Birch trees, found throughout Canada and temperate regions of the United States, are graceful trees known for their beautiful white bark. They are also susceptible to a variety of insects, some of which can kill a tree. Some insecticides will combat the insects most notorious for attacking birch trees.
Bronze Birch Borer
The bronze birch borer, Agrilus anxius, is the most lethal insect to attack birch trees, and it is the most difficult to control. The adult borer is a small, bronze-colored beetle up to 2 inches long. Its eggs, laid on the bark, turn into larvae that bore into the vascular tissue in the bark and the tissue between the bark and the tree. This tunneling by larvae interrupts the flow of the sap, weakening and sometimes killing the tree. From egg to beetle the live cycle of the bronze birch borer takes one year. The crown will begin to thin beginning at the top. A birch will die after two or three years of infestation.
The gray birch and the European white birch are most vulnerable to the bronze birch borer; the canoe and river birches seem better able to resist.
Researchers at the University of New Mexico recommend insecticides containing bendiocarb, chlorpyrifos, and dicrotophos permethrin, used according to manufacturer’s directions.
The birch leafminer (Fenusa pusilla) lays eggs in slits on the top of birch leaves in April to mid-May. The larvae, small whitish grubs begin feeding in May or June causing black splotches on the leaves. The leaves will ultimately turn brown and fall off. In June or July the larvae drop to the ground where they morph into sawflies, small, black insects that look like flies. The adults lay more eggs in a cycle that can repeat four times in a growing season.
Horticulturalists at the University of Minnesota recommend spraying insecticides containing acephate or dimethoate on the leaves or applying undiluted dimethoate to the bark when the buds are breaking in the spring.
Oyster scale have no eyes or legs. The insect moves only once during its life cycle; that it is in the spring when the nearly invisible nymphs emerge from the female shell to infect new areas of the tree. They cause small brown bumps that look like oyster shells. They suck sap and nutrients from a tree and over time can cause dieback in a birch tree.
Spray horticultural oil on the tree in late winter before the buds begin to break in the spring. The horticultural oil suffocates the scale. If you see crawling nymphs, Birch-tree.com recommends spraying the trees with a spray containing pyrethrin.
A birch tree can withstand small numbers of aphids--small, pear-shaped insects also called plant lice. Large populations of aphids can suck the sap from birch leaves, causing puckered marks on the leaves, turning them yellow and making them twist. This can eventually result in falling leaves and dieback of twigs and branches. As they eat, aphids excrete a substance called honeydew that attracts ants.
The Insect Diagnostic Laboratory at Cornell University recommends insectical soap, horticultural oil or insecticides containing cyluthrin or permethrin.