Galls on trees are caused by insects

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Tree Talk, Joanna Angle

 While walking to the mailbox one sunny morning, I saw odd-looking spheres hanging from one of the young white oaks. 

About the size of golf balls, they were white with a few brown spots and fuzzy.

Several days later I saw more on another tree. Having no idea what they were, I made a note to ask our S.C. Forestry Commission Project Forester Roy Boyd to take a look on his upcoming visit.

Roy immediately said he thought they were “galls” caused by wasps.

Galls are unusual growths on plants most often caused by three insects: gall wasps, gall midges and gall mites.

 Other less common gall-producing insects are aphids, psyllids (jumping plant lice) and gall flies. 

Galls may form on any of the plant’s tissues from buds, flowers, leaves, twigs and stems to roots.

To confirm his diagnosis, Roy sent a photo to Laurie Reid, forestry commission resident entomologist. 

“It’s called the Wool Sower Gall and is caused by wasp grubs,” Reid replied. “This particular wasp only lays eggs on white oaks and by the time we see the gall, it’s too late for any control, but since they don’t cause any damage, there really wouldn’t be a need for control.”

I have since learned the Wool Sower Galls (also called oak seed galls) only occur in the spring and early summer when wasp eggs laid in midwinter hatch. 

These galls contain seed-like structures in which the immature wasps develop, giving off powerful enzymes as they grow. 

Fortunately, this type of gall is usually never numerous enough to threaten the health of infested trees. 

Also, gall wasps have parasites so their populations often decline naturally. 

Gall wasps are not dangerous to humans.


Joanna Angle is a Master Tree Farmer and 2012 South Carolina Tree Farmer of the Year. Her Cedarleaf Farm in Chester County is a Certified Stewardship Forest and part of the American Tree Farm System.