Managing the Flatheaded Appletree Borer- Department of Entomology at Texas A&M University - Extension Publications
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September 2005

Managing the Flatheaded Appletree Borer

Noel Troxclair, Extension Entomologist, District 10

The flatheaded appletree borer, Chrysobothris femorata, is one of a complex of over 600 species in U.S. belonging to the family Buprestidae. The flatheaded borers, or metallic wood boring beetles, as a group, are perhaps the most serious pests attacking a wide range of tree species. The larval stage of the flatheaded borers is the most damaging to trees as they feed in the cambium layer just under the bark of the trunk and scaffold branches. They create galleries which may eventually completely girdle the tree under the bark. This type of feeding severs water- and nutrient-conducting vascular tissues and results in die-back of twigs and branches, and if severe enough, results in the death of the tree.

Flatheaded appletree borer adults - The adult beetle (Fig. 1) is about 1/2-3/4 inch long, has short antennae, large conspicuous eyes, and a noticeable tooth on the forelegs. The adult is blunt at the head and tapers to a rounded point at the back end. In cross section, it is oval-shaped and somewhat flattened on top. The adult coloration is a dark, metallic brown to green; the upper surface of the body, under the wing covers, is a bright metallic blue, while the undersurface is a coppery/bronze color. The wing covers are marked with two pale, gray, wavy bands.

Eggs - An adult female lays about 100 eggs, singly, that hatch in 15-20 days. Eggs are firmly attached under bark scales or in bark crevices on the main stem or larger branches. The eggs are pale yellow, wrinkled, flat, and about 1/20 inch (1.5mm) in diameter.

Larvae - The larvae (Fig. 2) are legless and when full-grown can be 1 1/4 inch long. They are usually a whitish-yellow color but may appear to be golden or a paler, light green. Larvae are legless, with several front body segments, just behind the head, that are enlarged and flattened. These segments are part of the insect’s thorax, but appear to be a large flat head, thus the common name. It is commonly found curved like a horseshoe, sluggish and inactive except in very warm weather.
Pupae - When fully developed, the larva bores, radially, deeper into the sapwood or heartwood, and excavates a pupal chamber where it overwinters. It pupates for 1-2 weeks the following spring and emerges, when conditions permit, to start the cycle again. Adults emerge by chewing their way out through the bark, leaving an oval hole. One generation per year is most common, but emergence cage studies have shown some emergence in the second and third years.

Biology and Behavior - Flatheaded appletree borer adults are present from March until November, being most abundant in May and June. Adults can be found throughout the summer feeding on foliage and at the base of twigs on partially defoliated trees. The adults can occasionally cause serious defoliation of trees.

Flatheaded appletree borers adults are most active in the heat of the day and prefer the sunny side of tree trunks and trees in full sun; they also may be seen basking in the sun on fallen trees or logs. They are very active, run rapidly and fly readily when disturbed.

Individual females are fertilized within a week of emergence and live approximately one month. Females lay eggs throughout the summer and place the eggs in bark crevices or under bark scales on apple trees. Females probe the bark for egg-laying sites and typically choose crevices in the bark which are deeper than others because this offers their eggs some protection and also places them closer to their food source beneath the bark. Although the females lay their eggs singly, one site may be visited several times by one or more females, so small groups of eggs are often found close together. The eggs are attached firmly to the bark surface and hatch in 15 - 20 days.

The legless larvae chew through the bark at the point at which they hatch and feed on the living phloem and outer sapwood. If the plant is healthy, larvae may be killed by the heavy sap flow, or alternatively, develop quickly in trees which are badly stressed. The weaker and less-vigorous the tree, the more damage the larvae will cause.

The larvae live, for the most part, just beneath the bark, where they excavate broad, flat, and irregular channels filled with powder-like frass. In late summer as the larvae approach maturity, they abruptly burrow more deeply into the heartwood and construct a pupal chamber.

Damage - Newly transplanted trees, those that have sustained bark damage, those newly exposed to full sunlight (from pruning), or trees that are stressed by changes in the environment (i.e., frost injury, drought, flood, soil compaction, etc.) and declining and dead branches in trees can all be susceptible to attack Hot, sunny portions of the main trunk and scaffold branches are favored for attack. Hail-damaged areas on trees are attractive egg laying spots for these borers.

The borers are particularly damaging to trees in the first 2-3 years after transplanting. The primary entry spot is the cut area above the bud that was grafted onto the rootstock. Once the main stem is cut, it requires 1-3 years for healing to occur. During this time the adult beetle of the appletree borer may deposit eggs in the exposed wood (left by the cut above the bud).

Young, thin-barked trees are most prone to be attacked due to the ease with which they may be injured. Once attacked, apple trees on dwarf rootstock are more susceptible to loss because of the smaller diameter of the rootstock, with a single larva sometimes capable of killing the tree. The frequency and severity of problems with this pest tend to decline as tree diameter increases and the tree ages. The borer also can be quite destructive in orchards or nurseries where grass and woody vegetation is allowed to grow around the bases of trees.

Native forests harbor significant populations of borers. Plantings in wooded settings, along wooded roads, and near pockets of native forest are at greater risk of attack. The following species serve as hosts for the flatheaded appletree borer: apple, basswood, beech, boxelder, cherry, maple, flowering crab apples, yellow poplar, oak, sycamore, hackberry, hickory, pear, peach, poplar, tuliptree, willow, rose, cotoneaster, hawthorns and serviceberry.

Symptoms of Tree Damage - Damage from these insect pests, whether as larvae or beetles, appears first as leaves wilting or turning brown and dying off. White, bacteria and yeast-infected liquids can be seen oozing from localized areas on the bark surface over newly damaged locations. Bark eventually darkens, appears wet and shiny. Little exterior frass or wood dust is evident except in bark cracks. Excavated galleries are broad, irregular, and packed tightly with frass. The living phloem, cambium, and last annual ring of sapwood are consumed.

In young, thin-barked trees, injured areas appear depressed or sunken with bark eventually splitting and falling away. In older, thick-barked trees, galleries are large round chambers just under the bark surface. The same wound may be enlarged by each successive generation.

The apple tree bark may become sunken or slough off in areas of flat-headed borer damage and you will see borer holes beneath this sloughed off apple tree bark. Tunnels which can be up to 3/8 inch in diameter may pepper the exposed tree trunk and heads of the borer larvae may be visible. Pathogens that utilize borer injuries for entrance to the tree may be more of a health risk than the borer damage.

Insecticidal Control - Three different methods may be used to apply insecticides for control of the flat-headed borers. Foliar insecticidal sprays may be applied when adults first emerge, shortly after bloom, with a repeat application 3 weeks later, using insecticides such as phosmet or carbaryl.

Alternatively, three insecticide applications of phosmet or carbaryl to the trunk and larger scaffold branches, timed at three-week intervals, beginning when the first adults are noticed on the trunks, should help to reduce borer infestations. Insecticides may be used to kill the beetles before they lay their eggs but they will not control the larvae once they are in the tree. A third method would be to apply a root drench of an imidacloprid-containing, systemic insecticide to help control the larvae.
A method for protecting wounds or cuts is to cover the exposed surfaces with one of the contact insecticides and then with a tree wound dressing. This practice is especially important when protecting the cut made on the rootstock to force bud growth on a newly grafted tree. The young apple tree is most susceptible to attack by apple tree borer adults when the wound made when cutting the top from the rootstock is fresh; it is essential to protect this wound.

