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Woody Ornamentals - Flatheaded Borers

Flatheaded Borers

Bronze Birch Borer
Bronze Birch Borer Adult
Rose Cane Girdler Larva
Flatheaded Borer Larva

David Keith, Extension Entomology Specialist

Fred Baxendale, Extension Entomology Specialist

Jeff Carstens, Graduate Student


Pest Identification & Damage

Common Name(s):
Flatheaded Borers, Metallic Wood-Boring Beetles, Buprestids

Scientific Name:
Family Buprestidae.

Identifying Characteristics for Damaging Stage(s):

Flatheaded borer beetles are often brightly colored, shiny metallic or bronzed, particularly on the belly (ventral) side, bullet-shaped, with tapered or pointed elytra (forewings), and short saw-like antennae. The body is somewhat flattened and may be uniformly colored or bear distinct, bright patterns. 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 over an inch in length. Flatheaded borers are yellowish to white, club-shaped "grubs" with brown heads and heavy-duty chewing mouthparts. The body is often long, flattened and worm-like, except for the strongly swollen area immediately behind the head. Galleries produced by these larvae are typically flat in cross-section and tightly packed with fine "frass" (sawdust and feces).

Damage/Nature and Symptoms:

Bronze Birch Borer Larva     
Bronze Birch Borer and Tunnels      
Larvae create tunnels and galleries that are flattened, sinuous and overlapping. They usually lie just beneath the bark, and infested areas may become bumpy or swollen over time, due to injury to the plant growth layer (cambium). Branches, twigs, canes or roots often become girdled and may die as flatheaded borers chew tunnels in spiral fashion around their perimeters. Adult emergence holes are somewhat "D-shaped" rather than oval, as in roundheaded borers.

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 of insect 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:

Flatheaded borers have varying ranges across the United State, depending upon the species. Many are host-specific and occur only where their host plants are distributed.

Summary of Life Cycle:

Representative Species:

Bronze Birch Borer:

The bronze birch borer, Agrilus anxius Gory, occurs in most species of birch grown in Nebraska, but is particularly destructive to paper birch. Adult beetles are about 3/8 inch long, slender, and metallic bronze. Beetles emerge from infested trees in late May and are present until July, feeding on birch leaves for several days after mating and before laying eggs. Eggs are deposited under bark and in cracks in the bark. The larvae burrow directly through the bark into the cambium layer. Heavy infestations cause raised burrows, easily detected on the bark surface. Adults prefer weakened trees for egg laying. Healthy, vigorous trees usually are less likely to be infested. Cultural practices are very important in the prevention of bronze birch borer injury. Birches should be planted where they are shaded in the afternoon - avoid southern or western exposures. They are best suited for shaded, damp situations and should be watered regularly. Plant a ground cover over the root area to keep roots cool and moist. If ground covers are not feasible, do not mow grass over the root area during summer. Except for river birch, most birches, and particularly the paper birch are not well adapted to Nebraska and are a questionable choice for home landscapes.

Oak Twig Girdler:

Oak Twig Girdler      
"Flagging" of Red Oak Leaves      
The oak twig girdler, Agrilus angelicus, has a two-year life cycle. In our area, it prefers red oak, but also attacks live oaks and several introduced oaks. The small, slender, bronze-to-black beetles emerge from May to September and deposit eggs on twigs at the junction between the current and previous year's growth. Larvae that hatch bore into twigs, and as they grow, they mine spirally so that terminal clusters of dead leaves ("flags") appear during August and September. During the next year, larvae continue to mine deeper into twigs and complete development, pupating in the autumn. While the damage is obvious, it is rarely severe, and there is no need for control efforts.

Rose Stem Girdler:

The rose stem girdler, Agrilus aurichalceus, attacks Rugosa and other bush-type roses. There is only one generation per year. The small, dark bronze beetles emerge from infested stems in May and June. Adult females deposit eggs on woody stems of newer growth. Larvae feed on the cambium beneath, producing a swelling of the canes as the plant tries to compensate for the injury. Eventually the stem or cane is girdled and dies, resulting in "flags" of dead leaves on infested bushes, which, if the attack is severe, can be severely set back or even killed. Spraying of the canes at least twice in late May and early June is suggested to prevent infestation. Infested canes should be removed immediately, run through a chipper and composted.

Management Methods:

Inspection/Survey Methods:

It is important to regularly check young trees and shrubs that are becoming established in the landscape. Check for chewing injury by beetles on new twig growth or leaves. 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.