Dorymyrmex insanus, Galls and Coccids

I was walking along a trail in Gardner Canyon, in Arizona's Santa Rita Mountains, when I noticed a 4-5' high shrub on the side of the trail. It looked like some type of scrub oak, which is not unusual. What was unusual, though was the shrub's leaves - about half of the leaves had a prominent cone resembling a witch's hat projecting from the ventral side. The witch's hat was a gall, a growth of plant tissue that is usually caused by insects. Wasps, flies, aphids or other insects "trick" the plant into making a house for the insect to live in while it matures. The insect tells the plant to build it a home, then the insect lives in the home - often while it is feeding upon the plant! While plant galls seem pretty improbable, they are surprisingly common. In fact, in one of these photos you can see a large round object that looks like an unripe orange - that is yet another type of gall that this oak tree is infected with.
Then I noticed that the tree was covered with ants - Dorymyrmex insanus. The ants were fascinated by slits on the tops of the oak leaves where the galls had formed. In fact, they were crawling into the slits - down into the gall itself. Usually one ant would crawl into the slit and stay for awhile, antennating with other Dorymyrmex insanus. It was hard to tell, but I think that the ants inside the gall were feeding liquid to the ants outside of the gall.
In some places, the entire leaf had been eaten away, perhaps by caterpillars  - everything except for the leaf midvein and the gall itself. But the galls were still attractive to ants, as in the photo to the left. Galls are often high in various chemicals, such as tannic acid,  secreted by the plant to make the plant tissue less palatable.
From what I can tell, the ants were responsible for creating the entrance to the gall. They were at least responsible for enlarging it - in this photo, you can see a Dorymrymex insanus worker removing woody particles around the slit, thus widening the entrance to the cavity in the gall.
I cut open one of the galls to see what was inside. At the bottom, in the point of the "witch's hat", was a disk-like creature - a coccid. Coccids are insects that are related to aphids and other true bugs. They typically feed on the plant's sap. The coccid was secreting honeydew - you can see the droplet at the point of the yellow arrow in the photo at left. While the gall was on the ground as I was photographing the gall, a Dorymyrmex wandered over and started drinking the honeydew (at right). And then another Dorymyrmex appeared, and another.
At left are closeups of the Coccid. I think it was pre-pupal - you can see legs in the ventral view - I outlined them in yellow.
One of the things I find to be fascinating about this interaction between the gall, the ant and the coccid is that I suspect the coccid may be communicating with the ant - probably through odor. The speed with which the ants found the gall I had opened was quite startling. It seems that the ants may be responsible for widening, if not creating, the entrance into the gall - how would they know to do this unless they could tell there was something inside that they wanted? I haven't seen them opening up other types of gall. It makes sense that the coccid would want the ants' attention - since the coccid is inside a closed gall, its honeydew waste would build up otherwise.

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