Has anyone ever used row covers (floating or built on a "structure", such as a PVC cage) to help prevent/discourage insects? How effective are they?
I've had mixed experience with row covers.
Two years ago (no row covers) I lost my entire cabbage crop to cabbage worms. The heads were completely chewed through like swiss cheese.
Last year, I used row cover supported by a PVC frame over a bed of cabbages and a bed of broccoli. I put on the row cover the same day I transplanted. It was held down on the sides by scrap boards and rocks, and on the ends with some rocks. This worked well: the leaves did not get eaten and I was able to harvest. I had uncovered brussels sprouts growing in a bed adjacent to the others, and these got munched to pieces. In midsummer I had to take the cover off the broccoli because the plants were too tall. At this point the plants were hardy enough that the bugs didn't hurt them too much. I did not spray at all and had a decent crop of both cabbage and broccoli.
This year I tried the same thing but I wasn't as diligent about getting the covers on immediately. I covered one bed of cabbage a few days after transplanting but had to remove the cover after a couple of weeks because the leaves were getting eaten. When I removed the cover I noticed it had a couple of small holes (I reused the covers from last year -- they looked like they were in good condition). I don't know if the holes or the delay was the problem with the bugs. I'm just spraying them all with Bt now and that is keeping the damage to a minimum.
No direct experience, but the following article talks about floating row covers being used to protect against frost and larger animals, and primarily insects:
The comments talk particularly about success with maggot and leaf miner prevention.
I finally got though an entire season using row covers and it worked out great. I planted carrots and spinach (and some cilantro snuck in there as well, somehow) back in December and we are still harvesting them now with only minor leaf damage on the spinach and no apparent damage to the tops of the carrots.
The only problem was that I didn't make the PVC cage quite tall enough. It was about 18" tall and both the carrots and spinach made a "tent" over the past month and the greenery starting pushing above the tops of the PVC. Very successful, though I'll need to purchase more covers as the ones I have are starting to look a little ratty (it gets really windy here at times; that plus the act of pulling them off and on for watering tore through the mesh eventually).