Lightning Fast Growing Painted Lady Caterpillar is Ready to Pupate

The Painted Lady caterpillars (Vanessa cardui) that we found at Tilden Park on 7/9 are developing at an incredible speed and the older of the two is already ready to pupate.

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When we first found two Painted Lady caterpillars (Vanessa cardui) exactly nine days ago on 7/9, the older of the two was in fourth instar and the younger one was in third. For the first few days after we started keeping them, we were struggling to provide them with an ample supply of fresh Milk Thistle (Silybum marianum) leaves, which was the host that we found them on. Milk Thistle is extremely common around here as a weed but for some reason most of it had either been pulled out by city maintenance or simply died on its own now that it is getting late in the season. The only thistle we could find were tiny little patches far from home regenerating from plants that had been cut down. They were so small that they were hardly identifiable as Milk Thistle without the characteristic tall stem and poofy pink flowers!

More than once, we had to let the two starve in their little plastic cup for up to half a day. Normally that would be pretty terrible, quite obviously. Molting in caterpillars is completely regulated by the endocrine system and juvenille hormone (JH) doesn't care whether the caterpillar is well fed or not; it will very slowly continue to drop regardless and eventually force the caterpillar into apolysis unless it has already died from starvation, of course. This would take a hefty toll both on the caterpillar's growth rate and overall size. Not to mention the weather has been at its coolest this whole summer this past week (low to mid 60's with fog) and, since caterpillars are cold-blooded, growth rate is positively correlated with higher temperatures.

But these Painted Ladies are crazy, to say the least, and are some of the fastest growing caterpillars in proportion to their size (usually larger caterpillars take longer to grow and vice versa) that we have ever witnessed regardless of the unfavorable conditions. In just nine short days, the older caterpillar powered through the fourth instar, molted into fifth, and is now in the "J" formation prepupa (typical of brush-footed butterflies [family Nymphalidae]). The younger has raced from third instar all the way to fifth and will also surely be a prepupa within a day or two. Now that is really fast. Most caterpillars of their size (about two inches but relatively narrow-bodied) usually take almost a week to get past the fourth instar and around one week for the fifth and final one. 

- Brian