ISSUE 8 JUNE 24, 1999
The poplar bud gall mite causes cauliflower-like galls to form at leaf buds of cottonwoods and other poplars. These galls are dark green in color early in the season and turn brick red by late summer. Old galls have a ridged and furrowed surface, are hard, and grayish in color.
Trees may lose aesthetic value quickly because the new galls are formed every year and old galls may remain active (growing and supporting development of new mites) for up to 4 years. Although trees are seldom killed, lower branches often become crooked, stunted, and are often killed. Continuous attack may weaken a tree, increasing its susceptibility to drought, frost, or other injuries.
Some of the greatest damage caused by poplar bud gall mites is seen on hybrid poplars. There appears to be a wide range of susceptibility of poplars to the mite. When planting hybrid poplars in areas where the mite has a history of causing damage, use more resistant cultivars. Northwest, Saskatchewan, and Brooks No. 5 are highly susceptible to the mite, while Walker, Wheeler, Griffin, and Dunlop are highly resistant to bud gall mite infestations. If chemical control is necessary, carbaryl can be sprayed as buds and leaves are expanding in the spring.
Aphids cause a variety of injury symptoms on leaves, twigs and stems of trees and shrubs. Many aphids cause leaf discoloration, disfigurement, and occasionally premature leaf drop. Often-times the leaves, stems and anything under the plant becomes sticky as the aphids excrete honeydew (a sticky substance) while they feed. Honeydew covered surfaces will often turn black as sooty mold fungi move in and colonize the sugary substance. Some aphids cause galls. The poplar gall aphids cause marble-sized galls at the junction between leaf blades and petioles on cottonwoods and other poplars. Aphids on twigs or stems can cause reduced plant growth, early leaf drop, and/or twig death.
High numbers of aphids can weaken trees and shrubs, but rarely kill them. The sweet honeydew is attractive to ants, flies and wasps. The attraction of undesirable insects along with the honeydew mess and damage to painted surfaces caused by honeydew are factors which often motivate people to control aphids on shade trees. If an aphid population is becoming unacceptable, take a close look to see if there are predators (ladybird beetles, lace wings, etc.) present. When predators and/or parasites are present, it may be better to allow nature take its course. If compelled to spray these plants, use an alternative product such as insecticidal soap to reduce the impact on beneficial insects. Other products labeled for aphid control in trees and shrubs are listed on the back of E-296.