Congratulations, you have a Danaus plexippus, or more commonly known as a Monarch!

The yellow things are aphids.

Will the aphids harm the plant?

They haven't hurt my plants. Last year. I killed a lot of them and used the spray nozzle on the hose to knock them off the plants. This year I haven't done anything and don't see any damage.

Congrats on the Monarch caterpillars!

Congratulations on the Monarchs Marilyn! So awesome that you cared enough to plant a butterfly garden. Your efforts are definitely paying off. :o)

I'm thinking I might have to plant more milkweed plants. I did see another cat on the plant yesterday and they are really stripping the plant. Are there another plants they might go too? I have some red Pentas, some butterfly plants and several salvias.

Marilyn, they only eat Milkweeds. Do you have any Butterfly Weed? It is a form of Milkweed.

No I don't have any butterfly weed. I don't have any more room to plant anything else right now. Hopefully the milkweed will sustain the couple of cats on it. Then when they go to the next stage I hope the milkweed will be able to recover.

Milkweed does recover rapidly, Marilyn. Your cats will start eating the branches when all the leafs are gone. It's best to have a few plants so that you have enough to feed them. Somehow there is almost always more room.