Last year the cabbage worms made a mess of our brassicas. This year we made row covers. Since the brassica family produces without the assistance of pollinators, it is safe to keep them covered with a material that will only let in light, wind, and rain. We're using agribon.
The construction couldn't have been easier. We purchased one-foot sections of rebar and hammered them in around the perimeter of the beds in question. We left about three inches or so poking out and these little nubs then acted as anchors for the eventual ribbing for the structure.
The ribs are eight and ten foot lengths of recycled 3/4 inch PVC. Each end of the PVC slips over the rebar forming a frame that looks similar to that of an old covered wagon.
We couldn't find agribon wide enough to span the width of the beds with the ribs so we fastened two lengths together with clothes pins. After draping them over the ribs we weighed down the edges with strategically placed rocks. The whole shebang cost about $25. All that's left to do is keep our fingers crossed that no other mysterious pests materialize.