Oak Cup Gall Wasp
Callirhytis
Observed in the riparian corridor of Florida Canyon, Sta. Rita Mts., Arizona, USA. 27 July 2013. Quercus emoryi. The big insect is a Polistes paper wasp, and not the topic of this web page!
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The large paper wasp in the photo at left is more like what comes to mind when people think of a wasp. But the gall wasp is a 100 times smaller and resides inside the vase-shaped structure (a gall) attached to this acorn cup, i.e. it's not even visible in this photo. In fact the gall probably contains several minute wasps in the form of larvae (grubs) - the immature stage. In a complex interaction between plant and insect, plant tissue is stimulated to grow in a specific but abnormal way. In this case the gall wasps caused the acorn seed to abort.
Some galls like this one produce a sugary secretion that attracts other insects. This tree, an Emory Oak, had many such galls and was the focus of much attention by the Bugguide 2013 Excursion because a great number and variety of insects were visiting the galls for the sugar energy. The paper wasp was likewise seeking some of the sugary sap. The interactions between the tree, the many insects, the gall maker and no doubt microbes offers a complex story for future ecologists to untangle.