Perhaps you’ve noticed green acorns on the trails. Oaks will drop acorns if stressed, but there’s also a more intriguing cause … the acorn pip gall wasp.
If you see many green acorns on the ground, stop and examine a few acorns. Look for acorns that look like the ones pictured below.
Specifically, look for a triangular shaped gall or a notch on the top of the acorn. The notch is present if the gall has fallen out. If the acorn retains its cap, the gall or notch is just below the cap (top row in photo). If you look very closely you might find the galls in the duff (bottom row in photo). (Note: a gall is an abnormal outgrowth of plant tissue usually due to insect or mite parasites or fungi.)
The pip gall is caused by the exceptionally tiny pip gall wasp. In our Woods, pip gall wasps laid eggs in acorns earlier this summer. Larvae hatched and caused the acorns to produce the galls. The larvae now live inside the galls, and feed on the gall’s tissue. This infestation causes the oak trees to drop the green acorns. Typically, adult wasps will emerge from the gall next year and start a complex life cycle that includes causing a woolly catkin gall on the red oak’s flowers. Intrigued? Read here for the details The The Peculiar Acorn Pip Gall Wasp.
References
Merriam-Webster Gall Definition
Williams, L. (2023, 7 July). Acorn Pip Galls, Woolly Catkin Galls All The Same. Wisconsin DNR Forestry News.