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Sticky traps for the adults, nematodes for the larvae. For a less organic approach, you could also use a bt product such as spinosad, though it would mean that trace amounts of the bt toxin would end up in your weed.
You can also use physical means to make it harder for them to lay eggs. I've heard of people putting layers of sand on top of their soil for that purpose.
It would also help to reduce their food source. For example, if you're top-dressing malted barley powder, you should quit doing so until the problem is resolved.
One more trap that works for the adults is a mixture of apple cider vinegar and warm water, with a drop of soap. You can add a bit of honey if you want to as well.