We came across these galls on a wild rose plant last week, and Eliza insisted I do a blog post about them. So here you go.
Galls like this are formed when an insect lays an egg on or in a plant and the feeding of the recently hatched larva stimulates excess growth of plant tissue. The result is that the plant creates a little structure that contains both food and protection for the young larva. The most familiar example of this in prairies is the goldenrod gall, which can be seen in just about any prairie containing goldenrod plants. In this case, a wasp laid eggs on this wild rose (Rosa arkansana) plant and inside each of the resulting galls is a tiny white larva.
For much more information on galls and the insects that create them on wild rose, click here.