Fruit of the Oak?

Fruit or gall?

On Saturday's Master Naturalist training field trip, one of the finds was this chewed specimen.  It was firm and had a spot that looked like the insertion of a stem.  There were some wild plums scattered about in the oak and hickory forest but no one obvious source.

Cutting it in half exposed either the single seed of a fruit (drupe) or a larva in a gall.  The color was distinctive, but of what?  Sad to say, Jay Barber called it a gall and nailed me with it.  My only defense is that I was half right - it is an acorn plum gall from an oak tree.  OK, he didn't buy "half right" either.

Gall- NCSU.edu

You can see how it gets its acorn name, as it arises directly from the cap of the acorn.  This is somewhat unique as other oak galls are twig galls such as oak apple and wooly sawyer galls, or leaf galls.  You can see pictures of these here.

Intact larva- MJ Hatfield

Galls in general are the products of eggs from gall wasps, gall midges, and gall mites, while a few others are produced by aphid and fly species.  The egg laid on the parent material alters the plant's metabolism to do its bidding, producing both food and shelter as the larva grows.  Eventually the gall insect emerges, breeds, and lays its eggs in or on a similar species to repeat the cycle.  In our example above the cut went through the larva.  The example to the left shows the larva intact.

The acorn plum gall is formed by a Cynipid wasp, Amphibolips quercusjuglans.  As galls seldom do any significant damage to the parent tree, there is little research on many species.  To quote backyardnature.net:

Gall wasp- MJ Hatfield

"We have over 750 species in 49 genera in the Gall Wasp Family. Furthermore, "Each species makes a characteristic gall on a specific part of the plant. Many make galls on oaks. Most have a complex life cycle with a parthenogenetic generation and a sexual one. Each generation makes galls of a different appearance and on different parts of the plant."

After Bugguide.net links, the trail grows cold.  I hoped to find the derivation of the species name quercusjuglans, a strange mixture of quercus - Latin for oak, and juglans - genus name of walnuts.  This is probably more than most people would care to know about the naming and sex life of this tiny wasp with the big nursery.