The news is not good. Getting rid of our cabbage root maggot problem is not going to be easy. Apparently, they're tough to beat once they're in your garden.
It seems that a floating row cover is the way to go if you want a healthy cole crop. These row covers provide a physical barrier to prevent damage from flea beetles, cabbage root flies and cabbage white butterflies. The theory is that if the flying insects can't get near the plant to lay eggs or eat the plant, there's no worry. Ideally, these covers should go on immediately after planting cabbage transplants. Oops, we'll know better next year.
This also explains the row covers we see in other gardeners' plots. Guess they've been there, done that!
Lee Valley has a floating row cover (7' x 50') for $19.95. Other gardening centres probably have them too.
For more info, check out some of these links.
Gardening Know How
Ecological Agriculture Projects
We'll have to go back to the plot soon to ensure we killed what we can and count how many plants are left to save.