Question
My children found an oak leaf with a strange little critter on it.  It is a small green ball with purple looking hairs on it.  We all agreed it looked like something from space, but I am very curious to know what it actually is.  It is actually a very pretty color combination.  Do you have a clue.  We live in rural Ohio.

Answer
It is one of the oak galls--a small insect (wasp) lays its eggs on the leaf and secretes a chemical into the leaf and the leaf curls over the egg, the egg hatches and the young feed in the gall and emerge as a tiny wasp. They cause no real harm to the tree and the population will vary each year from non existant to exteme. The most they do is cause the tree to drop its leaves pre maturely. There are many different ones. Here is a web link to pictures of some of the different galls.   http://www.hastingsreserve.org/OakStory/Oakgalls/LeafGallsHNHR.html