Roundheaded Borers

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Locust Borer Adult Roundheaded Borer with Holes and Frass

Pest Identification & Damage
Common Name(s):
Roundheaded Borers, Longhorned Beetles, Cerambycids

Scientific Name:
Family Cerambycidae.

Identifying Characteristics for Damaging Stage(s):

Roundheaded borer adults are beautiful beetles, often brightly marked, large-bodied (sometimes referred to as "broad-shouldered"), a cylindrical body shape, with very long antennae, often as long as the body or longer, and with notched eyes. These beetles are very alert and capable of swift movement.

Size of wood borer larvae depends on the species and type of insect and on stage of development, and can therefore range from tiny to almost 2 inches in length. Roundheaded borers are yellowish to white, club-shaped "grubs" with brown heads and heavy-duty chewing mouthparts. The head area is somewhat rounded and therefore is referred to as a "roundheaded" borer. Galleries produced by these larvae are typically smooth in texture, quite rounded or oval in cross-section. They are often tightly packed with rough sawdust or wood shavings.

Damage/Nature and Symptoms:

Damage in Cross-Section

Borers attack nearly all trees and shrubs to some degree, but certain species are more susceptible to infestation. Those trees which seem somewhat more prone to borer damage in Nebraska are ash, birch (paper or white), cottonwood (seedless), black locust, hickory, linden and willow. Newly set tree plantings, including fruit orchard species are highly susceptible during the first years when they are under stress. Establishment and root system development is a critical time for new trees and they must be carefully tended, well-watered, fertilized and protected from borers during this period.

To understand how boring insect damage trees and shrubs, it is essential to know the basic fundamentals of tree growth. Trees grow in height because of apical buds, or meristems at the tips of twigs, which results in extending these stem tips. Trees grow in width, or girth because of a special ring-like meristem, consisting of several tiers of cells, perhaps just 8 - 10 cells in thickness, the cambium layer, which surrounds each stem, including the main trunk itself. If these cells are killed, for example by cutting around the girth of a tree with a chainsaw, or by the feeding of insects beneath the bark we know that the tree is "girdled" and will die. These meristems are the living parts of the tree both wood and bark cells are technically dead, although they may continue to function by conducting water and nutrients. The cambium produces wood (or xylem) cells to the inside and bark (phloem) cells to the outside. The cambium grows rapidly each spring, when day length is increasing and water is more plentiful, producing large wood cells to the inside. Later in summer, when day length is beginning to shorten and moisture is in shorter supply, growth rate slows and the cells produced are smaller. As winter approaches, growth stops altogether, to be resumed in the spring. These differential growth rates in spring, summer and fall, producing differently sized cells, accounts for the annual growth rings. The outermost (most recent) growth rings constitute the sapwood, whose cells are usually higher in starch or sugar content and are therefore often more attractive to certain types ofinsect larvae. In the middle of a mature tree, the oldest growth rings appear darker and the cells may be more thickened or lignified and less attractive for insect feeding. This heartwood is sometimes chosen for use as posts and other wood that may have soil contact because it is somewhat more resistant to the feeding of certain insects.

Distribution and Life Cycle

Distribution:

Longhorned beetles have varying ranges across the United State, depending upon the species. Certain species occur only where their host plants are distributed, while others that have multiple hosts may be distributed throughout the U.S.

Summary of Life Cycle:

Roundheaded borers have a complete metamorphosis, therefore they begin life as an egg deposited on the host tree or shrub, which hatches into a larva, or grub stage that feeds for a period of time, molting its skin several times. Depending on the species, larvae may take several years to develop to maturity. Each time the skin is shed, the larva increases in size until it reaches full growth, when it stops feeding and transforms into a quiescent stage, called the pupa. During this stage, the insect rebuilds itself into the mature stage, referred to as the adult. The adult stage is winged and disperses to find a mate. Adult females then deposit eggs on the host tree for the next generation.

Representative Species:

Redheaded Ash Borer:

The redheaded ash borer, Neoclytus acuminatus (F.), attacks several species of shade trees, but causes the most serious damage to green ash. The adult is a long-horned beetle that is 1/2 to 1 inch long and reddish brown to black with transverse white or yellow stripes on the wing covers. The beetles are attracted to weakened trees where they deposit eggs in cracks in the bark. The newly hatched larvae (caterpillars) initially feed under the bark and later tunnel into the sapwood. The redheaded ash borer generally has a one year life cycle. The larvae feed during summer and pupate in the fall. Adult beetles emerge in the spring and deposit eggs from May until August. For this reason ash trees must be protected from early spring until autumn.

Cottonwood Borer:

The cottonwood borer, Plectrodera scalator (F.), infests the bases of cottonwood and willow trees. The adults of this long-horned beetle are 1 to 1-3/8 inch long and black with numerous patches and transverse white stripes. Adult beetles emerge in late spring and early summer and feed on tender new shoots of young trees. They deposit eggs in openings chewed into the bark at the bases of trees below the soil line. The larvae burrow into the bases and roots of trees, pushing out frass, a sawdust-like excrement, at the entry points. Severely infested young trees may be badly damaged. Larger trees tolerate light to moderate infestations without apparent serious effects. They have a two-year life cycle.

