Bronze Birch Borer 

S.A. Katovich1, A.S. Munson2, J. Ball3 and D. McCullough4 

The bronze birch borer (figure 1), Agrilus anxius Gory (Coleoptera: Buprestidae), is 

native to North America. Records from the 

late 1800’s and early 1900’s describe 

widespread damage to ornamental birches, 

especially in the Northeastern United States 

and Canada. In the 1920’s and 1930’s, the 

bronze birch borer was widely reported to be 

associated with dying birch trees in forests and 

woodlands. Today, this borer often 

contributes to mortality of woodland birch 

during severe drought or other stress. It is also 

an important pest of landscape birches (figure 

2). 

The bronze birch borer occurs throughout the 

range of birches in Canada from 

Newfoundland to British Columbia but is most 

common in the southern portion of the 

provinces. In the United States, it is 

distributed from Maine, across the Great Lakes 

region to the Cascade Mountains of Oregon 

and Washington, and from Maryland to 

Kansas. 

Hosts 

Bronze birch borers are known to attack all 

native and introduced birch species, although 

birch susceptibility varies. Many varieties of 

birch species as well as numerous crosses 

between species are currently planted as 

ornamentals in North America. Although 

some varieties are more resistant tha n others, 

Forest Insect & 

Disease Leaflet 

111 

Figure 1. Bronze birch borer adult and 

two D-shaped exit holes. 

none are immune to birch borer attack. 

Generally, the white-barked 

birches are more susceptible than those 

without white bark such as river birch, sweet 

birch and yellow birch. Within the whitebarked birches -- paper birch and gray 

birch -- show more resistance than do many 

of the introduced species. 

1Entomologist, U.S. Department of Agriculture, Forest 

Service, St. Paul, MN. 

2Entomologist, U.S. Department of Agriculture, Forest 

Service, Ogden, UT. 

3Associate Professor, Department of Horticulture, Forestry, 

Landscape & Parks, South Dakota State University, 

Brookings, SD. 

4Associate Professor, Departments of Entomology and 

Forestry, Michigan State University, East Lansing, MI. 
Native birch species and a few of the more 

common non-native species can be grouped as 

follows: 

Severely attacked 

European white birch, Betula pendula 

Whitebarked Himalayan birch, Betula 

jacquemontii 

Commonly attacked 

Paper birch, Betula papyrifera 

Gray birch, Betula populifolia 

Sweet birch, Betula lenta 

Yellow birch, Betula alleghaniensis 

Rarely attacked 

River birch, Betula nigra 

Damage Caused 

Larvae damage trees by feeding on the phloem 

(the inner bark) and the cambium (the growth 

layer producing both phloem and xylem). 

Repeated attacks and the construction of 

numerous galleries by larvae eventually disrupts 

nutrient transport, killing roots. The damaged 

root system of an infested tree can’t supply 

leaves with adequate water and branches die, 

further reducing the ability of the tree to produce 

food and transport nutrients. Most often, the end 

result is a dead tree. 

Tree Condition 

The condition of host trees is the principal factor 

in the birch tree and borer relationship. Adult 

bronze birch borers primarily attack birches that 

are weakened or stressed by drought, old age, 

insect defoliation, soil compaction, or a stem or 

root injury. Birch trees prefer cool, moist 

growing locations and have a shallow root 

system that is easily injured by disturbance or 

dry, hot conditions. Many landscape birch trees 

are planted in unsuitable habitats and are 

stressed on a regular basis. Birch trees growing 

within a forest are stressed by old age, weather 

events such as drought, or by repeated 

defoliation. Widespread birch mortality often 

occurs after these events. 

Figure 2. A landscape birch tree infested 

with bronze birch borers. Branches in the 

upper canopy are usually affected first. 

Vigorous trees can be attacked, but larvae 

rarely succeed in completing development. A 

healthy tree will produce callus tissue around 

the feeding gallery of a larval birch borer. If 

the callus can be produced quickly, the larva 

will be overtaken and die. If the tree is 

stressed, however, callus production will be too 

slow to prevent the larva from expanding its 

gallery and completing its development. 

Callus tissue often appears as swollen bumps 

or ridges under the bark on the branches or 

main stem (fig ure 3). Repeated attacks, even if 

unsuccessful, result in localized areas of 

damage that can weaken even vigorous trees 

over time. Many fast-growing landscape trees 

eventually succumb to damage from numerous 

unsuccessful attacks. 

