I am guessing that your kale and collard greens are attacked by whiteflies. Of course I cannot be completely sure, since you are referring to "butterflies", but since nothing can be seen on the plants whiteflies are a likely choice. Whiteflies are very small and have usually white wings. The larval stage is extremely small and can only be seen with a hand lens.
Short term solutions can be neem oil based products and insecticidal soaps. These products need to be applied to the underside of leaves where whitefly larvae are mostly found. Furthermore this should be done during the early morning hours when temperatures are lower and adults are less active. Long term solutions are aimed at preventing outbreaks or alleviating the pest's impact. Plants that are healthy will be able to better withstand damage from insects and diseases.
Therefore proper fertilization and irrigation are the first line of defense. Natural enemies, such as lacewings and parasitoid wasps, that can cause mortality are attracted by flowering plants. Good choices in a vegetable garden are herbs and vegetables that are allowed to flower, such as fennel, basil, cilantro, dill, coriander, and others.
Once the crop has been harvested, left over parts of the plants should be destroyed so whiteflies cannot continue to reproduce and build up and migrate to subsequent crops. Hosing down plants with water may also help to destroy adults and rinse of honeydew, which is the excrement produced by whiteflies.
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