Look closerPlants
Oak galls: the bumps and balls on oak leaves, and what makes them
When: June to October
Bumps and balls on eastern Massachusetts oak leaves are galls, tissue the tree grew around a tiny wasp or other gall-making insect, and most leave the tree unharmed.
Why this time of year
By late summer a whole season of gall-building is on display on Sudbury's oaks, and the shapes hold still long enough to study.
How to notice it
- Pick one low oak branch and turn over ten leaves.
- Look for balls, blisters, discs, and fuzzy patches; each shape comes from a different gall maker.
- Compare two oaks: does one carry more galls than its neighbor?
- Leave every gall where it is; some still house a developing insect.
Try this with kids: How many different shapes can one oak tree make?
a gall is leaf or stem tissue a plant grows around an insect egg or larva
Safety
- Look and compare; do not open galls or strip leaves from the tree.
Sources
- umass.eduTiny Cynipidae wasps induce oak leaf galls; leaf galls do not reduce the overall health of the host plant and very rarely if ever require management; hosts include white, red, pin, scarlet, black, and bur oak
In the guide for June, July, August, September, October