Poplar Borer:

The poplar borer, Saperda calcarata Say, is a long-horned beetle that attacks cottonwood, poplar, and willow trees. The adults are approximately 1 inch long and are dark grey with small orange spots on the wing covers. They emerge in summer and lay eggs in slits cut in bark, usually near the middle portion of trees. Larvae, which are white and about 1-1/4 inch long, bore into the heartwood. The larvae take three years to mature. Damage appears as swollen areas on trunks and larger branches. Holes where larval excrement is pushed out and where adults have emerged are also signs of infestation.

Asian Longhorned Beetle:

The Asian longhorned beetle, Anoplophora glabripennis, is a native of China, Korea and Japan, and is not presently found in Nebraska. However, it does constitute a serious threat to our native trees and landscape plantings. The beetle, first found in New York in 1996, was found in Chicago in 1998 and now seems well established at both locations. Given it's distribution in Asia, and proximity in Illinois, there is little reason why it would not expand its distribution and do well in the northern and central U.S. This species is a devastating pest of various tree species, particularly maples (Norway, red, sugar, silver and boxelder), horse chestnut and ash, but can also attack poplar, willow and other tree species. The adult beetle is 1 to 1 1/4 inch in length and bullet-shaped, with long antennae. The basic color of the insect is shiny black, with white spots on the wing covers. The antennae are black and circled with white bands. Female beetles chew niches, small holes or crevices into tree bark, then deposit an egg near or on the surface of the cambium, a layer of cells that constitutes the living part of the tree. Young larvae feed under the bark in the cambium layer, gradually girdling the tree before turning inward to bore in the wood and water conducting vessels. Nearly mature larvae produce large holes, pushing coarse sawdust fragments to the outside and weakening the trunk. Sawdust often accumulates at the bases of infested trees. Once mature, larvae move just below the bark surface, where they transform into adult beetles and emerge, coming out through dime-sized exit holes. Since most of the life cycle occurs in the tree, and the attack can occur almost anywhere on the tree, the timing of controls is almost impossible. Management strategies so far consist of locating infested trees and destroying them by burning or chipping them into small fragments.

Pine Sawyers:

Pine sawyers, Monochamus spp., consist of a several species of long horned beetles that are relatively common. Adults are mottled gray and brown. In our area, the most common species is M. titillator (F.), the southern pine sawyer. These insects attack felled logs or trees under stress. Sawyers get their name from the noisy, saw-like sound that feeding larvae make as they gnaw away at the wood, producing coarse fragments which they often pack into their galleries or may push outside. The life cycle is more or less continuous, with adults emerging continously and all stages of the life cycle present throughout the summer. This makes timing of control actions almost impossible. As feeding beetles chew into the bark of both healthy and weakened trees, they transmit the immature stages of pine wood nematodes, which cause the pine wilt disease which has destroyed plantings of Scotch and other pines in the Midwest and southern Plains states.

Management Methods:

Inspection/Survey Methods:

Eggs in Bark Crevice

It is important to regularly check young trees that are becoming established in the landscape. Check for chewing injury by beetles on new twig growth. Also, inspect the bases of trees for signs of oviposition (egg clusters in bark crevices or wounds), or for emergence holes from which adults recently emerged.

Non-Chemical Management Strategies:

Plant trees and varieties that are less prone to borer attack, such as oaks, lindens, crab apples, and conifers. Choose trees and shrubs that are suitable for the climatic zone in which they are planted. Those more vulnerable to the extremes of winter and summer in the Great Plains will ultimately suffer damage and be attractive to borers.

Several cultural practices can reduce borer infestations. Trees that are strong and maintained in vigorous growing condition are not as attractive to borers. Trees should be properly watered, fertilized, and protected from pests, particularly during the first two or three years of growth and during drought periods that cause extreme stress.

Trimming damaged trees and eliminating weak ones is are very important management tools. Infested limbs, branches and trunks should be trimmed in the fall, burned or chipped and composted during the winter to reduce emerging borer populations.

Borer damage must be prevented because once borers gain access to cambium, sapwood, and heartwood, little can be done to control them. A few worms may be destroyed by probing active tunnels (with emerging sawdust) with a stiff wire or by injecting some fast-penetrating petroleum material (kerosene, penetrating oil, diesel fuel) into the sawdust.

Chemical Management Strategies:

Chemical treatments are effective only if applied as residual sprays prior to egg-laying activity by beetles or moths. If residuals are in place, young larvae are killed while attempting to invade the wood. While proper timing is critical, it is extremely difficult to achieve. The difficulty is in determining which trees may be infested. Normally the infested tree population is quite low, therefore routine preventive treatments are not feasible. Recently the cancellation or banning of effective residual treatments has further made the issue of timing even more difficult, since long residual products are no longer legal to use.

Follow label instructions carefully so that good control is achieved, and that humans, animals and the environment are protected.

For the latest information on available chemical controls, consult the Pesticide Selection Guide.

References:

Baker, W. L., 1972. Eastern Forest Insects. USDA Forest Service Misc. Pub. No. 1175, US Forest Service, 642 pp.

Solomon, J. D., 1995. Guide to Insect Borers in North American Broadleaf Trees and Shrubs. USDA Forest Service Handbook #706, 735 pp.