Source: http://www.lianginsects.com/timeline/a-painted-lady-caterpillar-mixed-in-with-the-west-coast-lady
Timestamp: 2019-04-22 14:59:04+00:00

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Today we realized we have a fifth instar Painted Lady (Vanessa cardui) caterpillar mixed in with out fifth instar West Coast Ladies (Vanessa annabella)!
Yesterday, while we were cleaning out the petri dishes that we our West Coast Lady (Vanessa annabella) caterpillars in, we took note of which ones were in apolysis as always and took care not to disturb them. We had quite a few -- maybe five or so -- fourth instar individuals in apolysis for fith instar. We noticed then, that one of them seemed much larger than the others, larger than perhaps a young fifth instar. At the time, it never once crossed our mind that it might be a Painted Lady (Vanessa cardui) caterpillar, which are normally significantly larger than V. annabella but today we found out!
The fourth instar in apolysis looks more or less identical to the V. annabella fourth instars, so it was no wonder that we didn't realize earlier. Even among different V. annabella caterpillars before the fifth instar, there is very little variation (there is incredible diversity in coloration in the fifth instar， though); every single caterpillar we had looked pretty much identical. We don't know exactly when the V. cardui caterpillar molted, but it must have been a while since we checked this evening because we almost jumped back when we opened our petri dish and saw a monstrous caterpillar towering over even the largest of mature V. annabella fifth instars. Besides the ungodly size, its appearance was now distinctly different than any V. annabella caterpillar. Its body much longer and less fat compared to V. annabella, more like a Red Admiral (Vanessa atalanta). In addition, its coloration actually is very similar to that of a typical black morph V. atalanta that we have never witnessed in any V. annabella; the base is completely black with not red or yellow striped down the back seen in all of our V. annabella, the spiracles are outlined in yellow, and the feet are reddish. At the same time, it also doesn't look quite the same as V. atalanta. See images below.
The strange part about finding this V. cardui caterpillar is that mallow (Malva) is not a particularly popular host plant. While V. cardui is probably one of the only butterflies in our area that are truly polyphagous, capable of feeding on low plants from a huge number of unrelated botanical families, it still seems unlikely that they would utilize mallow unless there is a population spike and food sources are in high demand, but V. cardui is certainly is not very common in our particular region at this particular time. A regional preferences for host plants is plausible but unlikely -- why would they compete with the abundant V. annabella which exclusively feeds on mallow? If anything, we would assume that V. cardui would more commonly utilize the milk thistles growing here but to this date we have never found one on this particular host plant in Albany.

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