Question
I moved into my house in October, the neighbor has a very large oak tree that hangs over the drive way. Before the tree lost all of its leaves for the fall it continued to drop Sap all over our cars, it stopped in December through February but star up again in March with the budding of new leaves. Is this common for the oak tree to do this or does it have a disease or insect problem? Is there anything I can do to prevent this from happening?

Answer
Sounds like the tree is infested with an insect called aphid. Aphids feed on plants by sucking plant sap from the leaves, twigs or stems. When abundant, aphids remove large quantities of sap, reducing plant growth and vigor.
Most aphids excrete large quantities of a sweet, sticky substance called honeydew. At times, excessive honeydew dropping from trees can be an extreme nuisance. Also, sooty mold fungus may grow on the honeydew, producing a gray, unattractive covering of the leaves. Sooty mold is not damaging to the trees except when it covers leaves and temporarily reduces photosynthesis.

For most aphid problems, particularly those associated with leaf curls, insecticides that move systemically within the leaf or plant provide the best control. The most common systemic insecticide available to homeowners is Orthene(acephate). Cygon (dimethoate) also may be available as a spray for use on evergreens. Some insecticides can be applied to the soil and taken up by the roots of the plants. These are called systemic insecticides. The most recent, Imidacloprid, is sold under the trade name Bayer Advanced Garden Tree & Shrub Killer Concentrate. (Merit is the trade name of imidacloprid used by professional tree care companies.) It is applied as a drench over the rootzone. An older梐nd much more toxic梥oil systemic insecticide that is stillavailable for some ornamental plant uses is DiSyston (disulfoton). DiSyston is sold as granules or in plant food mixtures for soil application. There are several insecticides effective for aphid control when sprayed on plants. Perhaps most effective are those with systemic activity that allows them to move through the plant. Acephate (Isotox, Orthene) is the most widely available systemic insecticide.

This spraying especially if the tree is large may take a professional pesticide applicator. You did not say where you were located--if you are in the U.S. check with your local Agricultural extension Service for recommendations on aphids on trees and they also should have a list of applicator. Their phone number will be in the local phone book.