I'm not familiar with the root maggot flies you have there but the cabbage root fly we get here deposits its eggs near the stem of the plant. So if you put a barrier on the surface of the ground around each stem they can't lay their eggs. You can buy the barriers or make your own.

Here is a link that might be useful:

Thanks Flora. The problem is that I have thousands of turnips to cover! :)

And you must plant in a new area. This because the adults emerge from the soil each spring.

You have thousands of turnips?
Are you a grower who sells produce?
If so, contact the Field Crop specialist at your local agricultural authority. (I don't know who or where that might be in Canada.)

Here's some info from University of California

Here is a link that might be useful: manage cabbage maggot

Insect barrier (aka Summerweight), a type of floating row cover that is much lighter and more airy yet still effective.

But an insect barrier that is both more sturdy than row cover AND still prevents small insects is an oxymoron I fear. :) Even fiberglass screening won't stop them unless you build a full enclosure.

Dave

Ah - I see. I thought you were referring to cabbage/kale/cauli type brassicas which have a stem. You didn't mention the scale of the operation. I've looked it up and it is the same fly we get here.

Agribon makes AR-15 row cover which is their lightest weight. I just bought a roll to experiment if it works better than the AR-19 i've been using.
I'm not sure what you mean by "flimsy", I think row cover is the best thing available for this purpose. I use it to keep out leaf miner, flee beetle, root maggot, rust fly, cucumber beetle, etc......
I couldn't grow organic veggies without it.

-Mark

When I leave lightweight row cover on rutabagas they get too warm and get aphids. I have heard people use ash on the surface to prevent the cabbage fly.

I am trying tulle this year. Can't say about results yet, but it is much easy to work with then insect barrier, you can see through it and it is not disturbed by wind - wind just goes through(better air circulation) I bought whole bolt 108'' X 50 yards. It will be enough for me at least for two, may be for 3 years and i cover EACH bed I seed, if not maggots, then leaf miners, or cucumber beetles, or neighbor's cat) If the tulle will survive whole season and will be effective for small cabbage flies - this is what I will end up with for the rest of my gardening experience))

How about beneficial nematodes? If they work on onions maggot I think they might work on cabbage maggots. Now, I've just used my first application so I can't reccommend them yet. But it could be something to look into.

Thanks all.

It looks like row cover may be the only way to go. I do have lots of AG-15 lying around.
Galinas, how small are the holes in your tulle?