Dec 13, 2013
On October 30 I was in the Hualapai Mts., near Kingman, AZ I picked an oak apple gall off a Sonoran scrub oak (Quercus turbinella) growing near our cabin and put it in a jar. The leaf it was on had died. On December 13th I found 3 tiny wasps had hatched, emerging from an even tinier hole in the dried gall.
Based on the type of gall (apple), where it is located on the plant (leaf) and the species of oak, Quercus turbinella, the wasp is probably Artrusca capronae (Weld, 1930) (Redfern, Askew, 1992), (Russo, 2007), (Fernandes, Espírito-Santo, Faria, 1999), (Weld, 1960). One source (Fernandes, Preszler, Grim, 1990) mentions that the larval chamber only holds one wasp but this clearly was not the case. Two of the three could be equilines, brood parasites which share the gall but don’t create the gall. All three look alike but equilines are often sister species. (Weld, 1960) provides a key but they are so small they are at the edge of my magnification to even get a good picture. I have preserved the three in alcohol and if anyone wishes to study them, they should let me know in comments.
The wasp itself has a complex lifestyle, the only females hatched from the gall crawl into the ground and lay eggs on the roots that form another type of gall. From this gall both male and female hatches and the female from this hatching lays eggs on the leaves which create the apple gall. This is done by the egg or the larvae somehow changing the development of the plant tissue. It is not known how (Ronquist, 1999).
The gall itself is pretty amazing, hollow with long strands radiating from the central brood chamber to the outer surface of the gall. These strands are threads of single cells that are believed to send nutrients to the brood. These strands can also hold plant crystals which may be some sort of defense against the gall (Fernandes, Preszler, Grim, 1990).