The reason I posted this picture is that these wasps are causing amazing damage to the trees in my in-laws suburban development. I wish I could post a follow-up picture to show the welts on the trees. After causing deep welts on one tree, now they are concentrating on the tree next to it. From a diversity perspective the development was not being smart by planting a concentration of these elms... as they are likely attracting more wasps.

My FIL suspected that the female wasps were laying eggs under the bark, but it seems like Eric is suggesting that the wasps are just looking for a sweet lunch? I'd like to know more about what they are doing and if there is anything reasonable to help stop the damage on the trees.

I seriously doubt the wasps are causing the damage, but instead benefiting from the oozing sap. Something else is at the heart of this, perhaps a disease, or even a natural phenomenon. Here in Arizona, some of our native plants and trees ooze excess sap (water) after heavy rains.

The smaller black and yellow wasp is an eastern yellowjacket, Vespula maculifrons. Many wasps will drink oozing sap....