100 hungry mouths to feed

A couple years ago I lamented that I didn’t see silkmoths much anymore.That was the year our friend Dr. Steve gave us a couple of Polyphemus (Anthera polyphemus) cocoons, so we got to see (and release) the adults that emerged. Dr. Steve has a big piece of property nearby, and his brother owns a tree nursery. They are able to find and raise several species of silkmoths every year. This spring, they gave us a bunch of eggs, mostly Cecropias, Hyalophora cecropia, but also a few Polyphemus and some Lunas (Actias luna).

The latter is, I believe, one of the most beautiful insects in North America. In all my time in the field these past decades, I’d never seen one, even though at work we have abundant black walnut, one of their favorite larval host plants. I intended to raise all three species and release them at work, with the hope of augmenting very sparse local populations.

We started out with about 50 Cecropias, 15 Polyphemus, and 20 Lunas. I know that doesn’t add up to 100, but as usual we were also raising some Monarchs, and Black and Giant Swallowtails. This year we’re also raising some Carolina Sphinx (Manduca sexta). You know these as tomato or tobacco hornworms. Purposefully raising what many people still consider the scourge of their garden may seem really weird, but they seem to have been wiped out in this area and I’d like to see a few adults again. So yes, I have been raising over 100 caterpillars this summer (so far!).

Luna eggs hatching

The silkmoth ranch starts out innocently enough. Once the eggs begin to hatch, the tiny caterpillars are put in small groups in tupperware containers with damp paper towels and some leaves of their host plants. I raised the Lunas on black walnut, the Cecropias on box elder and lilac, and the Polyphemus on maple and dogwood. Everything but the walnut was available in the yard.

Cecropia eggs and newly hatched larvae.

Twice a day the containers have to be cleaned out and restocked with fresh food. This takes longer than you might think. It’s pretty common to lose a percentage of young larvae to bacterial or fungal infections, mishaps, or parasitoids (if you raise them outside, more on them later). I was careful to make sure no frass (poop) was clinging to the tiny cats when I cleaned the containers. When they were all spiky little things only a few millimeters long, this required a soft brush.

They outgrow sandwich-sized containers pretty quickly, and graduated to plastic shoeboxes within 10 days. At this point, they were still stacked up all over my office.

The next upgrade was to aquariums and empty cat litter buckets in the garage. Food was kept fresh by inserting branches into plastic carry-out cups of water — the lids with the holes for straws are perfect for holding the plants while not allowing the larvae to fall or climb into the water.

We kept some on the bench by the open window, and the rest we moved outdoors in dappled sunlight during the day, and back inside at night. It’s important they get some light and be exposed to natural cycles of daylight. This, apparently, influences whether multi-brooded species will emerge after pupation the current year, or overwinter. At this point, fortunately, I was home on vacation, because it took about 2 hours a day to tend to all these things!

The Cecropias in particular were voracious, and there were a lot of them. I think we only lost around ten. By the time they were 3 inches long, I took a dozen to work and let them go, hoping they’d fend for themselves. By the time they started spinning cocoons, they were the size of large sausages. This species is supposed to be single-brooded here, so we’ll keep these cocoons overwinter and release the adults next year.

Polyphemus cats. There was much more variation in their individual
growth rates compared to the other species for some reason.

We lost quite a few Polyphemus to some pathogen when they were still very small. Once they reached second and third instar, they had been moved to mesh-covered containers outside. One day, we found two very tiny wasps had made their way through a small opening in the mesh. They were some sort of female braconid wasp. These are common parasitoids of many types of caterpillars. Within days a few more Poly cats died. The remaining ones seem fine and are starting to spin cocoons, but we shall see.

We ended up with 11 Lunas spinning cocoons. I’d heard conflicting reports of whether they were double-brooded or not in Michigan; most people told me these would overwinter. But they were wrong.