Q: I found this growing on a wild grapevine off my deck in late summer at my cabin in Rabun Gap. Very curious looking spikes.
A: They are grape tube galls. They are caused by a small flying insect called a midge. Adult midges lay their eggs either on the backside of leaves. Orange, maggot-like larvae hatch from the eggs and enter the leaf tissue.
The larvae secrete hormones that affect how the leaf tissue grows: like a horn in this case
Truthfully, not much damage is done to the grape vine so no control is needed.
If you’re curious, use a sharp knife to split some of the galls to see what’s inside. If you open them before the insects emerge from the gall, you’ll see yellow immature insects inside.