Expert: Jim Hyland - 8/10/2006

Question
I live in the south of New Jersey (shore region). I have an 85 year old oak tree in my yard that has sprouted galls. I have looked up almost everything I could about galls, but have yet to find the answer to this questions. Everyday my deck is covered with galls which appear to have been eaten. I find wood chips from the galls in piles on the deck and yard. Is this normal? Rarely do I find an un-eaten gall on the deck. It is usually in pieces. I can even see the pieces falling from the tree. Can you tell me what this is? Are squirrles eating them or is something more sinister going on?

Answer
I would guess it is squirrel.
Oak galls, the larger ones are also called oak “apples”, are some of the most fascinating things in 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.  
Each 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 habitats for other insects like ants, bees and wasps.