Excerpts from Jim Conrad's
from the April 15, 2012 Newsletter issued from the woods of the Loess Hill Region a few miles east of Natchez, Mississippi, USA
GALLS ON HICKORY LEAVES
It's a good time to see various insect-caused galls on many kinds of plants. A Pignut Hickory down in the bayou had a particularly heavy crop, as shown below:
These galls are caused by a small, aphid-like insect of the genus Phylloxera. Many Phylloxera exist, causing various kinds of galls on different kinds of plants, and I'm unsure which species caused this is.
The galls are actually pretty when viewed with sunlight backlighting them, as seen below:
That picture shows the leaf's top surface. The lower surface viewed with different light shows a different world below: