My milkweed seems to be a magnet for aphids every year and yet I don’t worry too much since they seem to disappear later in the year. One of the reasons is that these aphids have Lacewings as predators. According to the video below Lacewing larvae can eat 200 to 300 aphids. It’s interesting that the Lacewing eggs are mainly where there are aphids and not on milkweed plants without aphids.
This picture comes from one of my plants, so you can see that these eggs alone could easily eat 2000 aphids.
Lacewings will eat also small caterpillars, but they certainly will have plenty aphids to eat before they find my caterpillars.
On my milkweed, if I find any Monarch eggs I usually put them in a cage to protect them.
Butterfly Gardening 
Tom, I saw one Great spangled fritilary in my garden the other day. He was beautiful, never had seen one. Also, Giant Swallowtail came through today. For a very little garden I guess I am doing ok.