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As anyone who follows this blog knows, I raise caterpillars into butterflies every summer, namely the Eastern Black Swallowtail (Papilio polyxenes) and the Monarch (Danaus plexippus). I raise those two because they’re the ones that show up in my garden, I have larval host plants for them, I can find the eggs/caterpillars, they’re native species, and I like them. This is in contrast to the cabbage white (Pieris rapae), which also shows up because I have larval host plants for it, but is an exotic pest here (and I destroy the caterpillars whenever I can find the little bastards).

Over the years I’ve noticed some interesting trends. For example, the EBS first showed up on my carrot plants (Daucus carota). In fact, I started raising caterpillars because of these, as a way to manage the damage to the carrots while preserving the caterpillars. At that time, I didn’t grow dill. Now, however, I’ve noticed that the EBS females prefer to lay eggs on dill (Anethum graveolens) rather than on carrots. Despite the fact that dill leaves are so feathery and fine, the caterpillars seem to draw a lot more nourishment from dill than from the much more substantial carrot leaves – I find they eat much less of it before they pupate. It’s amazing to think that on some level, the EBS females are aware of this.

This year I’ve raised two generations of EBS so far, and this year I’ve noticed that the first batch (about a month ago) was mostly female and the second batch (just released) was mostly male. I don’t know if this is coincidence or just something I never noticed before.

Moving on to the monarchs: well, I haven’t raised monarchs in two years and it’s nice to be doing it again. Last year was very hot and dry and my milkweed (Asclepias incarnata) patch wasn’t really up to scratch, so I guess any monarch females that came visiting weren’t impressed. The year before that was the year of no monarchs, for some reason.

This year it’s been better. I even had the rare opportunity to see a monarch couple in a courtship dance. Right now I have seven monarch caterpillars in captivity and going strong.

I did have a bit of a scare with them, though. When I was gathering eggs, I found a caterpillar on one leaf. It wasn’t a monarch; if anything it looked more like a rose slug (larval form of a type of sawfly; Hymenoptera sp.). At any rate, it didn’t look like anything I knew feeds on swamp milkweed. It also didn’t react to being poked, which in most larval insects usually signifies one of three things:

  1. It’s in ecdysis, i.e. about to moult and shed its skin.
  2. It’s sick.
  3. It’s been parasitised.

…turns out it had been parasitised by a wasp. There are many species of parasitoid wasps that lay their eggs inside caterpillars (and other things, such as spiders) and the wasp larvae literally eat their hosts from the inside out. In fact, I rely on them to control tomato hornworms (Manduca sexta). However, I just put all the leaves with the monarch eggs and this mystery caterpillar inside one box because I didn’t have an extra container to isolate the mystery caterpillar and didn’t know at the time why it was non-responsive. Until a few days later when I saw the adult wasp in the box and the mystery caterpillar was just an empty shell.

Normally this wouldn’t bother me, because just one of anything can’t reproduce, so my monarch babies were safe, right?

No. Some of these parasitoid wasps are parthenogenetic, i.e. the females can reproduce without the males (usually via a form of cloning; aphids are notoriously good at this). I’m certainly no expert on these wasps, so I had no idea what species it might have been. On the other hand, I actually was freaking out about nothing, because the monarch caterpillars then were so small that the wasp simply couldn’t have laid eggs in them (at least, I hope so).

In case you’re wondering about the famed monarch resistance to predation because of the toxins in milkweed, well, it actually doesn’t stop predation by other insects or spiders. I’ve never looked it up, but it’s certainly possible for milkweed caterpillars to have a wasp that parasitises them.

Anyway, it’s been a few days now and all the caterpillars seem to be fine, so thus endeth the saga.