egg gallery between the bark and wood. These galleries may girdle the tree. Different bark beetle species excavate unique egg galleries. Black turpentine beetles make a blotch mine. Southern pine beetles excavate a winding tunnel up to a foot long. Native Elm Bark Beetles and smaller european elm bark beetles excavate a relatively short egg gallery across the grain and with the grain respectively. Eggs are deposited in niches along these egg galleries, and the larvae, usually feeding in the inner and outer bark, tunnel at right angles to the egg galleries. After a few weeks, the larvae molt into pupae, and new adults emerge in a few more weeks. These elm bark beetles are devastating to our native elms because they transmit Dutch elm disease to healthy elms.
Since weakened trees are more susceptible to borer attack, the best way to avoid borers is to keep trees in good health. Provide plenty of water during drought periods. Have the soil analyzed and apply the correct kind and amount of soil amendments. Try not to injure roots or above ground parts of plants. Remove and destroy dead wood and prune during the winter months for most trees (prune peach and apricot trees in spring). Choose trees suited to your growing area. Where possible, select varieties less susceptible to borer attack. Newly transplanted trees with thin bark should have the trunks wrapped for a year or more with burlap or other material (wrapping prevents egg-laying of many borers and reduces injury from sunscald).
To prevent borer infestation, use one of the following pesticides and follow label directions carefully. Do not apply to wet bark. Note that the pesticides Dursban and Lindane are no longer available for use around homes. Imidiacloprid products are being commonly used for flatheaded borers. This is a systemic and not just a protective bark spray. The North Carolina Agricultural Chemicals Manual has more pesticide choices.
|Pesticide (Trade Name)||Formulation and Amount|
cyfluthrin + imidacloprid (Discus)
|bifenthrin (Onyx, Talstar)||*Some homeowner garden pesticides containing bifenthrin are available.|
|*pyrethroids (Pyrellin, Pyganinc, Pyrenone or other compounds)||Weekly sprays may be required|
|Chlorantraniliprole (Acelepryn)||flathead borers; follow label directions|
|imidacloprid (Merit, Marathon II, and others)||flathead borers; follow label directions|
|permethrin (Astro, Perm-up, Permethrin Pro and others)||*Some homeowner garden pesticides containing permethrin are available.|
* Suitable for homeowner use.
Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the N.C. Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical.
- Act Now to Control White Pine Weevil. Boggs, J. 2017. Buckeye Yard & Garden onLine. The Ohio State University.
- An Identification Tool For Bark Beetles of the Southeastern United States. Baker, J. R., J. LaBonte, T. Atkinson & S. Bambara. 2009. Lucid® v. 3.4.1, December, 2009, Latest update [month/year] North Carolina State University, Raleigh, NC. 27695.
- Common name: cypress weevil, scientific name: Eudociminus mannerheimii (Boheman) (Coleoptera: Curculionidae). Mayfield III, A. E. 2005. Featured Creature, Division of Forestry, Florida Department of Agriculture and Consumer Services (FDACS). Publication EENY-360
- Elm Bark Beetle, Smaller European Krischik, V. and J. Davidson. 2013. on pp. 111-112 In IPM (Integrated Pest Management) of Midwest Landscapes. University of Minnesota Center for Urban Ecology and Sustainability.
- Fallen oak branches signal the work of the twig pruner. Russell, H. 2008. Michigan State University Diagnostic Services, Department of Plant Pathology. Michigan State Univ. Extension.
- Longhorned Beetles/Roundheaded Borers. Day, E. R. 2014. Virginia Tech, Virginia Cooperative Extension. ENTO-100NP.
- Roundheaded Borers. Anonymous. 1983. Florida Department of Agriculture and Consumer Services, Bulletin 196-A.
- Twig Girdler and Twig Pruner. Barrett, B. A. 2012 (reviewed). University of Missouri Extension.
- White Pine Weevil. Day, E. R. and S. M Salom. 2015. Virginia Polytechnic Institute and State University publication 444-270 (ENTO-113NP).
- Extension Plant Pathology Publications and Factsheets
- Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local Cooperative Extension center.
Publication date: Aug. 3, 2018
Revised: Oct. 22, 2019
N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.