Birch trees that have died, broken in wind 

storms, or been cut or mechanically girdled 

may be unsuitable for larval development once 

the cambial tissues turn brown. Many 

developing larvae die in trees when the inner 

bark tissue dies. 

 Page 2 
Figure 3. Swellings or bumps formed where a 

tree has healed over a gallery of the bronze 

birch borer. Photo by D.G. Nielsen. 

Evidence of Infestation 

Typically, borers infest trees that have already 

begun to decline slightly. The first symptom is 

often yellow, sparse, stunted foliage in the 

upper crown which may progress to twig 

dieback and then to branch dieback (figure 4). 

Trees may decline for several years before 

dying, although they may die during a single 

year if conditions are hot and dry. The decline 

of a birch tree may be reversed in the early 

stages, but trees are unlikely to recover once 50 

percent or more of the crown is damaged. 

The bronze birch borer leaves a permanent 

visible record of its visit. Adult borers chew 

distinctive D-shape d exit holes 1/5 inch (5 mm) 

wide in the bark (see figure 1). These D-shaped 

holes may be stained with rust colored sap. Exit 

holes are not found on Class 1 or 2 trees and are 

rarely found on Class 3 and 4 trees. Most exit 

holes will be found on Class 5 trees. 

Larvae feed under the bark in meandering 

galleries (figure 5) or in a series of zigzag 

patterns that grow increasingly larger as the 

larvae develop. Galleries are packed tightly 

with frass (excrement mixed with boring dust). 

Callus tissue that grows over the larval feeding 

galleries results in ridge-like swellings or bumps 

on the bark of previously infested trees 

(see figure 3). 

Figure 4. Decline caused by the bronze birch borer. 

Class 1 trees are healthy. Class 2 trees are in the 

early stages of birch borer infestation. Class 3 and 

Class 4 trees have more advanced stages of borer 

infestation. Trees in Class 5 are very close to death. 

Trees in Classes 2 and 3 may be treated for birch 

borer with some success. Trees in Classes 4 and 5 

are generally beyond recovery. 

 Page 3 
Figure 5. Meandering or zigzag galleries 

formed under the bark by tunneling larvae. 

Photo by G. Heaton. 

Identification of Life Stages 

Adult bronze birch borers are slender, olivebronze beetles, with a coppery reflection (see 

cover photo). Males are about 1/4 to 3/8 inch (6­

9 mm) long, while females are generally larger, 

1/3 to 1/2 inch (8-12 mm) in length. On males, 

the head is greenish, while females have a 

copper-bronze colored head. 

The egg is oval in outline and flattened, about 

1/20 inch (1.5 mm) long by 1/25 inch (1 mm) 

wide. Eggs are creamy white when first laid, but 

they yellow with age. 

The larva is somewhat flattened, slender, and 

pearly white (figure 6). The head is light brown, 

and at the tip of the abdomen are two short, 

brown, spine-like structures. Larvae range from 

1/16 inch (2 mm) to 1-1/2 inches (38 mm) at 

maturity. 

The pupa (figure 7) resembles the adult; it is 

initially creamy white and then darkens as it 

assumes adult pigmentation. 

Life History and Developmental 

Stages 

The life cycle of the bronze birch borer may be 

either 1 or 2 years long depending on host 

condition, geographic location, temperature, and 

time of emergence. In the northern portion of 

the borer’s range, a 2-year cycle is common; in 

warmer regions, a 1-year cycle occurs. 

Development generally occurs at a faster rate in 

stressed or dying trees. In vigorous hosts the 

life cycle generally requires 2 years, or more 

commonly, the borer dies before completing its 

life cycle. 

Adults emerge from previously infested trees 

between early May and early June in the 

southern part of the borer’s range and from late 

June to mid-July further north. Adults in any 

one location may emerge over a 5- to 6-week 

period. Adult beetles live about 3 weeks. 

Therefore, adults may be found on trees during 

most of the summer. 

Figure 6. Larva of the bronze birch borer. 

Photo by G. Heaton. 

Figure 7. Pupa of the bronze birch borer. 

Photo by G. Heaton.

 Page 4 
Egg laying occurs in bark crevices and cracks. 

A common oviposition site is the roughened, 

dark triangular patch at branch origins. 

Females lay eggs singly or in clusters. The 

average is about 6-7 eggs in a cluster, 

although as many as 14-16 eggs have been 

recorded. Each female can lay as many as 75 

eggs during her lifetime. Females begin 

laying eggs about 1 week after they emerge. 

