Question
I have a 20 year old red oak which "weeps" tiny sticky droplets. Usually this has happened in the spring for a few weeks and stops, but this year it has not stopped but gotten worse. It is over my driveway so the car windshield has to be washed every day from the sticky mess. Can anything be done about the tree? I live in the San Francisco-Oakland Bay area.

Answer
The dripping is a substance called honeydew it comes from an insect called an aphid. They suck the plant juices from the leaves and twigs and excrete the honeydew. It is stick because it is high in sugar content. You will need to control the aphids to stop the honeydew.

When natural enemies are not abundant enough to provide aphid control, insecticides sometimes are needed to prevent plant injury. 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 root zone. An older梐nd much more toxic梥oil systemic insecticide that is still available 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. Dimethoate (Cygon) is less commonly available and is mostly used for aphids on evergreens.