All my caterpillars (Black swallowtails) sudde…

All my caterpillars (Black swallowtails) suddenly died. They just… quit eating. One was writhing in the ground, refusing to eat. We had at least 25 very healthy caterpillars and once they ate one plant to the nubbins, they just refused to move to another, fresher plant. Do you have any idea what caused this?

Oh no, I’m sorry to hear about your loss! For moth caterpillars, they will go prepupal, where they stop eating, wander around, generally look sick, then stop moving before they pupate. But for butterfly caterpillars, you wouldn’t expect this, since they need to make their silk pad and dangle for a bit before they pupate.

Where did you get their first plant vs their second plant? Are there any differences between the two?

I have a couple ideas about what might have happened:

1. The plants weren’t the same exact species. A lot of caterpillars will eat similar, though closely related, plant species. Monarchs and milkweed are a common one—as far as I’m aware, any milkweed will do. Swallowtails eat citrus plants (right?), but I don’t know if they will eat all similar plants, or only a few. Additionally, although some caterpillars will eat whatever they find, some will only eat what they ate when they first hatched or before their last molt. I raise Unicorn Prominent moths, and I always find them on my baby plum tree. But, they will eat elm, so I try to move them over to elm. My last batch got big enough by the time I tried to move them that they would only eat plum.

2. If the plants were exactly the same, is it possible one had been sprayed or coated with something? Pesticide, oils from another plant, animal urine? Or is it possible the first plant had been sprayed? I had the unfortunate experience of feeding my Polyphemus caterpillars plants that had been upwind from pesticide applications. They grew slowly, and many developed severe constipation and died. It was very sad. But now I’m able to recognize pesticide poisoning in caterpillars.

Be aware: even if you buy your plants from a natural nursery that doesn’t spray, their supplier may have sprayed the plants. Your best bet is to grow the plants yourself, either from seed, or grow them enough that you can be sure you’re feeding fresh leaves only, or to get them from a trusted source who grew them from seed.

July 8, 2018