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Photo#603760
Rhopalomyia californica gall with odd hole - Rhopalomyia californica

Rhopalomyia californica gall with odd hole - Rhopalomyia californica
Carpinteria Salt Marsh, Carpinteria, Santa Barbara County, California, USA
December 26, 2011
Size: 11 mm (gall)
I was examining Rhopalomyia californica galls on coyote brush (Baccharis pilularis) at the local salt marsh today, and noticed this gall with a large hole in it. I'm curious if anyone has ideas about what may have caused the large hole. Is it an exit hole from a parasitoid? A hole opened up by some predator hunting larvae?

The hole is about 1.8 mm in diameter. You can compare it size-wise to the R. californica exuvia protruding from the gall in the lower righthand part of the image. I'll also upload crops of the large hole and the pupa.

Update: After bringing the gall home and examining it more carefully, I've revised the dimensions above, and have some additional images that I'll upload.

Later update: See also this additional set of images, which show an empty dipterid egg case, a leaf mine, and a brownish object (perhaps a pupa?) that I found on the leaves and stem at the base of this gall:



Still later update: Stuff has started emerging from the gall. See:



Still yet later update: I've gone ahead and cut the gall open to see what the inside looked like; see attached photos.

Images of this individual: tag all
Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica Rhopalomyia californica gall with odd hole - Rhopalomyia californica

Moved
Moved from ID Request.

Only a guess, but
I wonder if some kind of moth larva might have bored in, feeding on the gall tissue but not specifically going after the midge larvae (i.e. an inquiline). It would be interesting to see what you find if you cut the gall open (but I would first wait a bit to see if anything else emerges). I found caterpillars feeding in a few different cynipid oak galls this summer, and a tiny caterpillar in an aphid gall (witch hazel leaf cone)--unfortunately the latter fell out of the gall when I discovered it, but the others are currently overwintering in jars.

1.8 mm seems large for a hole of a parasitoid wasp (though maybe not completely out of the question), and I don't think that would explain the brown, dying tissue around the hole.

 
Some possibilities per Tilden
In terms of the identify of whatever it was that made the large hole, I've been doing some digging, and read James W. Tilden's 1951 paper, "The Insect Associates of Baccharis pilularis de Candolle", Microentomology 16:149-188. In a section of the paper dealing with competition, he mentions two moth species that sometimes bore into Gnorimoschema baccharisella and Rhopalomyia californica galls: 1) Oidaemotophorus grandis [sic], which I think was probably a typo referring to Oidaematophorus grandis, which I gather is more commonly referred to these days as Hellinsia grandis, and 2) Aristotelia argentifera. So, I guess it's likely I'll never know what actually made the hole in this gall, but both of those seem like possibilities.

 
If you cut the gall open...
the presence of frass pellets would confirm that a moth larva had bored in. But I agree, the only way you'd know what kind of moth (if it is a moth) would be if you found a pupa inside and saved it until an adult emerged.

 
What about the absence of frass?
I cut the gall open a few days ago (see the latest photos attached here), but did not see anything that looked like what I expect caterpillar frass to look like. What would the absence of frass suggest? Might we be looking at something else (a stem-boring adult beetle, maybe?) having chewed its way in?

 
Hmm...
Well, there are some caterpillars that push their frass out rather than filling their living space with it. I could also be completely on the wrong track. Looks like you'll just have to search for this phenomenon earlier in the season this year, and see if you can find galls with the culprits still inside.

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