Larvae begin to hatch in about 2 weeks and 

immediately begin to mine into the phloem 

and cambium. Larvae make meandering or 

zigzagging tunnels or galleries (see figure 5). 

They pass through 4 instars. 

When the weather cools, the larvae stop 

feeding and overwinter in the galleries. 

Because of prolonged adult emergence and the 

long egg-laying period, all larval sta ges can be 

present during winter, but most will be fullgrown 4th instars. In the spring, small larvae 

resume feeding. Full-gr own larvae construct 

shallow cells in the xylem and pupate (figure 

7). Most pupae are found in May and June. 

Management Techniques 

In both landscapes and forests, control 

strategies should revolve around practices that 

optimize tree health. Prevention should be the 

cornerstone of any management program. 

Landscapes 

In landscapes, strategies that improve tree 

health will reduce susceptibility to the borer. 

These strategies start with selecting suitable 

trees and sites for planting. Proper cult ural 

practices include watering during dry, hot 

periods; mulching; avoiding wounds or injury 

to the tree; and promptly removing dead and 

dying trees to reduce local borer populations. 

Species selection: 

Several species of birch as well as numerous 

horticultural crosses and selections are available 

for planting. Many of these vary in their 

relative susceptibility to bronze birch borer. In 

general, the white-barked birches are more 

susceptible to attacks than those without white 

bark (river birch, sweet birch, and yellow birch). 

Of the white-barked birches, native paper birch 

and gray birch are more resistant than many of 

the exotic species such as European white birch. 

Soil temperature: 

Birch trees thrive in cool, moist soils. Plant 

birch in locations where the soil will be cool and 

moist such as on the north or east side of 

buildings. However, birch trees do require some 

full to partial sunshine on their leaves to grow 

well so do not plant them under dense shade. 

Mulching and regular watering as described 

below can help maintain cool soil temperatures. 

Mulch: 

Mulching landscape birch trees will moderate 

soil temperature and conserve water in the soil. 

If the soil tends to dry out quickly or if there is 

an extended period of hot, dry weather, regular 

watering may be required. Most of the roots are 

in the top 18 inches (45 cm) of soil; a thorough 

watering will allow water to soak to a depth of 

at least 2-3 feet (1/2 to 1 m). 

Sanitation: 

To reduce the number of adult borers in a 

localized area, cut and destroy recently killed 

and heavily infested trees (Classes 4 and 5) 

before adults emerge in the spring. In Class 3 

and 4 trees, most adults that complete 

development will emerge from the base of dead 

and dying branches. Therefore, prune dead and 

dying branches slightly below the point of dead 

wood. Pruning out these branches will not 

significantly reduce the number of larvae 

already in a tree because many of the larvae are 

located in the main stem.

 Page 5 
Insecticides: 

Insecticides can be applied to the surface of the 

tree bark to kill larvae emerging from eggs. 

Thoroughly drench the larger branches and 

trunk. This treatment will not kill larvae already 

under the bark, but it will help prevent new 

attacks. Depending upon the insecticide, as 

many as three applications are generally 

required during the spring and summer (midMay, mid-June, and mid-July). Contact local 

county extension agents for site-specific 

treatment dates and a list of registered 

insecticides. 

Landscape birch management is discussed in 

detail in a Forest Service brochure titled, “How 

to grow and maintain a healthy birch 

tree” (Katovich et al. 1997). 

Forests 

Silvicultural practices that increase stand health 

and vigor should reduce bronze birch borer 

attacks. However, management practices that 

cause significant stand disturbance can initiate 

periods of birch decline and lead to increased 

problems with bronze birch borers. Thinning 

should be done with care in birch stands. Birch 

root systems are easily damaged and opening 

the stand can increase the sunlight and heat 

intensity on the forest floor. Further, birch 

should not be considered an inherently longlived tree species, although yellow birch can be 

moderately long-lived. Older stands are more 

likely to undergo periods of decline than 

younger stands. Silvicultural prescriptions 

designed to enhance age class diversity will 

reduce susceptibility to birch borer over large 

areas. 

The practice of keeping birch trees as “leave 

trees” during harvest activities should be 

discouraged. Because birch root systems are 

easily damaged during harvesting and opening 

of the stand, isolated birch trees or small groups 

of trees generally die quickly after harvest cuts. 

