By far and away the best stuff to use is Debris Netting, you can get it on Ebay, its about £28 for a roll 2m x 50m. Fine enough to stop practically every thing down to a fruit fly, but doesnt stop the rain ,and only slightly shades the sun. I build frames 2.2m x 1.2m x 0.6m out of free 2.2m x 1.2m pallets covered in it and move them around the plot as needed. Its nylon, so is indestructible and weatherproof.