Answer: Caterpillars such as rose budworms sometimes attack rose bushes. Chewed leaves and/or buds, and telltale webs or nests among the foliage indicate the presence of a caterpillar of some kind. Pick off the caterpillars and destroy the infested buds, leaves, and nests. Spray the rose bush foliage weekly with a product containing Bacillus thuringiensis (Bt) while the caterpillars are feeding. They will ingest the bacterium, sicken, stop eating and die in a matter of days. Repeat the Bt sprays after rains and until the caterpillars disappear.
If you haven't already, prune the affected rose bushes back to encourage healthy new growth and remove any insect eggs that might be lurking there. Yellowing leaves can indicate overly wet soils so be careful with your watering. One deep soaking per week is about all your Knock Out roses will need.
Good luck with your plants!
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