River Birch & Leaf Insects

Insect Resistance

• River birches resist most damaging leaf insects. Those that do invade this tree species rarely cause permanent or serious damage, especially with mature, healthy individuals. Among the most damaging insects the river birch resists are the bronze birch borer and the birch leaf miner, which cause considerable damage to other birch species. Aphids and sawflies are the primary insect invaders affecting river birch leaves, but the trees recover quickly and the insects do not spread to neighboring plants.

Aphids

• The spiny witch-hazel gall aphid feeds exclusively on river birches and in the spring and witch hazel in the summer. Aphids, small, soft-bodied insects about the size of a grain of rice, reproduce quickly and feed on leaves by inserting a needlelike structure into leaves that sucks out sap. This distorts the plant's leaves and stems. Aphids also secrete a sticky, sweet substance called honeydew that attracts ants and wasps. Honeydew also provides an excellent environment for the harmless sooty mold fungus.

Aphid Signs and Treatment

• Spiny witch-hazel gall aphids cause crinkled or discolored leaves with pink, corrugated ridges on the river birch. The tree often drop infested leaves but quickly grows replacements. Look at the undersides of the leaves. Aphids tuck into the ridges. Ant and wasp infestations, as well as sooty black mold, which covers leaves in an unattractive, fuzzy black substance, are common and obvious secondary signs of an aphid infestation. Treatment is unnecessary unless the tree is already weakened by disease or is very young, as the insects rarely cause permanent damage. Ladybugs are naturally effective aphid predators. Purchase a supply of ladybugs from a gardening center and release them into the tree.

Sawflies and Treatment

• Sawflies are related to bees and wasps and look much like them in their adult stage but without the stinger. Their larvae resemble caterpillars with green bodies, black dots and black heads. The dusky birch sawfly invades the river birch. The larvae feed heavily on the leaves for four to five weeks in the spring, and then spin cocoons where they spend the rest of the year. Adults emerge the following spring and lay eggs on the tree. While they can eat many leaves in their feeding period, river birches quickly recover, and treatment is unnecessary unless the tree is unhealthy or young and suffers complete defoliation. In this case, pick the larvae off and drop them in soapy water or apply an insecticidal soap intended for leaf eaters.