Oak galls - the larger ones are also called oak "apples" - are some of the most fascinating things in the Open Space. These growths are the result of the defensive reaction of the oak to attack by specific parasitic insects called "gall wasps." These black wasps are small, only one quarter to one third of an inch long, and their eggs are even smaller. These gall wasps lay an egg just inside the skin of an oak leaf and the oak responds by growing more, and larger, plant cells to cover this irritation. This is a pathological response by the oak tree to the intrusion of this wasp egg. The egg ripens, turns into a small larva, and this larva then feeds on the tissue inside of the oak gall.
The interesting thing about these galls is that the larvae that emerge in the fall, when there are no leaves (the oaks on top of Shell Ridge are deciduous), then create galls on the roots of the oak. When these "root larvae" emerge, they turn into gall wasps that lay their eggs on the new leaves. Thus, these wasp generations alternate, with one creating galls on leaves and the next generation making galls on the oak root system. Alternating generations may be the adaptation of the gall wasp's "reproductive system" to the seasonal appearance of leaves in deciduous trees. Also, the larvae that emerge in the fall turn out to be all females who have reproduced parthenogenetically (without the aid of males!).
Galls are home to many thingsEach gall may contain one or more larvae and many of these larvae have parasites on or in them. These parasites, in turn very likely have their own set of parasites, called hyperparasites. When an egg is laid, the gall wasp may also inject digestive enzymes that break down plant cells into constituent amino acids. This process makes it easier for the larva emerging from the egg to subsequently feed on the gall tissue. In addition to eggs and larva, many galls contain visitors that either nest in the gall or are parasitic to the larva. These visitors are called "inquilines," or animals that live in the abode of another animal. When the gall ripens, birds and squirrels easily eat the soft gall tissues. Eventually, after the larvae emerge, the gall dries out and can become suitable habitat for other insects like ants, bees and wasps.