Black Twig Borer

Black Twig Borer (Xylosandrus compactus) is a species of a scolytid beetle and is one of the few ambrosia beetles that will infest healthy or stressed plants. The adult beetle is solid black, shiny, and about 1/16^th-inch long. Although ornamental shrubs and shade trees seldom die from black twig borer infestations, the damage to twigs can substantially affect their appearance.

General Information

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

Hosts: Flowering dogwood, redbud, red maple, magnolias, willows, live oak, pecan, grape, and black gum are common hosts, though this borer is known to attack over 200 different host species. The 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.

Symptoms: Wilting of twigs and branches usually becomes evident within one or more weeks after beetles enter the twigs. Generally, branches that are attacked are 7/8^th-inch in diameter or less. Entrance holes are small (1/32^nd of an inch in diameter, or about the size of a pencil lead) and are located on the underside of twigs and branches.

Cankers from ½- to 8-inches long are common around the attacked area on larger twigs where multiple female black twig borers have entered the twig. These twigs die from the point of infestation outward, as the resulting cankers disrupt water movement to the ends of the twigs. Since leaf death is rapid, the brown leaves may stay attached. Dark stains from the ambrosia fungus are found in the central pith as well.

Insect Life Cycle: Adult beetles primarily are 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. The females then re-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.

A magnolia twig gallery of black twig borer (Xylosandrus compactus) with white fungal ambrosia on which they feed.
Forrest L. Oliveria, USDA Forest Service, Bugwood.org

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, the fungus clogs the xylem tissue of the twig, resulting in wilting and death of the end of the twig.

Management

Cultural Control: 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, and then burn or dispose of them immediately.

Good tree care practices should be applied to promote tree vigor and health to aid in recovery from beetle damage. During drought conditions in summer, 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. 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. However, in the coastal counties of Beaufort, Charleston, and Horry, there is typically sufficient phosphorus that naturally occurs in the soil. Therefore, in these areas, use a 15-0-15 fertilizer around the trees during the spring. Avoid over-fertilization.

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: 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 the infested trees following pruning because once a tree is stressed from an attack, it becomes more attractive for further attack. Trees generally become less attractive to beetles by the time their new leaves are fully expanded.

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

Spray every 2 to 3 weeks during late winter and early spring (late February through March) with permethrin or bifenthrin. Spray both the infested as well as other susceptible trees for best control. For spraying large trees, homeowners may wish to hire a landscape professional. 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.

The 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) with imidacloprid are not effective.

Table 1. Preventative Insecticides to Protect Trees from Black Twig Borer Attack

Pesticides are updated annually. Last updates were done on 8/24 by N. Jordan Franklin.

Originally published 01/99