Question
What causes the leaves of knockout roses to become riddled with little holes that appear white?

Answer
Female sawflies (the adult life stage of rose slugs) lay individual eggs in slits along the margins of leaves. When the larvae hatch they begin feeding on the undersides of leaves. Once they are fully grown, they drop to the ground and pupate in cocoonlike chambers in the soil, then emerge as sawflies. Rose slugs look more like caterpillars than slugs. They are the larvae of primitive wasps called sawflies.  For  large infestations you may need to spray the leaves thoroughly on both sides with insecticidal soap, neem, or horticultural oil. Most insecticides are not effective against rose slugs because thy need to be smothered.  Safers Insecticidal Soap will solve the problem.  Don't forget to also spray all around the base of the rose bush as that is where they are coming from. When these catterpilars start they can only make small holes but as they get bigger then the damage is increased.