Agrilus anxius

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Elateriformia
Superfamily	Buprestoidea
Family	Buprestidae
Subfamily	Agrilinae
Tribe	Agrilini
Genus	Agrilus

Scientific Name

Agrilus anxius

Scientific Name Synonyms

Agrilus gravis

Agrilus torpidus

Common Name

bronze birch borer

Authors: Kamila Baranowska and Laura Timms, Faculty of Forestry, University of Toronto

Identification

Adult beetles are small with a flat head and elongated bodies. They range in colour from olive green with bronze reflections to black with bronze reflections. They are about 6.4 mm to 12.7 mm long with the females being larger than males.

The eggs are creamy white initially but turn yellow with time. They have an oval shape and are 1.5 mm long by 1 mm wide.

The larvae are white, legless, and have flattened, elongated bodies. They are 12.7 mm to 15.2 mm long. They have a small enlargement in the second thoracic segment. There are two brown forcep-like spines extending from the last segment of the body.

The pupae are initially creamy white but take on the adult colour as they develop.

For information on how to identify a bronze birch borer attack, see the damage section below.

Geographic distribution

The bronze birch borer is native to North America. It is found in the southern portions of all Canadian provinces. It’s also common in the northern United States from Maine to Idaho, Colorado, and Utah.

Hosts

The bronze birch borer attacks all species of birch. Although no birch species are completely resistant to attack, some are more susceptible than others. Specifically, white-barked birch varieties (such as European white birch and paper birch) are generally less resistant than non-white bark birches (such as river birch)

The bronze birch borer prefers to attack trees that are weakened, stressed or standing in fragmented landscapes. It is also known to attack healthy trees but in these cases, tree wound tissue can fill larval tunnels and kill the larvae.

Damage

The bronze birch borer is considered a serious pest of birch species. The adults cause minor damage by feeding on the leaves. The severe damage is caused by the tunneling larvae. The feeding galleries interrupt the flow of sap which reduces tree growth and can cause tree mortality

The attacks usually start on branches in the upper crown. Sparse and discolored foliage are the initial signs of an attack. This is followed by twig and branch death. Each year the insect moves further down the tree. The tree may recover if less than 50 % of the crown is damaged; if greater than half the crown dies off it is likely that the tree will die.

Attacks on trees are also indicated by raised swellings on the tree bark where the tree has healed over larval tunnels. In the late stages of an attack, D-shaped holes can be seen on the bark created by the adults exiting the tree.

The most obvious evidence for attack by the bronze birch borer is the presence of larval galleries which can be seen if the bark is peeled back in areas below the dead branches. The galleries are often filled with digested and packed sawdust. They usually meander in various directions.

Life Cycle

The life cycle takes one to two years to complete depending on the climate. Insects in more southern regions of the distribution complete the cycle in one year, while insects in the northern region require two.

Adults are visible from mid June to August. They live for three weeks during which they fly around feeding on foliage. After they mate, the females lay eggs in crevices that they make with their jaws or within cracks in the bark. They prefer to oviposit in unshaded areas. The eggs hatch in approximately two weeks and the larvae penetrate the bark to start creating tunnels in the wood. The larvae tunnel near the surface for most of the summer but later move deeper into the wood where they overwinter in hibernation. In the spring, mature larvae create pupal cells where they pupate. Immature larvae continue to feed near the surface of the wood and may create pupal cells near the wood surface in the fall and pupate the following spring.

Population Dynamics

The bronze birch borer does not have periodical population cycles. Outbreaks usually occur during periods of drought or other stress¹. Records of widespread damage to birches by this insect date back to the late 19th century and early 20th century. The bronze birch borer is associated with a massive birch dieback which occurred from the 1930’s to the 1950’s in northeastern United States and Canada .

More recently, a drought during the 1980’s weakened the birch trees of the Great Lakes region, many of which were over 60 years old The weakened trees were later infested and killed by bronze birch borers.

Control and Management

The best way to control the bronze birch borer is to keep birch trees healthy and vigorous by fertilizing and watering them. Stressed branches should be cut below the level of the dead wood and burned before adults emerge. Entire dead trees should also be destroyed before adults emerge. The destructive larvae are hidden under the bark and are not affected by pesticides . However, a stomach or contact pesticide sprayed in June can kill the emerged adults that are feeding and prevent females from laying eggs. The bronze birch borer also has natural enemies such as parasitoids that attack eggs or larvae, as well as woodpeckers which eat the larvae.

Management practices that are known to prevent bronze birch borer impact include planting more resistant varieties of birch, as well as planting species better adapted to their environment. Having a good mix of old and young birch trees is also helpful to reduce bronze birch borer loss in a stand .

Bronze Poplar Borer

For many years the bronze poplar borer (Agrilus liragus) has been confused as the bronze birch borer. It was not identified as a separate species until 1949. Both the morphology and biology of the bronze poplar borer are similar to the bronze birch borer. The two species can be distinguished by slight differences in larval morphology and by certain features of the adult male genitalia.

Images

''Agrilus anxius'' larval gallery with larva inside	''Agrilus anxius'' pupal chamber with pupa inside	Tree mortality from bronze birch borer attack	Bark pulled back showing larval tunnels of ''Agrilus anxius''	Adult bronze poplar borer (''Agrilus liragus'' )
''Agrilus anxius'' adult and exit hole	Raised ridges on the bark caused by the tree healing over the larval tunnels of ''Agrilus anxius''	Dead branches as a result of ''Agrilus anxius'' attack	Larva of bronze poplar borer (''Agrilus liragus'')