Swallowtail butterfly caterpillar

[sunshine]

More future butterflies!

[dondc]

It may be a monarch caterpillar. 
Looks like it is feeding on milkweed.
Swallowtails do eat toxic plants for protection, but more likely carrot greens and fennel.
Their main defense is they look like monarch caterpillars.

I'm no expert but I'd say that is a monarch.

https://owlcation.com/stem/caterpillar-identification-2

[sunshine]

I meant to correct that yesterday but got a phone call and forgot to fix it. You are correct, it's a Monarch.

[dondc]

It is hared at work on the milkweed leaf.  when they get that big they grahold of the leaf at the stem, they start at the end of the leaf and eat the whole thing up to the stem.  The plant will recover in a couple of weeks.