Non-Insecticidal Control - The best defense against flatheaded appletree borers is to keep the apple trees healthy and vigorous to begin with, being careful not to inflict mechanical wounds and treating other infections and infestations which may occur as soon as they start. Prevention of wounds, sun scald, or drought stress can greatly reduce borer attacks. Trees should be mulched and fertilized in order to keep them growing vigorously. Vegetation should be removed from around the base of trees and accompanied with clean cultivation. Mechanical damage from cultivation equipment such as plows or hoes should be avoided. Any wound made by a hoe or plow is an open door to apple tree borers. It is best not to hoe close to the base of the tree since wounds will inevitably occur. The use of organic mulches or herbicides is recommended to control weeds close to apple trees.

Cultural Practices - New trees should not be planted in areas in close proximity to the borers' wild host plants such as flowering crab apples, hawthorns or serviceberries. Removal of all such plants within 300 yards of an orchard is recommended. Keeping the bases of trees free of vegetation makes it easier to detect and remove borers, but also exposes them to natural enemies such as ants and other predators, parasitic wasps and flies and the golden and downy woodpeckers. When given the opportunity, there are several natural parasites and predators that keep this insect under control.
Trees that are heavily infested and beyond recovery should be removed and burned before the spring to prevent developing borers inside from completing their life cycle. Sanitation of fallen and standing deadwood, and removal of any pruned materials, is critical.

Shading the trunk of newly transplanted trees with a board driven into the ground on the southwest side of the tree, close to the trunk, will reduce egg laying. The board should be about six inches wide and slightly longer than the trunk.

Oviposition (Egg-laying) Barriers - In March, protective coverings of various materials can be wrapped around the trunks, up to the level of the branches, to exclude flatheaded appletree borer females from their preferred sites for egg-laying. Mosquito netting, fine mesh hardware cloth, tree wrap, tar paper, cotton batting, or even layers of newspaper should be wrapped loosely around the trunk, tied at the top with twine and covered at the bottom with soil. Barriers should be removed in September, after all egg-laying activity is finished.

Tree wrapping that covers all cut surfaces will prevent appletree borers from entering the tree; however, numerous sprouts will usually start to grow under the paper that gives the tree a "warty" or knotty look. Most growers have found tree wrapping to be unfavorable due to the many sprouts that occur.

An alternative method is to paint the lower surface of the trunk using white latex paint; this approach tends to work better on the smooth trunk surfaces of younger trees, and should be repeated each year, as the paint layer tends to crack with normal tree growth.

Surface Deterrents - Additional protection from egg-laying females may be obtained by applying a deterrent wash on uninfested trunk surfaces using a paintbrush. An alkaline mixture of insecticidal soap plus caustic potash (lye) mixed to the consistency of thick paint is recommended. This should be applied every 2 - 4 weeks, depending on rainfall, from April through August to deter egg-laying. Caution: Proper protective clothing and eye wear should be worn when working with lye.

Trap Logs - Large numbers of flatheaded appletree borer adults can be trapped out by placing newly cut posts or felled logs of almost any kind in the orchard, either driven in upright or simply lying on the ground and exposed to the sun. These can be coated with tanglefoot to catch the beetles, or else left uncoated until the egg-laying period is past, and then removed to another site and burned.

Worming - Destruction of larvae by hand is labor-intensive, but this can be effective, depending on the magnitude of the infestation. During bloom and again in August, inspect the bark surface for small pinholes with sawdust exuding from them, checking the lower 24" of the trunk to just below the soil surface. Use a sturdy knife to cut through the bark at any such points until the burrow is reached; use caution not to further damage the tree. Insert a thin, stiff wire that is slightly hooked at the end, to reach and impale the borer if possible.

The information given herein is for educational purposes only. Reference to trade names is made with the under-standing that no discrimination is intended and no endorsement by the Texas Agricultural Extension Service is implied.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Edward G. Smith, Director, Texas Agricultural Extension Service, The Texas A&M University System.

Chrysoborthris femorata. Flatheaded applietree borer, adult.
Fig. 1
Chrysobothris femorata, adult.
Click on image to enlarge.

Chrysobothris femorata.  Flatheaded appletree borer. Larva.
Fig. 2
Flatheaded appletree borer, larva.
Click on image to enlarge.