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Hello Robert and gneve,

No male needed. This is a 100% S. loewi! Very nice find! The black andtennae in combination with the abdominal markings leave no room for S. ruepelli.
All Sphaerophoria are presumed to be aphidophagous on grass and such. S. loewi is probably dependent on a aphid from a rush. It has been found in both brakish and freshwater habitats. It's probably rare because adults are active above the watersurface and removed from the shore. Not a place where Syrphidologist tend to collect...
Mayhaps you can tell some more about the locality and time of capture?

Greetings,

Ectemnius