Quite often people will ask why a caterpillar that is known for hosting on one plant is eating a different one. Even today, people are discovering new host plants for species. Some eat it only in captivity and others eat it in the wild. But the question we first must ask is, “IS it eating the plant?” Quite often, it isn’t eating it at all.
Caterpillars move off a host plant for several reasons. Some move off to molt, to pupate, or simply are lost, working their way back to their host plant.
We’ve seen many species of caterpillars on plants that weren’t their host plants. Almost every time, they were molting or pupating.
Much to our amazement, we found a Buckeye chrysalis on a snail. You never know where they will choose to pupate.
We had Buckeye butterflies in a screened garden room. To my surprise, they stripped a butterfly bush (Buddleia davidii) of leaves, pupated, and emerged with no problem. Later we found caterpillars on butterfly bush in the wild. Evidently it happens but rarely.
Although there was no question whether the Buckeye caterpillars were eating butterfly bush leaves, we did tests, feeding that particular bush to caterpillars in captivity. They ate it with no problem. We tried another butterfly bush, one that bloomed a different color, and they all died. We bought the lavender blooming bush from a nursery without a label. When we returned to the nursery, they had no idea which cultivar it was.
Monarch butterflies often pupate on milkweed. They also often pupate off milkweed.