Birch dieback is usually caused by a lack of water. In their native habitat, birch grow in moist locations, usually near lakes and streams – conditions not found in most yards. The first sign of damage usually starts in midsummer with the leaves turning yellow and dropping off. Dieback generally begins towards the top of the tree and at the ends of the branches. The following spring, some branches may be dead and the tree will have sparse, unhealthy looking foliage. Under severe drought conditions, the tree may die or become infested with insects.
Trees under moisture stress are also more susceptible to winter injury. It is important to water the tree separately from the lawn to make sure it receives enough water. If there is no rain, the tree should be watered every two weeks during the summer. The soil should be soaked to at least 20 cm (8 inches), but 45-60 cm (18-24 inches) is preferable. The area to be watered is halfway from the trunk to the drip line and the same distance beyond the drip line. The drip line is an imaginary line on the ground directly beneath the outermost tips of the tree’s branches. It is more important to water this area rather than next to the trunk. It is advisable to reduce the amount of water in late August and September to allow the tree to harden off.
The birch leafminer is a small sawfly that is active between May and September. In most of Canada, it has two or three generations per year. The first generation of birch leafminer adults emerge in the spring, at the same time as new leaves are being formed on birch trees. The female will deposit her eggs on these new leaves as they are expanding. The eggs hatch in 4 to 14 days. The newly hatched larvae feed within the leaf for 8 to 12 days before emerging from the leaf as mature larvae. These larvae drop to the ground and create a cocoon in the soil, emerging 1 to 2 weeks later as a mature adult.
A complete generation can take 5 to 6 weeks, or slightly longer at higher latitudes. The final generation overwinters in soil cocoons as mature larvae. Damage appears as a small brown or reddish-brown, irregular-shaped patch on the upper surface of the leaf. The leafminer attacks before the leaf has fully expanded, which interferes with normal development and results in a deformed leaf. Leafminer damage can be easy to find on immature trees or trees with many new shoots. On older or more mature trees, damage may be less apparent or restricted to leaves on the outside of the crown. Mined leaves will stay on the tree for some time, often not dropping until early fall. The birch leafminer is an aesthetic pest. Small populations can be tolerated and often go unnoticed. The damage does not kill the tree because it occurs later on in the season. However, if the same tree suffers heavy defoliation for a number of years it may be weakened and more susceptible to attack by other insects.