Oak Bullet Galls: Protected by Hornets?

During my recent walks in Detroit’s Eliza Howell Park, I have been observing what strikes me as a fascinating phenomenon. On two different Swamp White Oak trees, there are many small round insect galls that are being regularly visited by Bald-faced Hornets.

The galls are known as Oak Bullet Galls and they encase the developing larvae of very small wasps. The types of cynipid wasps that produce them on the twigs and stems of Swamp White Oak and Bur Oak trees are best known for these galls.

Inducing plants to grow in a manner that provides space and food for larval deveopment, as insect gall makers do, is amazing enough. What these Oak Bullet Gall wasps do in addition is to have the galls exude drops of nectar.

Bald-faced Hornets seek nectar at this time of the year and they return repeatedly to these galls seeking the latest drop.

What a wonderful strategy for protecting the galls from predators that might try to get to the larvae — have the galls patrolled by large (stinging) wasps that don’t want anything happening to their food source!

Bald-faced Hornets are the most common of the large wasps that seek out gall nectar. There are a variety of others, three of which are shown here.

I stopped by the trees last week to check out the size and number of the acorns. It’s a good acorn year, but an even better bullet gall year. The buzzing of the hornets quickly pulled my attention away from acorns, and I have been returning frequently since to watch and try to photograph the insects

Nature study tends to reinforce my sense of wonder. What is happening with Oak Bullet Galls is pretty incredible.

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