Promethea Moth (Callosamia promethea) caterpillar.
Photo: Mark Conboy.
This morning found me scouring the mosquito-infested forests of the Norfolk sand plain (Norfolk County, Southwestern Ontario) for botanical treasures like Dwarf Chinquapin Oak (Quercus prinoides), a rare species in Ontario. Happy as I was to find this diminutive oak, I was equally as excited to come across a chunky Promethea Moth (Callosamia promethea) caterpillar.
Promethea Moths, and their decorative larvae are fascinating for countless reasons, one of which is that they are becoming ever more uncommon throughout North America's eastern forests. The reason? Well, it may have to do with a most interesting but unfortunate link to the infamous Gypsy Moth (Lymantria dispar) and an obscure parasitoid fly, Compsilura concinnata.
This link harkens all the way back to the days of the fledgling, and ultimately unsuccessful, North American silk industry. Domestic oriental Silk Moths (Bombyx mori) had been brought North America in the early 1800's in an effort to kick start silk production that could rival China's monopoly. Over the years, various efforts were made to employ our native silk moths in the industry as well, but none of those succeeded. Eventually, the Gypsy Moth was introduced from Europe in 1869 as part of a hybridization experiment wherein domestic Silk Moths would be crossed with Gypsy Moths to create a silk-spinner like no other. The experiment failed, but the Gypsy Moth went on to have a much bigger impact in North America then it ever would have as a mere silk producer.
Domestic Silk Moth (Bombyx mori) caterpillar and cocoon.
Photo: Srithern (Wikimedia Commons).
Gypsy Moth (Lymantria dispar) caterpillar.
Photo: Materialscientist (Wikimedia Commons).
The Gypsy Moth is infamous. It's cyclic population explosions are legendary, sometimes causing the defoliation of thousands of trees across vast patches of countryside. Various attempts have been made to eradicate it, including the introduction of no less than 10 foreign insect species, the most notable of which, as far as Promethea Moths are concerned is Compsilura concinnata.
Compsilura concinnata is a parasitoid, it searches out a host, infects that host with a maggot, and ultimately that maggot kills the host. This behaviour is in stark contrast to a traditional parasite, which usually doesn't kill its host, it just takes advantage of it, either for food, a place to reproduce, or a free ride, etc. Compsilura concinnata is a small, grey, hairy-looking tachinid fly (family Tachinidae), one species among many in that are parasitoids of caterpillars. Some tachinids lay eggs on their caterpillar hosts, and when those eggs hatch the fly maggots burrow into the caterpillar, eating it from the inside out, saving the vital organs until last, so the caterpillar lives as long as possible; tachinid flies don't like spoiled meat like their blow fly relatives. Other tachinids, like Compsilura concinnata, larvaposit on their hosts, they lay maggots directly onto caterpillars. Those maggots burrow in and selectively feed on certain organs, just as their egg-borne relatives do. In the end, the result is the same, a dead caterpillar, and a new parasitoid, ready to strike its next unsuspecting victim.
So, after some digression, we return to the Promethea Moth caterpillar I spotted this morning. It's been suggested that the population declines apparent in Promethea Moth and some of its relatives, such as the Tuliptree Moth (Callosamia angulifera), in eastern North America, are likely attributable to parasitoidism by Compsilura concinnata. Promethea Moths don't seem to have any defenses against Compsilura concinnata, but that's not to say that it has no defenses at all. After all, the Promethea Moth did just fine when faced only with native parasitoids, including several species of tachinid fly. For example, the tuberacles behind the caterpillar's head and near its tail may actually mimic fly or wasp (many wasps are also caterpillar parasitoids) eggs, giving the appearance that this host has already been taken, and possibly heading off a real parasitoid attack. Promethea Moths snip off partly eaten leaves from their food plants, letting them fall to the forest floor. This may be primarily a defense against avian predators that may search for caterpillars by looking for partially eaten foliage, but it may also be a way to avoid the attentions of parasitoids, which could conceivably cue in on damaged foliage. Other moths, particularly sphinx moths (family Sphingidae), will writhe back and forth when approached by a parasitoid, in an attempt to scare it off. But for whatever reason, none of the strategies that have worked for Promethea Moths against native parasitoids seem fit to keep the depredations of Compsilura concinnata at bay. Hopefully, despite the additional stresses Compsilura concinnata comes to bear on our Promethea, Tuliptree, and the other native silk moths, hopefully we won't lose these magnificent denizens of the night.
Adult female Promethea Moth.
Photo: Mark Conboy.
As for Compsilura concinnata's effectiveness against the Gypsy Moth, well, it wasn't! Whatever control measures it did provide probably weren't significant enough to warrant its introduction. Perhaps a more successful biological control has been the fungi Entomophaga maimaiga, which has been repeatedly introduced and seems to have made a fairly significant impact on the Gypsy Moth population, slowing its westward spread. What native organisms the fungi might adversely affect remains to be seen...
