Hi
It is fine with both at any time but I think the monarchs can't switch from A.tuberosa to swamp milkweed because of the toxicity level of swamp milkweed is so high compared to A.tuberosa. Right now I have three black swallowtails and I have golden alexzamder rue dill parsley and fennel in the same cage and they all eat it. So it is probability ok I just got a swamp milkweed from a friend so I have always had common so you should be ok.

Thanx for the info :) I think I'll keep Monarch cats on syriaca just to be safe, but switch my BST cats to Fennel slowly when they're at #5 I still have the original Dill plant right next to my Fennel in the veggie garden, out of reach of critters. I'll let them work that down to zero and march onto the Fennel. Last year I switched them to carrots and they took longer to mature, plus it was the last flight of the season. They spent a week in their 5th instar.
I still have four BST eggs due to hatch tomorrow.

John

I think you can switch host plants at any time, assuming it's a legitimate host plant. If the two plants are very different, the cats might not like the new plant and might not eat it.

I feed my palamedes swallowtails sassafras all the time, and they don't seem to know the difference. You can feed spicebush swallowtails sassafras or spicebush without them even knowing it, but then these plants are all in the laurel family, closely related.

Sherry

I'll try one or two on my Purple Milkweed, which seems to be very similar to Common. I still have one more Common Milkweed to "work", so I'll stick a couple there. I'm afraid to place them back where I found them due to regular spraying. I happened upon these between spraying, which I believe is monthly. (They always place little warning flags after spraying)