Scrub Oak Swellings

On March 19 I walked up a hill in my town called Mount Lincoln.  When I got to the top I remembered why I never go there; like many high points around here it is denuded by an access road, and I could feel the electromagnetic field emanating from the radio tower so I didn’t stick around for long.  Before I took off, on the side of the cleared area I spotted a little patch of scrub oak (Quercus ilicifolia), which is not a species I run into often in western Massachusetts.  A few of the twigs had pronounced swellings, galls caused by Callirhytis quercussimilis (Cynipidae).  Weld (1959)* lists four other hosts (all in the red oak group) for this wasp, none of which are found in Massachusetts, although Felt (1940)** also lists the common red oak (Q. rubra), which if correct would make it less miraculous that the wasps were able to find these isolated host plants.  I have never noticed swellings like this on red oak.

Gall of Callirhytis quercussimilis on a scrub oak stem.

In any case, I broke off a twig with three of these galls, and yesterday a wasp emerged from the one pictured above.  Like her cousins from the swamp white oak leaf, she wouldn’t hold still for even a fraction of a second, but I managed one decent shot.

Adult female Callirhytis quercussimilis (2.5 mm).

This species, incidentally, appears to have no documented parasitoids, and Neuroterus floccosus has just one (Eulophidae: Aprostocetus verrucarii), so I guess my chances of getting the expected cynipids out of these galls were good.

* Weld, Lewis H. 1959. Cynipid Galls of the Eastern United States. Privately printed in Ann Arbor, Michigan.
** Felt, Ephraim Porter. 1940. Plant Galls and Gall Makers. Ithaca, NY: Comstock Publishing Company, Inc.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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