I've found that caterpillar (and most other pest) problems go away if you grow near some established fennel (the leaf type, not the bulb type) somewhere near where you like to grow. When in flower, fennel attracts hoverflies, dozens of species of parasitoid wasps, and several species of predatory wasps - all of which attack caterpillars. The plants will literally be teeming with them. I manage a large commercial grow and I've found that a fennel "hedge" on one edge of the field completely eradicates caterpillars 50-100 feet away and results in only very minor caterpillar damage up to 250 feet away with no spraying at all. Fennel should be established in the fall prior to your growing season and requires next to no care. It is hardy in USDA hardiness zones 4-9 which covers almost the entire continental USA and Europe. It will be in flower from early- to mid summer through frost. Once established, fennel will typically naturalize permanently. Mine are 6-9 feet tall right now.