Monarch eggs

I have a few butterfly weed plants that I have lovingly planted and willed to grow. Lo and behold a couple years ago I noticed they were really chewed up. As I looked closer I noticed monarch caterpillars just munching away on them. Hey wait a minute! I had no idea that mom monarch would lay her eggs on the butterfly weed as well as on the milkweed. However, they obviously are related when you look closely at the plants. I have seen only one monarch flying around a few weeks ago but apparently someone was busy when I wasn't looking.
I counted at least 12 eggs on my plants. It takes about 4 days for the eggs to hatch and I'm not sure where in the cycle these are. But as soon as they hatch and I'm able I will transfer the caterpillars to actual milkweed. We have quite a bit of it around. I love the whole process but I don't want my butterfly weed eaten up!