Further, dead birch quickly decay and their use 

as snag trees is questionable 

Natural enemies: 

Natural enemies are not thought to play a vital 

role in bronze birch borer dynamics, although 

they may be important in some situations. The 

most important predators are apparently 

woodpeckers. In some places they have 

removed more than 90 percent of the borer 

larvae in a single tree. Woodpeckers feed 

mainly on larvae during the winter and spring, 

foraging primarily on open-grown trees or 

where undergrowth is either short or sparse. 

When the effects of insect parasitoids on bronze 

birch borers were studied in New Brunswick and 

in Pennsylvania, larval parasitism in both 

locations was found to be approximately 18 

percent. At least 13 parasitic species reportedly 

attack birch borer larvae. Egg parasitoids 

killed 50 percent of borer eggs in several New 

Brunswick sites, but only 7 percent of eggs in 

Pennsylvania sites. 

Cultural control: 

Bronze birch borer larvae are sensitive to rapid 

drying of the host tissue and rarely complete 

development once the cambial tissues turn 

brown. Felled birch seldom produce adult 

beetles unless the trees are infested with late 

instar larvae or pupae. As a general rule, borers 

are likely to survive and emerge in trees cut after 

November 1. Rapid drying of cut material will 

further reduce adult emergence. Exposing slash 

and cut debris to full sunlight will enhance 

drying. Bark removal will kill most immature 

life stages but is generally not required. 

 Page 6 
Implications for Ecosystem 

Management 

The bronze birch borer is a native insect and 

should be viewed as a normal component of any 

birch-dominated ecosystem. Since the borer is a 

“secondary” species (one that requires weakened 

host trees), high populations may indicate that 

the local or regional birch resource has some 

underlying stress. Most birch stands in the Great 

Lakes region originated in the 1920’s and 1930’s 

after extensive logging and fires. In the late 

1980’s, when these birch stands were more than 

60 years old, a severe drought weakened trees 

throughout the region. 

Bronze birch borers infested and ultimately 

killed many of these trees (figure 8). A massive dieoff of birch was observed throughout 

the Great Lakes region in the early 1990’s indicating that many of the existing birchdominated ecosystems throughout the region 

were aging and more likely to be stressed by 

weather events. Developing a mosaic of age 

classes should reduce bronze birch borer impact 

over a large area and contribute to the stability 

of the birch resource within that region. 

Figure 8. Paper birch killed after a severe regional drought in northern Minnesota. 

 Page 7 
References 

Anderson, R.F. 1944. The relation between host 

condition and attacks by the bronze birch borer. 

Journal of Economic Entomology. 37:588-596. 

Ball, J.; Simmons, G. 1980. The relationship between bronze birch borer and birch dieback. 

Journal of Arboriculture. 6:309-314. 

Barter, G.W. 1957. Studies of the bronze birch 

borer, Agrilus anxius Gory, in New Brunswick. 

Canadian Entomologist. 89:12-36. 

Katovich, S.; Wawrzynski, R.; Haugen, D.; 

Spears, B. 1997. How to grow and maintain a 

healthy birch tree. NA-FR-02-97. USDA Forest 

Serv., Northeastern Area State and Private Forestry, Newtown Square, PA. 21 p. 

Loerch, C.R.; Cameron, E.A. 1984. Within-tree 

distributions and seasonality of immature stages 

of the bronze birch borer, Agrilus anxius 

(Coleoptera: Buprestidae). Canadian Entomologist. 116:147-152. 

Solomon, J.D. 1995. Guide to insect borers in 

North American broadleaf trees and shrubs. Agric. Handb. 706. USDA Forest Serv., Washington, DC. 735 p. 

Revised — 2000 

Pesticides used improperly can be injurious to humans, animals, and plants. Follow directions and 

read all precautions on labe ls. Consult your local county agricultural agent or State extension 

agent about restrictions and registered uses of particular pesticides. 

The U.S. Department of Agriculture (USDA) prohibits discrimination 

in all its programs and activities on the basis of race, color,

national origin, gender, religion, age, disability, political beliefs, sexual orientation, and 

marital or family status. (Not all prohibited bases apply to all programs.) 

Persons with disabilities who require alternative means for 

communication of program information 

(Braille, large print, audiotape, etc.) should contact 

USDA's TARGET Center at 

202-720-2600 (voice and TDD). 

To file a complaint of discrimination, write USDA, Director, 

Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, 

Washington, DC 20250-9410 or call (202) 720-5964 (voice or TDD). 

USDA is an equal opportunity provider and employer.