Maple Leaf Galls: Insect-Caused Bumps on Autumn Blaze Maple

Our 5-year-old Autumn Blaze maple in Wisconsin has tiny balls on leaves—pale green, reddish, or dark black. Is this disease or insects?

Diagnosis: Leaf Galls (Insect-Induced)

These are galls—abnormal plant growths triggered by tiny insects (mites, midges, aphids, or wasps) laying eggs or feeding. The plant walls off the irritant, forming a protective structure around the developing larva.

Key Facts

• Cosmetic only—galls rarely harm tree health
• Species-specific—maple gall mites (Vasates spp.) are common
• Colors change as galls mature: green → red → black
• No treatment needed for tree health

If You Want to Treat (Aesthetics Only)

Systemic soil drench: Imidacloprid (Bayer Advanced Tree & Shrub) applied at bud break next spring. The insecticide moves into new foliage, killing gall-formers as they feed.

• Existing galls won't disappear
• Prevents new galls for ~1 year
• Follow label rates exactly

When to Worry

• Severe early defoliation
• Branch dieback
• Signs of verticillium wilt (sudden branch flagging)

Your tree is fine. Galls look alarming but are a fascinating plant-insect interaction, not a threat.