Fuzzy Balls on Fallen Oak Leaves: Gall Wasps, Not Fungus

Southern Minnesota oak leaves with fuzzy balls underneath. These are leaf galls from tiny gall wasps (Callirhytis), not fungus.

Wasp Life Cycle

• Egg in leaf vein → larva secretes chemical → gall forms.
• Larva develops → adult emerges → drops to ground.
• Female burrows, lays eggs in roots → next generation.

No Worry

• Wasps don’t sting; galls don’t harm tree health.
• Population varies wildly yearly.
• Rake leaves to slightly reduce numbers.

Next year may have zero galls. Enjoy the biology.