Q: We have an elm tree that is beset with problems. Each spring, sticky sap drips on the sidewalk and our automobiles. Later in the year, at least one entire branch will die. Are these things related?
--C.S., Panorama City
A: The sticky honeydew that drips from the tree is probably the result of an infestation of European elm scale. Because of its size and the conspicuous cottony-white fringe, the adult female scales are easy to see and identify. Because sooty mold fungus grows on the honeydew deposited on the elm's leaves, badly infested elms appear to be covered with black dust. The insecticide sevin (carbaryl) will control European elm scale. Applied in the spring, both malathion and diazinon are also effective. Thorough spray coverage is important. Because ants are attracted to the honeydew, and repel the European elm scale's natural enemies, ant control is also important.
The death of an entire branch can usually be traced to the boring and girdling of the elm bark beetle. Small holes in the bark on the limb are a sign of infestation, and when the bark is peeled back, the beetles' tunneling is readily apparent. A bark-beetle-infested elm can be saved with a series of heavy waterings and an application of sevin (carbaryl).