Question
The leaves of the Liquidambar trees in our neighborhood in the South Bay Area of San Francisco seem to exude a sticky substance that sticks to cars parked underneath them and gets stuck on soles of shoes. It's very annoying. Some of my neighbors think that it is the excrement of aphids, another says that it's sap from the tree. Who is right, or is there another explanation? Thanks.  

Answer
More than likely it is a scale insect not aphids. Both will suck the plant juices from the tree and secrete a substance called honeydew which is high in sugar content and sticky. Aphids move around and you should be able to see them on the leaves and small stems. Scales are attached and look like small turtle shell. or bumps on the small twigs.

On large trees I would treat the soil around the tree with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soul around the tree and the roots will carry the insecticide to the foliage and small twigs and when the insects suck the plant juices form the tree they are killed.

Sweetgum can exude sap but this is usually in gobs that runs down the trunk not in the fine mist that gets on everything.

Here is a web link to this product. Check with your local garden type store for it.

http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect...