[hubballi]

I have many onion sets sitting in shallow compost in plastic trays in the cold greenhouse waiting to plant out. I found on the inside on the greenhouse ceiling a fly that looked like an onion fly. It resembles a house fly but slightly orangey looking. I wonder if this had been in looking for my onions ? The reason I am paranoid is because a couple of years ago I got maggots on all my onions. Even ones inside the greenhouse sitting in shallow trays. It was definitely onion fly but later in the season.

Could it be it ? If not, how can I avoid getting it outside when I plant out ?

Thanks.

[hubballi]

50 views and no reply 

[aelf]

maybe no one is sure what the answer is? This link gives info on the onion fly:
 
http://bayer.warinteractive.com/pestspotter_newf558.html?fn=details&con=2303&pestId=926

I think the onion fly is grey rather than orange but would need to see a picture of your fly to be sure.

[mumofstig]

I'm not sure if what you saw was an onion fly 

Delia antiqua, as the onion fly is known to science is only 5-7mm (0,2in) in length. It is a fairly non-descript ash grey and while males do have a 'go-faster' stripe along their abdomen, females do not so a seriously expert eye is needed to identify one of these guys.

http://expertscolumn.com/content/know-your-enemy-onion-fly

To stop them outside, covering with fine mesh is the only way.

[hubballi]

What about mulching the ground with something ?

[pdblake]

I would have though Enviromesh would be your best bet, though I don't usually cover onions, only while the sets are first taking root. Enviromesh is too expensive to use where it's not really needed, but I don't have an onion fly problem, yet anyway.

[hubballi]

All my sets are still in the greenhouse which is hitting up. All have quite tall shioots already.