It seems that they lay eggs only on milkweed plants. The caterpillars use the sap and leaves, which I think contains some toxic materials. This could be why Monarch Butterflies have their orange and black motif. Bright colors on creatures usually mean they are poisonous. 

Rothbardian_fanatic

I told my son about your comment, and he has a question for you: why do monarch butterflies like milkweed plants specifically? 

cryotosensei

science

Life cycle of butterflies

I like seeing them stopping by my milkweeds and flying around them. There are not as many around as when I was young.