Woolly Apple Aphids

The white fuzzy areas are actually insects - Woolly Apple Aphids.
Photo: William M. Ciesla, Forest Health Management International
through Forestry Images.

Individual woolly apple aphids produce white-bluish waxy filaments on their body.  The waxy filaments protect the aphid.  Controlling the aphid with sprayed insecticides is difficult because of the protective covering of waxy filaments.

The woolly apple aphid will generally overwinter in egg stage on elms.  The eggs hatch in the spring and the wingless aphids feed on the elm buds and leaves causing the leaves to become curled and stunted clusters at the twig ends.  The curled leaves enclose a colony of the aphids.  After two generations, a winged generation will fly to the summer hosts of apple, crabapple, or mountain-ash.  On the summer host they will feed on the roots and on trunk or limb injuries.  Most damage occurs on the roots of the summer host where galls will form, stunting root growth.  Frequently, colonies of woolly apple aphids are visible on above ground roots appearing like a mold or fungus growth.  Frequently, colonies will be found in the edge of old pruning wounds or on small branch unions.  Sucker growth at the base of the trunk or suckers growth from the roots will sometimes be covered with the woolly aphids.

Control of the woolly apple aphid is difficult.  A systemic insecticide can be injected with water into the soil.  The insecticide moves throughout the tree, killing the sap-feeding insects.  If there is a chance the fruit will be eaten, systemic insecticides should not be used.  The easiest way to get rid of above ground infestations is to use a strong stream of water to wash them off.

Links:

Colorado State University Cooperative Extension

University of Wyoming - Cooperative Extension Service, publication B-1035 is particularly helpful for tree care. This online publication contains information on: Aphids, Borers, Cottonwood blotch leaf miners, Cytospora canker, Fireblight, Gall makers, Aspen leaf spots, Oystershell scale, Pear slugs, Powdery mildew, and Spider mites.