Knapweed Gall Fly

2013 August 7

I spotted this small fly on our window on August 18, 2012. It was a bit under a quarter inch long (about 4 mm), and it was a bit hard to see details with the naked eye. I didn’t realize until after taking some photographs that it was kind of an odd-looking little fly.

The abdomen tip was narrowed and extended, like some kind of spigot.

The eyes were kind of an odd color, appearing orange in some spots grading to a pale, iridescent green on the edges.

The abdomen tip, the color and shape of the eyes, the striping on the wings, and the pale-yellow dot on the back between the wings makes me think it is a (female) Knapweed Gall Fly, Urophora affinis. They are originally from Europe. This is one of the relatively small number of non-native species whose introduction date is known with some precision: they were first introduced in 1971 to attack Spotted Knapweed, which is a strongly invasive plant species. The introduction is ongoing, knapweed gall flies are still for sale commercially and from state weed-control offices.

These flies don’t kill the knapweed plants, but by making galls in the seed-heads they can eliminate seed production from the buds they infest. Their galls also reduce the overall vitality of the plant, so that even uninfested seed-heads on a given plant end up producing fewer viable seeds. Of course, a single uninfested plant produces several hundred seeds, so there are still plenty of knapweed seeds available even after the gall flies knock down the numbers. These are more a part of an “every bit helps” philosophy, and are intended to go along with other knapweed control measures.

So, since we have a lot of knapweed about, I expect we’re going to be seeing more of these flies in the future.

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