Back on April 7, Noah stopped by to get some eggs and some seeds for his vegetable garden. Just as he arrived and Julia was walking up to meet him, she spotted a caterpillar with a tiny wasp on it. I dashed into the house to grab my camera, and the wasp was still hanging around when I returned.
Amazingly enough, the wasp still clung to the caterpillar after it was picked up and placed on the driveway for a better view.
The caterpillar proceeded to go running across the driveway at a steady clip, so it is somewhat miraculous that I was able to get a few sharp close-ups of the 1.5-mm eulophid wasp.
Now, I was assuming the wasp was inserting eggs in the caterpillar, but as I reviewed the photos on my camera after the wasp finally flew away, I realized she had actually been laying eggs on the surface.
Twenty-six of them, to be precise. (One of them is obscured in the photo below, but see the following photos if you want to check my math.)
So naturally I scooped up the caterpillar once more and put it in a jar so I could chronicle the development of the wasp’s progeny. To do this, I’d need to keep the caterpillar fed. All I knew was it was some kind of owlet moth (Noctuidae), but Sam Jaffe of The Caterpillar Lab recognized it as a middle instar Leucania species. He said if I could rear it through another instar or two it might be possible to narrow down the species possibilities. I wasn’t sure how much time this caterpillar had left, but I gathered from the Wagner et al. guide to owlet caterpillars that Leucania species are generalists on trees and shrubs, none of which had leafed out yet, so I collected an assortment of leaves from around the yard and hoped for the best. The caterpillar became pretty inactive and didn’t seem to have much of an appetite, but it ultimately munched a bit on some dewberry and strawberry leaves.
The next day, the caterpillar had contracted, and I thought maybe it was getting ready to molt. Would it be able to shed the eggs before they hatched?
The eggs had already darkened noticeably.
The following day, the caterpillar had stretched back out, without having molted.
With the caterpillar sitting still and with its skin unwrinkled, I was able to get a photo clearly showing all 26 eggs.
On April 12, nothing much had changed.
On April 15, the eggs had begun to split open.
On April 18, the caterpillar had become very active again. The white larvae were now mostly emerged from their darker eggshells, but had not moved away from them. I presume their mouthparts were now latched onto the caterpillar, so it was understandable that it had become a bit restless.
The next morning, I was surprised to find the caterpillar dead, with 26 fuzzy white cocoons spun on a nearby strawberry leaf.
But these were braconid cocoons! The poor caterpillar already had 26 braconid larvae living inside it when that eulophid wasp showed up to lay another 26 eggs on it. The eulophid larvae still clung to the caterpillar’s corpse, which was now riddled with dark spots where the braconid larvae had emerged.
I kept the caterpillar to see if the eulophids would be able to keep developing now that their host was dead. They had grown considerably by the next day (April 20).
On April 21, none had grown further and several of them had begun to shrivel up. Mold had also started to grow on the caterpillar.
On April 24, a few were still hanging onto life, but still weren’t growing any.
A few days later, the caterpillar was consumed with mold, and all hope was lost for these few lingering larvae. I believe these eulophids belonged to the genus Euplectrus, the larvae of which continue to feed in a tight cluster throughout their development, turning a striking bluish color when mature*:
When finished feeding, Euplectrus larvae disperse and spin a mass of loose cocoons—which could easily be mistaken for mold growth—beneath the remains of their host caterpillar.
Anyway, on May 4, all 26 of the braconid wasps emerged from the cocoons that had appeared on April 19. They were very zippy but I managed to get a few decent photos.
They belong to the subfamily Microgastrinae, and I’m guessing the genus Cotesia. Someday, when the staff at the Canadian National Collection of Insects are allowed to go back to work, I’ll send them to José Fernández-Triana and he can tell us exactly what they are. Assuming they have a name.
I suppose now is a good time to follow up on this post from eight years ago that included a pile of Cotesia cocoons with a sprinkling of Eulophus pupae on top—evidently the result of a second wasp ovipositing on an already parasitized caterpillar before it was too late for both sets of offspring to complete their development.
I collected this in September 2012, and the wasps emerged the following spring—a dozen or so Eulophus between April 28 and May 11…
…and about 20 Cotesia between May 6 and 23.
From one of the Cotesia cocoons, a hyperparasitoid emerged—an ichneumonid in the genus Itoplectis.
Whether the mother of the Itoplectis laid an egg in the Cotesia cocoon or had to locate a Cotesia larva within the host caterpillar, I don’t know. Either way, these were two unlucky caterpillars.
* Added 5/6/2020: The Caterpillar Lab has been rearing similarly parasitized Leucania pseudargyria / ursula caterpillars this spring, and these particular Euplectrus larvae are evidently pale yellowish throughout their development, turning a little pinkish when mature.