Q: I am having trouble with my Knockout Roses. I have had these plants for quite a few years. Last year I lost two plants and was not able to see any reason why. They just began to turn yellow and died. Today I was pruning and observed that on the canes with leaves turning yellow and brown. I could see some small holes in the dead cane. When I cut the cane, in the green area I see a hole in the center of the cane.. I continued to cut this cane about every inch back to the main trunk. The hole was the entire length of the cane.
A: I think you have rose cane borers. Several species of insects are attracted to pruned roses, where they lay eggs on cut twigs. The eggs hatch into larvae which bore into the soft pith of the rose stem. The stem and foliage around their home dies.
There are no effective rose borer sprays. Most folks just prune out damage when seen. The best way to prevent them is to get a small bottle of white glue, put two drops of food coloring in it, stir it up with a thin stick and put a dollop of colored glue on stems when you prune roses in spring. The glue prevents the insect from boring into the cane.