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The key only mentions features which are useful in distinguishing between the two options in the couplet. For D. planipalpis it reads:
Hind femur with av setae in apical half only, no strong pv setae. Hind tibia with weak av present, a few small pv
in basal half. Wings paler. Abdomen with median vitta distinct, sharply defined. Scutum greyer with only faint
vittae.
The comparisons are with D. floralis, and there's nothing about frontal width.

From the hind femur D. radicum could be possible. I don't know if the frontal width varies within species - that would account for its not being used in the key. From what we can see I don't think we can rule out D. carduiformis (assuming the fore-tibial spur is an illusion). I think we need an expert ;)