Borers are a group of insect pests that spend part of their adult or larval life stage feeding inside roots and branches, or tunneling beneath the bark or into the heartwood of many trees and shrubs. Many species of boring insects are capable of causing internal damage to a wide range of plants. The larvae of many wood boring beetles can be classified as either flatheaded or roundheaded borers. As adults, they may be either beetles or clear-winged moths.
Plant injury caused by borers can be long lasting; moderate to heavy infestation can cause death of the plant. Any plant can be susceptible, but it is generally believed that wood borers are secondary pests that develop because trees and shrubs are stressed, injured, or dying from other causes. Plants most susceptible to borer attack are those stressed from recent transplanting, drought, repeated defoliation, mechanical injury (mowers, grass trimmers), or other causes. (Also see plant information leaflet on bronze birch borer.)
The location of damage on the bark and the species of tree attacked aid in the identification of the insect involved. Most larvae are shallow-boring species and tunnel just beneath the bark of the trunk, branches, or twigs. Boring activity often starts a flow of tree sap or results in sawdust-like excrement (frass) which is visible in cracks and crevices. Foliage may be off-color or drop prematurely, portions of the bark may crack, and branches and twigs are often girdled and die because of borer activity. The overall health of the plant declines rapidly, often resulting in the death of the tree or shrub.
Adult flatheaded borers are generally known as metallic wood borers. They range in size from 1/3 to 1-inch long. Their bodies are flattened, oval, or elongated, and typically they have grooves on their wing covers. The larvae are yellowish- white, 1 to 2-inches long, broad and flat in the front and tapered toward the rear. Many have segmented bodies.
Females lay their eggs in bark crevices in the spring and early summer. After hatching, the larvae bore shallow tunnels packed with frass beneath the bark and sometimes into the heartwood. Their tunnels are long and winding, forming oval galleries beneath the bark. Dark, dead areas of bark, often with sap, are evidence of larval activity. A wet spot usually forms around affected bark. Many species complete their life cycle in one year, while others require 2-3 years.
The flatheaded apple tree borer (Chrysobothris femorata) is a common species that feeds on many deciduous shade and fruit trees and shrubs. Favorites include maple, oak, hickory, willow, sycamore, rose, tuliptree, and cotoneaster. Adult beetles are dark metallic brown to dull gray, about 1/2 inch long. The adult female lays eggs in bark crevices. The newly hatched larvae bore into the inner bark, forming tunnels and filling them with frass. If a plant is healthy, the larvae may be killed by sap flow. When nearly mature, the larvae tunnel into the heartwood where they pupate. Newly transplanted trees and shrubs are particularly susceptible.
Two-lined chestnut borer (Agrilus bilineatus) primarily attacks oaks. Some of the more frequently attacked species include white, red, scarlet, northern pin, black, bur, and chestnut oak. The adult beetle is slender, 1/5 to 1/2-inch long, with 2 yellow stripes along its back. Adults emerge in late May to early June, feeding on foliage. After mating, females lay eggs in small clusters in bark cracks and crevices. Eggs hatch in 7 to 14 days. Larvae burrow through the bark, constructing galleries tightly packed with frass. These feeding galleries cut the flow of food and water in the plant’s vascular system, causing bark to crack and split. Larvae pupate in the outer bark in a pupal cell. The following spring the adult beetle chews a D-shaped emergence hole, similar to bronze birch borer. There is one generation per year. Attacks from twolined chestnut borer begin in the crown of a tree and proceed downward in each succeeding year of infestation.
Honey locust borer (Agrilus difficilis) attacks large and small honey locust trees, as well as branches more than 2 inches in diameter. Trees under stress are more susceptible. Adults are 1/2 inch elongated beetles, with a metallic, greenish-black abdomen with a distinct yellowish- white band on the sides. Adults emerge in June and feed on honey locust foliage. Females lay eggs that are covered with a frothy substance that hardens. The larvae bore into the trunks or branches causing sap to ooze at the site. Once exposed to air, this sap hardens into a mass of gum. Fully-grown larvae are flat and segmented. Repeated infestations gradually cause decline and dieback of twigs and branches in the crown.
