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Does anyone know how long I need to fleece my onions for in order to keep the buggers at bay?

I believe there is more than one generation each year.

I don't think there's any need to worry once the onions start to grow, but I don't know when they reach the relevant stage. Three years ago my spring-planted onions were devastated (first time I'd come across the fly) but the overwintering ones were unaffected, and gave me a good crop.

As you know , Our area is prone to the pest. We keep our onions covered at all times. April is supposed to be the first bout and I think august/ september is the next one. We don't take chances leaving them uncovered just in case.
We had alovely crop of overwintering last year, so was well worth the trouble of covering them. We are growing maincrop this year as well and they are covered now.