A very serious pest on apple and other plants in the same family, including pyracantha, cotoneaster, pear, and others such as hawthorn or elm. The aphids may infest stems or roots, and when feeding on the roots may feed undetected as their population grows. On tender bark of stems the feeding causes large swollen areas that may eventually girdle that stem and cause death of the plant part. When large aggregations feed on the roots the foliage of the plant may turn yellow, and as colonies grow they may begin to feed on foliage as well as the stems. Eggs are rarely produced, with nymphs usually over-wintering on the roots deep in the soil. In some cases nymphs or adults may over-winter in crevices on the bark as well. In the spring the females produce live “crawlers” that may move to the roots, to stems, or be carried by the wind to other plants.
A mature group of wooly aphids has enough wax to repel light mistings of insecticide, so either a powerful spray or the addition of a wetting agent will be helpful to get the material to the aphids. Treatments early in the spring will be more likely to contact the exposed, susceptible crawlers. Once the aphids are infesting the roots of a plant control is nearly impossible with insecticides.