Mary left the sticky card traps in the onion patch until the last onions came out, clustered them around the leeks, and collected them long after the season was over.
I count maybe twenty flies that might be onion maggot flies or cabbage maggot flies.
The cards protected the onion crop, failed miserably for the leeks, and did nothing for the nearby cabbages. Deploying the cards while planting worked very well, refreshing them after a month continued the protection, but the main fly season seems to end shortly thereafter.
All the sticky cards as a slideshow, starting with the three along the border fence:
The cards remain sticky to my fingers, but an adroit fly could skate over the debris field and emerge unscathed.
The Smell of Molten Projects in the Morning 