Friday, August 19, 2011

Ants Still up a Tree

A Shingle Oak is not supposed to turn brown in the middle of August. I’ve been watching this tree grow for 25 years and even during the worst of our droughts it has never turned brown this early in the year. A sight like this tells me that something is wrong.

There are very few branches low enough to be easily examined, but these showed the same pattern of dead leaf clusters as the rest of the tree. This is the tree that attracted so many ants in early July. The ants have not changed their behavior and are still pouring up and down the tree trunk. They’ve got to be responding to something and I’ve got to believe that it has something to do with the mysterious dead spots.

I could find no leaves in the intermediate stages between brightly healthy and dead. Perhaps the catastrophic event has passed and the tree is no longer at risk. That’s possible, but the presence of the ants kept me searching.

Then I found some mottled brown nodules growing on a stem. Galls had developed in response to insect eggs laid on the oak stem. Inside each gall would be a developing larva, safely contained within the growth produced by the oak. But galls on an oak are a common occurrence. I think it would be hard to find an oak tree that didn’t have at least a few galls. I continued my search.

An ant tending a gall. This ant would not be so protective if it was not getting something of value from the gall. Certain Cynipid Wasp larvae produce galls that ooze a sweet liquid. The ants stay on duty collecting the sweet substance and in so doing, protect the gall from predators. It’s hard to believe that a few galls could cause a massive leaf die-off on the tree.

Well, there turned out to be more than a few galls. Gall villages were scattered along every branch. This amount of gall activity could certainly hurt the tree. It would also explain the thousands of ants traveling up and down the tree trunk. It’s also possible that the ants are inadvertently responsible for the clusters of dead leaves scattered across the tree. Cynipid Wasp galls don’t normally reach this density because they are destroyed by a parasitic wasp species that lays its eggs inside the gall. The parasite consumes the Cynipid larvae which reduces the stress on the tree. The ants are such good protectors that the parasitic wasp can’t get close to the gall, so the Cynipid numbers grow out of control.

Many dead stems can be found on the ground around the tree. These dead sections don’t show any signs of galls. Maybe galls located further down the stem effectively girdled the branch and caused the tip to die. I’ll just have to keep watching and see what develops. I’m sure the tree will be able to recover next year, but I don’t know if it can survive too many consecutive seasons of this kind of stress.


  1. After just finding one on the ground and posting a photo, a blogging friend in 'Denmark' identified it for me. The gall was firm, almost solid in structure, not papery and thread-like inside, ones I'm more familiar with. I read the purplish moisture it oozed can be used for dyeing, the making of ink and leather working.

  2. Hi, Wanda. I opened one of these to see if I could see the larva within, but what I finally released was more like scrambled egg than worm.