Question
What can you tell me about the insect that causes a large sticky/slippery spot underneath trees adjacent to pavement?
I ride a motorcycle, often on tree-lined roads, and have experienced significant loss of traction in these areas, especially in wet conditions, when it's difficult to see the spots.

Answer
These are more then likely Aphids. Aphids, or plant lice, are small, soft-bodied insects which are common pests of nearly all indoor and outdoor ornamental plants, as well as vegetables, field crops, and fruit trees. There are hundreds of different species of aphids, some of which attack only one host plant, while others attack numerous hosts. Most aphids are about 1/10 inch long, and though commonly green and black, they may be gray, brown, pink, red, yellow, or lavender. A characteristic common to all species is the presence of two tubes, called cornicles, on the back ends of their bodies. The cornicles secrete defensive substances. In some species they are quite long, while in others they are very short and difficult to see.

Aphids feed in clusters and generally prefer new, succulent shoots or young leaves. Aphids feed by sucking up plant juices through a food channel in their beaks. At the same time, they inject saliva into the host. A distinctive feature of aphids, as well as some scales and other bugs, is the production of honeydew. Honeydew is the clear, sticky dropping that lands on the leaves or anything below the plant or tree that aphids are feeding upon. A sticky glaze of honeydew may collect on lower leaves, outdoor furniture, cars, and other objects below aphid feeding sites. Honey dew coated objects soon become covered by one or more black or brown fungi known as sooty molds. This glaze of honeydew is what you are experiencing with the loss of traction. This area should be sticky but I would expect when it rains these areas will have a different traction than the other road surface.

Aphids are controlled with the use of insecticides.