something is killing my knockout rose bush
The leaves of the bush have brown spots and it looks like something is eating them but I couldn't see any bugs on the bush.
This sounds like a night feeding worm or perhaps a fungus if the spots are more black. If it is the might feeding worms you can usually catch them at their work by going out well after dark with a flashlight and investigate the foliage. Spraying with a good insecticide being sure to get some spray up under the leaves will usually get them. You can also get some Bayer Tree and Shrub granules and sprinkle the correct label amount around the rosebushes and work it in lightly, then water well.