We have a tall oak tree (which was trimmed this past winter), and now it is suddenly dropping sap. It looks like the leaves are being eaten by something. We've been parking 2 cars under the tree since January, and about a week ago we started seeing sap droplets all over the car overnight. The sap is sticky and light brown, like maple syrup.
Can you tell me what might be causing this? I have attached photos of the tree, the partially-eaten leaves, and the sap. The photo of the sap is on a plant below the tree. Is the tree diseased or infested? It looks healthy otherwise. Any help identifying what is going on and what we should do about this would be greatly appreciated.
Kent County Michigan
The sap-like substance is probably honeydew, which is a liquid excrement from certain types of insects. One of the common insects on oaks that produces large amounts of honeydew is a scale insect called lecanium scale. These appear as small bumps on the stems and branches. For most of their lives they are legless, just sitting there and sucking sap from the oak tree. Small populations do very little harm to the tree, heavy populations can slow the tree growth and cause some dieback of branches, but they are not usually a serious problem for an otherwise healthy oak. There are many natural enemies of the scales and some weather conditions will also reduce their numbers over time. Sometimes the higher populations persist for a few years.
Thank you for all your information. That is very helpful. Can you tell me what some of the predators are for these scale insects?
The primary predators are a few species of lady beetles and a very odd insect called a brown lacewing. One of the lady beetles is much smaller than the typical ones, and its color is black with two small red spots. Brown lacewings as larvae look like tiny alligators, but their long jaws move side-to side rather than up and down. They have sticky hairs on their back, on which they place the partially eaten bodies of their food insects. The adults are a very delicate winged insect that is not often seen. There are also tiny wasp "parasitoids" which use the scale insects as hosts, and they can be very effective control agents. On rare occasions, cued in by certain weather conditions, a pathogenic fungus can kill nearly 100 percent of the scales n a matter of days.