Several native birches grace Wisconsin forests and waterways. Homeowners often plant these trees because of their attractive bark, dappled shade or soft spring catkins. Unfortunately, birches are afflicted by a number of pests. These pests may not pose a problem in forests but where birches are used as cultivated specimens, they can cause costly damage.
The White Birch
The European white birch is one of the most beautiful birches. Unfortunately, it is also most susceptible to the birch leaf miner, a pest that eats leaves from the inside out, producing a network of dead brown tissue in its delicate leaves, opening the way for the more dangerous bronze birch borer. Although considerable effort has been expended in developing more resistant cultivars of the white birch, Wisconsin's river birch and yellow birch are both more resistant to the borer than white birches. The paper birch, native from the middle of the state to the north, is available in bronze borer-resistant varieties. The first step in pest management is to plant one of these native varieties.
Birch leaf miners lay their eggs under the surface of the soil where larvae develop and emerge as black sawflies to lay eggs on leaves in May. The young larvae eat between the surfaces of the leaves for 10 to 15 days before falling to the ground to pupate. Leaf miners can produce as many as four generations in a summer. The miner attacks only new foliage so the spring attack is the most serious. Miners must be managed with chemical sprays or granular systemic applications in the ground around the tree.
Bronze birch borers frequently follow a leaf miner attack and capitalize on weakened immunity in birch trees. Beetles emerge from the ground in late May to early June, feed on leaves and lay eggs in the soft phloem tissue underlying the bark from May to July; young and freshly pruned branches are particularly at risk. Larvae then eat away at the phloem, permanently damaging the tree. Good cultural practice in watering, planting in full sun and keeping tree roots cool divert borers. Chemical insecticide for borers must be applied to leaves and bark in combination with cultural controls every two weeks to kill eggs laid in the bark.
Several other pests stalk Wisconsin birches. Aphids, birch catkin bug and fall webworm are all nuisance pests. Aphids and birch catkin bugs will not do any harm to trees that are watered regularly. Fall webworms build filmy tents that can be cut out of trees and destroyed; as webworm populations grow, so do those of predators so the problem will resolve itself in a few years. Birch is a favorite food of the caterpillar phase of life for the gypsy moth. Contact a local University of Wisconsin extension office to report this pest; the extension system, Department of Natural Resources and Wisconsin Department of Agriculture, Trade and Consumer Protection operate a gypsy moth suppression program.