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Photo#1075168
Fly on yellow starthistle, Santa Clara County, California - Urophora

Fly on yellow starthistle, Santa Clara County, California - Urophora
Los Trancos Open Space Preserve near Palo Alto, Santa Clara County, California, USA
May 29, 2015
Size: approx. 0.5 cm long
I originally thought this was a false peacock fly since they are usually dancing on the yellow starthistle buds this time of year, however, when I looked at the photo up close later, I was surprised that the thorax was solid black instead of spotted. Do they have a life stage or newly hatched form that has a solid thorax? Could possibly be the angle of the sun.

Moved
Moved from Fruit Flies.

Moved
Moved from Flies.

Moved
Moved from ID Request.

Possibly Urophora sirunaseva…
This strongly resembles the yellow starthistle gall fly. We do not have a BG reference for this as yet, however. Their larvae feed in the flower heads, and it is definitely established in California as one of a number of biological control agents released to help control this invasive weed.

See reference here.

 
Possibly Urophora sirunaseva
Thanks Ross, that was a fast response and very helpful. We will be looking for biocontrol insects on yellow starthistle plants of central coastal California over the next few weeks and will look sharp to see if we see more of these gall flies.

 
More yst gall flies
Through June 2015, we have been checking more locations for yellow starthistle (yst) biocontrol insects and have been frequently finding hairy weevils, false peacock flies and yst gall flies in dozens of locations in San Mateo County and Santa Clara County, California.