The adult roundheaded borers are also known as “longhorned” beetles because of their long antennae. They have hard-shelled bodies that are sometimes colored with stripes, bands, or spots. Body length varies from 3/4 inch to over 3 inches. The larvae are white to yellow, have no legs, and are round and fleshy. Roundheaded borers are more destructive than flatheaded borers, feeding on dead or dying trees and rarely on healthy trees. Females lay eggs under bark scales, in crevices, or in tree wounds. After hatching, the larvae feed beneath the bark for a while before entering the wood. Some species do not tunnel into wood. Life cycles can vary from several months to several years.
The locust borer (Megacyacyllene robiniae) is a serious pest of black locust (Robinia pseudoacacia). It does not attack honey locust (Gleditsia spp.). Larvae tunnel into a tree’s trunk and branches, weakening the tree and making it susceptible to wind breakage. The most obvious sign of attack is a knotty swelling on the trunks, many broken limbs, and irregular grub holes.
The adult locust borer is a slender, “longhorned” beetle, about 3/4 inch long, with reddish legs and black antennae. Bright yellow bands circle its jet-black body. A distinctive W-shaped band extends across the wing covers. Often the brightly colored adults are found feeding on pollen of goldenrod (Solidago spp.) during September. Egg laying occurs in late August through October in bark cracks and scales of host trees. Eggs hatch in 7 days and small, white larvae tunnel into the inner bark causing sap to ooze around small holes. Through the spring and early summer, larvae enlarge in their tunnels. By late July into August, they have completed pupation and emerge as mature beetles through holes created by larvae.
The roundheaded apple tree borer (Saperda candida) takes 2-3 years to complete its life cycle. Adult beetles are 1 inch long and brown, with two white longitudinal stripes on its back. Larvae overwinter in various stages feeding on sapwood and heartwood. Pupation occurs in late spring of the second year, and emergence begins in early summer.
The presence of piles of frass at the base of trees or on bark of larger branches is evidence of attack. Trees become weakened and heavy infestations can kill a tree in one season. Members of the rose family are favorite hosts of the roundheaded apple tree borer.
Lesser peachtree borer (S. pictipes) adults have clear wings and metallic blue bodies with yellow markings. It is a pest of flowering cherry and almond. Larvae feed on branches, near or in trunk crotches, and at old trunk wound sites found above ground level. An amber-colored gum (gummosis) is frequently found where borers have attacked.
Trees and shrubs of low vigor or in a weakened state of health are especially susceptible to borer attack. Prevention is the key in controlling wood-boring insect pests. Keep plants healthy and vigorous.
- Grow only trees and shrubs that are adapted to the area and site, and select resistant varieties.
- Keep plants healthy and vigorous through proper planting, mulching, watering, fertilizing, pruning, and winter protection practices.
- Protect trees from injuries to roots and trunks.
- Remove dead limbs or trees promptly to avoid infestations. Remove bark from felled trees if stored for firewood.
Generally, chemicals have little value in controlling borers already established within trees and shrubs.
Insect/ When to Treat
Ash borer/ Spray trunks and limbs in early June, repeat in 4 weeks
Dogwood borer/ Spray in mid-May
Flatheaded appletree borer/ Spray in late May, repeat in 3 weeks
Honey locust borer/ Spray in early June
Lilac borer/ Spray in early June, repeat in 4 weeks
Locust borer/ Spray in late August
Peachtree borer/ Spray in late August
Roundheaded appletree borer/ Spray in early June, repeat in 3 weeks
Two-lined chestnut borer/ Spray in mid-May, repeat in 3 weeks
Viburnum borer/ Spray in June, repeat in 4 weeks
Refer to the Illinois Urban Pest Management Handbook (University of Illinois Extension Service) for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions