Snails and slugs will have a field day eating the new shoots on your plants. There are various methods of controlling them, depending on how Nature friendly you want to be. There are commercial products available and I think none of these will affect your Monarch caterpillars as they go on the ground where they’re eaten by the molluscs.
Ants also enjoy your swan plants as they “milk” the aphids which will soon come and enjoy the plants – watch out for little orange crab-like things around the new shoots. Now these are more tricky, as if you use a commercial product it will affect the Monarchs. There are plenty of suggestions elsewhere in the forum on how to control aphids – and a handout which is downloadable under Species / Pests . Personally to control the ants I use an ant poison which I place in tubing in the ground at the foot of the plants. It’s an ongoing task though, and I’m sure other people will suggest other ideas.