Contact insecticides can be used to control this pest. They must be sprayed when the insect is feeding outside the plant. This form of control won't impact insects that feed inside the plant.
Organic insecticides can be useful. However, it must be applied more regularly. These contain natural horticultural oils or pyrethrin and pyrethrums. Again, it must be sprayed directly on the insect to work.
Synthetic insecticides including either of the ingredients cypermethrin, phenothrin, lambda-cyhalothrin, and deltamethrin are thought to provide sufficient control of these insects. These can still be toxic to some wildlife and a pollutant to aquatic life if residues leach into water systems, so please take care to research products and read instructions carefully before using them.
Never spray plants in flower.
Removing caterpillars by hand, coupled with cutting off infested leaves is the best treatment for controlling this pest.