By Whitney Cranshaw, Colorado State University Cooperative Extension Specialist, Entomology
Not very much can be done to control aphids that affect trees. Most aphids on trees and shrubs have complicated life cycles, switching between different plants during the growing season. This habit, plus their phenomenal ability to reproduce and their great sensitivity to myriad natural controls, causes great fluctuations in their numbers from season to season. Very little correlation between problems exists from one season to the next.
Aphids that were present on trees during autumn lay eggs near buds. A spray of horticultural oil before bud break can kill these eggs, and this will help prevent aphid problems in the spring. This, however, will have little effect on subsequent fall aphid problems, which are dependent on conditions during summer and fall. At best, certain persistent insecticides that move systemically in the plant may provide control through the fall. Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.
Photos: Judy Sedbrook
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Date last revised: 01/05/2010