Once milkweed has been treated with pesticide, it becomes toxic to insects, including the Monarch caterpillars that use it as a food source. The pesticide needs time to dissipate, usually months, during which time the plant either needs to be covered with netting or similar, or if in a pot, moved indoors so that Monarch butterflies will not try to lay their eggs on it. Remove caterpillars already on it and place them on safe foliage; they may or may not survive but at least you're given them a chance.