I have used a mix of approx 1 teaspoon sodium bicarbonate and 2 teaspoons (a squrt ) of fairy liquid, which contains salt ,in 500ml of water. Warm water will dissolve fully. I used a cheap trigger spray but expect it to rust in the spring ballvalve. The trick to bring these back to life is a single sharp knock on the back of the trigger, to free the ball from its rusty socket. Additional salt if the varmints walk away! If you are early in detecting these, as hatchlings, you may be especially effective. Fully mobile catapillars tend to drop off ,to relocate if allowed to.
Be cautious if treating other plants because they may be susceptable to salt, which is not a problem (in moderation) with brassicaes