Hickory leaf-stem gall

I was doing some clean-up with the brush mower today when I noticed this strange yellow/green object on a hickory tree.



After additional inspection of the hickory tree I found several more of the objects including this larger group of 4. After some Internet research I've determined the objects to be Hickory leaf-stem galls.



The inside of the gall was somewhat hollow with a dark material in the center.



The dark material in the center of the gall was very small Phylloxera aphids. These aphids cause very little damage to the hickory tree.