GardenLine | Common Saskatchewan Pests | Bronze Birch Borer

Russell Friesen

The bronze birch borer is an insect of increasing importance. It is the most important pest of ornamental birch trees, and to a lesser extent poplar trees. The insect targets old and stressed birch trees, boring underneath the bark and feeding off the sap conducting tissue. The borers start at the top of the birch tree and eat their way down. A sure sign of a bronze birch borer infestation is a leaveless dead tree top. Unfortunately, once a tree is infested with bronze birch borer, controls are largely unsuccessful. Complete death of the tree results. Prevention of infestation by maintaining tree health is the best course.

Description

The adult borer is olive green to black with metallic coloured reflections. The beetles are about 6 to 11 mm in length and are very slender in profile. The females are quite strong flyers. The adult beetles feed on leaves of birch and poplar trees. The larvae are cream coloured and about 35 mm at maturity. The larvae have a pair of spines at the posterior end. Larval galleries are from 75 mm to 125 mm in length.

Damage

The feeding activities of the borer larvae girdle tree trunks and limbs cutting off sap flow, resulting in their death. The larvae tunnel underneath the bark causing a ridge in the bark. The ridging may be apparent visually, but is more likely to be found by feeling along, the normally smooth, bark and noting a slight rise. The bark covering the raised area can be gently sliced away, revealing a serpentine trail of darkened material. The darkened material is excrement and can be flicked out of the trail with a knife.

Emerging adults will chew a hole in the bark shaped like the letter "D". This distinctive "D" hole is about the same size as the "D"s on a typewritten document.

Life Cycle

The bronze birch borer has a two year life cycle. The adult borers emerge, and chew their way out of the tree bark in June. The adults lay eggs near the top of the birch tree. The eggs hatch and the new larvae chew their way into and underneath the bark. The larvae fed and grow working their way down the trunk. The larvae over winter and pupate in the spring. The pupa mature into adults and the cycle is repeated. As attacks are repeated the egg laying takes place on lower parts of the tree.

Control

The best control for bronze birch borer is to keep the tree healthy as the borer larvae cannot live in health trees. One of the most important factors in a birch tree's health is sufficient watering. The birch trees are native to rivers, lakes and other areas with high water availability, therefore trees are not likely to do well in areas subject to drought. In the spring a birch tree may utilize hundreds of gallons of water a day. If the water reserve is not in the soil, the tree is put under a great deal of stress. If this lack of water is a chronic condition repeated year after year, the tree is almost certain to be targeted by the bronze birch borer.

It is because of the birch tree's high water use in the spring that fall and spring pruning is not recommended for birch trees. The wounds resulting from improperly timed pruning will bleed sap. This sap bleeding can be so great that even if there is sufficient soil water, branches higher on the tree will be drought stressed. Yellow-bellied sapsucker and woodpecker injury can also induce drought conditions. The damage, these birds cause, may result in top dieback if their holes girdle the trunk.

Birch trees should be watered quite heavily in late October, so that in spring there is a reserve of soil water present as the ground begins to thaw. Watering at a lighter rate through out the summer (until mid-August) is also suggested to maintain tree health. The best way to water a tree is to place a soaker hose upside down at the tree's dripline (the outside edge of the leaf canopy). During the spring and summer, a biweekly overnight watering should suffice.

Control of the birch leaf miner and other pests will prevent additional stress to birch trees. A soil drench application of the chemical (Cygon) is recommended for the birch leaf miner. See the birch leaf miner fact sheet for further information. It is possible to prevent the adults from laying eggs by spraying the trunk of the tree with chloropyrifos (Dursban) in May. Precautions should always be taken with any agricultural chemicals. Be sure to follow the label instructions.

Once a birch tree is infested by the bronze birch borer, control is very difficult and the chances for a cure are very slim. It is possible to remove dead or infested branches and trunks. Wood should be removed well below the last sign of a borer. The signs to look for are the ridging of the bark and the adult's "D" emergence holes. Trees with a great deal of dieback from the bronze birch borer should be removed. There are no regulations regarding the disposal of wood from infested trees, but as the larvae overwinter underneath the bark, firewood can serve as a source of infestation for other birch trees. A good neighbour would either dispose of the wood, or be sure to burn the resulting firewood before the next spring.

References:

Ives W. G. H. Ives, H. R. Wong. Tree and shrub Insects of the Prairie Provinces: Information Report NOR-X-292. Edmonton: Northern Forestry Centre. 1988

Johnson, Warren T., Howard H. Lyon. Insects that Feed on Trees and Shrubs. Ithaca: Cornell University Press, 2nd Ed. 1988.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.