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Saturday, June 22, 2002

Q. We have a 'Heritage' river birch (Betula nigra 'Heritage') that stands about 15 feet tall. I noticed that about a third of the leaves have started to shrivel. On closer inspection, there is a white, crusty powdery substance on the underside of all the shriveled leaves. Other leaves look normal, but I am concerned this problem will spread. Do you know what the problem could be and what steps I can take to correct it?

	Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

A. There are a number of species of aphids that feed on most species of birch trees, including river birch. Your description of damage is typical of spiny witch-hazel gall aphid (Hamamelistes spinosus). Infested leaves crinkle up and take on a corrugated appearance in response to the aphids' feeding. Secondary or lateral veins in the leaf often take on a yellow (chlorotic) appearance. Aphid feeding on river birches causes the leaves to take on a reddish coloration. The aphids are found on the undersides of the leaves, caked into the corrugations with a white granular material.

Spiny witch-hazel gall aphids have an interesting life cycle. They alternate between witch-hazel (Hamamelis spp.) and birch (Betula spp.) trees. They survive the winter in one of two ways: as eggs laid on witch-hazel twigs or as hibernating females on the bark of birch trees.

Eggs laid on witch-hazels hatch in spring. Newly hatched aphids are called stem mothers. Their feeding results in the development of bud galls on witch-hazel twigs. The bud gall is hollow and contains many reddish, immature aphids. The aphids exit the gall and move to their secondary host, birch trees.

Hibernating females on birch trees become active in spring at about the same time birch tree leaves begin to expand. They move from the bark to the newly expanding leaves and begin to feed. The aphids reproduce rapidly, and soon leaves begin to show the characteristic corrugations. Winged aphids develop and fly in search of witch-hazel trees to lay their eggs and complete their life cycle.

Spiny witch-hazel gall aphids feed with piercing-sucking mouth parts. They insert their mouth parts into the leaf veins to feed on the sugary, nutrient rich sap. This deprives the leaf of the nutrients it needs to remain healthy and productive. Severe infestations can cause premature leaf drop, twig and even branch death. Before spraying, scout for lady beetles, lacewings and other predators that are feeding on the aphids. Predators often get minor infestations under control without any assistance from us.

More significant infestations do require control. Dormant applications of horticultural spray oil suffocate overwintering females on the bark. This helps reduce the population of aphids, but will not eliminate all of them. Be sure to look for horticultural spray oil rather than dormant oil. Horticultural spray oil is more highly refined and less damaging to sensitive species than dormant oil.

Labeled insecticides include Orthene (acephate), BioNeem (azadirachtin), Cygon (dimeth-oate), malathion, and products that combine pyrethrins and rotenone. Repeat applications according to label directions are necessary to control this pest.

An alternative to spraying is to hire a certified arborist to inject Merit (imidacloprid) into the soil around the base of the tree. Merit will provide season-long control with a single application. Merit moves slowly through the tree and would take approximately 30 days for uptake, based on the size of your tree. The treatment should be done early enough in spring that it is in place before the leaves begin to expand.