Subject: Mysterious milkweed eating caterpillar
Location: Morgantown, West Virginia
September 14, 2012 7:21 am
Hello! I am an avid observer of bugs, but this is a new one to me. I am familiar with the common bugs that eat milkweed – monarch and milkweed tiger moth caterpillars, etc. But this one is completely different. I have found several in a large pasture with a high concentration of milkweed plants. Curiously, I have seen no milkweed tigers at all on these plants, but there are many of these guys. I would be very grateful to learn what they are. Thank you so much for your WONDERFUL website.
Signature: Bug Watcher Guitry
Dear Bug Watcher Guitry,
Your unknown caterpillar is most likely one of the Woolly Bears or Tiger Moth Caterpillars in the subfamily Arctiinae, but your claim that it is feeding on milkweed has us puzzled. We couldn’t turn up any likely species candidates that would feed on milkweed in our initial search, and the plant in your photo does not look like common milkweed. Please look at this marvelous page with photos of common milkweed as a reference.
Thank you Bugman,
I am almost absolutely certain that this caterpillar is feeding on Milkweed. It isn’t ‘common milkweed’ but there are so many species of milkweed. And I have also observed monarch butterfly caterpillars feeding on these same plants, (I mean, not the exact plant – the caterpillars don’t seem to share except with each other). I found two wooly ones on the same plant, but I have seen monarch caterpillars on them, and I have even seen adult female monarchs laying eggs on them. I am no botanist of course – but maybe you could point me to a good plant ID site… THANK YOU so much for your response. I am always excited to learn about new bugs.
Hi again Bug Watcher Guitry,
Try breaking the stem of the plant to see if it oozes a milky sap. If you are certain Monarch Caterpillars were feeding on this plant, then we are pretty certain it must be a milkweed despite our own doubts. The Butterfly Encounters website has a milkweed gallery with several species. The plant in your photo does not appear to be among those in the gallery. Seeing a photograph of the bloom might also help.
Update from Bug Watcher Guitry
Hello again, Bugman,
I did make it back to break the stem of the food plant. It did indeed ooze a milky liquid (see attached photo) but I think it is likely that I am mistaken about the plants being milkweed. When you mention the flowers, I realized I did not recall ever seeing or smelling their flowers. I also attach a picture showing the seed pods that have formed on these plants. I looked at the milkweed gallery you linked me to, and I agree, there isn’t a matching plant. The red stems are quite different from any of the plants shown there. I was wondering if I could perhaps keep one of these caterpillars in captivity to see what kind of moth it turns into? Would you recommend this, or is the caterpillar unlikely to survive if I keep it in captivity? (I have raised Monarch butterflies from egg to adult successfully quite a few times.) Do Arctiinae spend the winter in cocoon? Would I need to keep him outside once he pupates? I am just very curious, and I am enjoying the mystery with this particular caterpillar!
Very many thanks.
Bug Watcher Guitry
Hi again Bug Watcher Guitry,
The milky sap would indicate that even if this plant is not a milkweed, Monarch Caterpillars might feed upon it. The pods do resemble milkweed pods. We actually remember seeing this plant growing up in Ohio, but we cannot tell you its identity. Try keeping the caterpillar outdoors in a protected area over the winter to see what moth emerges.
Update: October 11, 2012
We just approved a comment that identified this as a Delicate Cycnia Caterpillar, Cycnia tenera, which according to BugGuide, does feed on milkweed. It is also called the Dogbane Tiger Moth and according to the photos on the Primitive Ways website, the plant it was found feeding upon is Dogbane, Apocynum cannibinum.