Black Twig Borer on Ornamental Trees and Shrubs in South Carolina

Quick Guide

• Affected Plants: Flowering dogwood, redbud, red maple, magnolias, willows, live oak, pecan, grape, and black gum. This borer attacks over 200 host species.
• Primary Disease Types: The ambrosia fungus (Fusarium solani), introduced into the insect galleries, is a pathogen of trees.
• Primary Insect Pests: Black twig borer (Xylosandrus compactus).
• Primary Damage Caused: Adult beetles introduce spores of Fusarium solani into galleries. As the fungus spreads, it clogs the xylem tissue of the twig, resulting in wilting and death of the end of the twig.
• Most Important Prevention Strategy: If only a few branches are attacked, prune twigs back 3 to 4 inches below the entrance holes and resulting cankers, and then burn or dispose of them immediately.
• Chemical Control: Protective insecticidal sprays on twigs and small limbs should be applied to infested trees following pruning. Spray every 2 to 3 weeks during late winter and early spring, from late February through March, with permethrin or bifenthrin.

About Black Twig Borer

Black Twig Borer (Xylosandrus compactus) is a species of scolytid beetle and is one of the few ambrosia beetles that will infest healthy or stressed plants. Adult beetles are solid black, shiny, and about 1/16-inch long.

Although ornamental shrubs and shade trees seldom die from black twig borer infestations, the damage to twigs can substantially affect their appearance.

Black Twig Borer Hosts and Distribution on Ornamental Trees and Shrubs

Black twig borer attacks more than 200 host species and has spread along the coastline of Georgia, South Carolina, and North Carolina.

Common hosts include:

• Black gum
• Flowering dogwood
• Grape
• Live oak
• Magnolias
• Pecan
• Red maple
• Redbud
• Willows

Black twig borer is an introduced species from Southeast Asia that entered Florida in 1941 and has spread upward along the coastline of Georgia, South Carolina, and North Carolina.

Black twig borer (Xylosandrus compactus) female preparing to bore into a twig of a susceptible woody host.
Lyle Buss, University of Florida, Bugwood.org

Black Twig Borer Symptoms on Ornamental Trees and Shrubs

Black twig borer infestations cause wilting, cankers, twig dieback, and other damage to small branches.

Twig and branch wilting becomes evident within one or more weeks after beetles enter the twigs. Generally, branches that are attacked are 7/8 inch in diameter or less.

Common symptoms include:

• Small entrance holes, 1/32 inch in diameter (approximately the size of a pencil lead)
• Holes are located on the underside of twigs and branches
• Cankers from ½- to 8-inches long around the attacked area on larger twigs, where multiple female black twig borers have entered the twig
• Twigs die from the point of infestation outward
• Brown leaves that may stay attached because of rapid leaf death
• Dark stains from the ambrosia fungus in the central pith

Black Twig Borer Life Cycle on Ornamental Trees and Shrubs

Black twig borer adults overwinter in infested twigs and branches before emerging and establishing galleries in new plant tissue.

Adult Activity and Overwintering

Adult beetles are primarily active in the early spring, but cold winters may reduce black twig borer populations or slow insect development.

Adult beetles overwinter in infested tree twigs and branches, and the adult females begin to emerge about the time dogwoods bloom.

Gallery Development and Egg Laying

Adult females enter new twigs and small branches, form their brood chambers or galleries in the stem pith, and then lay eggs.

It takes approximately 28 to 30 days to complete the life cycle (from egg to adult) within the host plant. During the spring, all stages of the beetle are found within their galleries, and 10 to 15 beetles may develop within a single gallery.

Ambrosia Fungus

The adult beetles also introduce spores of a fungus (Fusarium solani) into the galleries that grow to produce white fungal ambrosia on which both adults and their immature grubs feed.

Unfortunately, this ambrosia fungus is a pathogen of trees, and as it spreads, it clogs the xylem tissue of the twig, resulting in wilting and death at the end of the twig.

Black Twig Borer Management on Ornamental Trees and Shrubs

Management of black twig borer focuses on pruning infested plant material, promoting tree health, and applying protective insecticidal sprays.

Black twig borer larva is feeding on the fungal ambrosia within the carved out gallery.
Doug Stone, Mississippi State University, Bugwood.org

Cultural Control

Pruning and Disposal

If only a few branches are attacked, pruning out and disposing of the ambrosia beetle-infested plant material is highly recommended.

• Prune twigs back 3 to 4 inches below the entrance holes and resulting cankers.
• Burn or dispose of the pruned material immediately.

Good tree care practices should be applied to promote tree vigor and health, aiding recovery from beetle damage.

Watering and Mulching

• During summer drought conditions, properly water established trees once weekly as needed at the rate of 1-inch of water per week.
• Apply a 3- to 4-inch layer of mulch around trees to conserve soil moisture.

Soil Testing and Fertilization

• A soil test is recommended to determine which nutrients are required in the soil for best plant growth and to determine if lime is required.
• In the absence of a soil test, fertilize trees very lightly in the spring with a slow-release tree and shrub fertilizer, such as a 12-6-6 or 16-4-8 fertilizer.
• In the coastal counties of Beaufort, Charleston, and Horry, use a 15-0-15 fertilizer as there is typically sufficient phosphorus naturally in the soil.
• Avoid over-fertilization.

Herbicide Precautions

• Keep broadleaf weed killers (including weed and feed fertilizers) away from the plants, as they can be very injurious to both trees and shrubs.

Chemical Control

Limits of Insecticidal Sprays

Once trees are infested, the beetles cannot be killed within the plant by insecticidal sprays, and fungicides are ineffective against the ambrosia fungus.

However, protective insecticidal sprays on twigs and small limbs should be applied to infested trees following pruning, because once an attack stresses a tree, it becomes more attractive to further attack.

Trees generally become less attractive to beetles by the time their new leaves are fully expanded.

Protective Spray Timing

Spray every 2 to 3 weeks during late winter and early spring (late February through March) with permethrin or bifenthrin.

Spray both the infested and other susceptible trees for the best control.

For spraying large trees, homeowners may wish to hire a landscape professional.

Systemic Insecticides

Black twig borer adults and grubs feed upon the ambrosia fungus introduced into the insect galleries; they do not consume plant material. Therefore, common systemic soil-treatment products (drenches or granules) containing imidacloprid are ineffective.

Products containing permethrin or bifenthrin that are labeled for spraying on limbs of landscape trees are listed in Table 1. Follow label directions for mixing, application, and safety.

Insecticides for Preventing Black Twig Borer Attack (Table 1)

The following products contain permethrin or bifenthrin and are labeled for spraying on the limbs of landscape trees.

Insecticides

RTS = Ready to Spray (a hose-end sprayer)
These products are typically sold in small containers at feed & seed or farm supply stores, or can be ordered online.

Pesticide Disclaimer

Inclusion of a product in this table does not constitute an endorsement by Clemson University or Clemson Extension. Always read and follow all label instructions for mixing and safe use.

Pesticides are updated annually. The last updates were made on 8/26 by N. Jordan Franklin.

Originally published 01/99