Some people are surprised to learn that tree holes are a common place where mosquitoes breed. Not all mosquitoes like tree holes, but unfortunately the species of mosquitoes in this country that are potential transmitters of the Zika virus are among those mosquitoes that love to lay their eggs in places where there is very little water. So filling tree holes has become even more important as a way to reduce the numbers of mosquitoes that can bite you, and potentially transmit mosquito-borne viruses, including Zika.
Holes in trees are left when trees are injured in any way—often when a tree limb rots out or falls off. When it rains some of these cavities fill up with water, and they may hold water for a week or much longer. This is long enough for mosquito larvae to hatch and develop into biting adult mosquitoes—a process that takes as little as four days in hot weather, or up to two weeks in cooler weather. There are several ways to fill a tree hole, but in most situations, arborists have stopped recommending concrete (which is heavy and causes other problems), or gravel or sand (both of which hold water and promote decay).
Expanding foam is being used more these days—you can smooth off the excess foam after it dries. If rats or mice are in the area, some kind of screening or other excluding materials can be added to prevent these pests from chewing into the foam. It is not necessary to clean out the wood decay before filling the cavity.