Conlin’s Corner – Giants of the Night – Unraveling our Giant Silk Moths

We’re excited to introduce you to a new monthly installment on our blog: Conlin’s Corner, written by guest writer for The Land Between, Basil Conlin!

Basil is a lifelong naturalist, amateur lepidopterist, and native plant gardening enthusiast. He grew up in the Black Oak Savannas of Toronto, the Drumlins of Peterborough, and recently relocated to the rolling hills and valleys of the Haliburton Highlands where he operates a small native plant nursery, 'Haliburton Micromeadows', dedicated to making native plants accessible to everyone. When he isn't collecting seeds or dividing plants, he can often be found roaming the sidehills of Haliburton County observing the seasons and looking for wildlife to photograph. Basil went to Trent University for Conservation Biology, has worked on projects with the Nature Conservancy in the Rice Lake Plains, and was a seasonal naturalist at Algonquin Provincial Park. This month, Basil tells us about the secret lives of Giant Silk Moths!

May is a month heavy with the rites of spring: from trilliums to sucker runs, The Land Between awakens with new life. Insects make a noticeable appearance with the return of blackflies and mosquitos, to the annoyance of many residents (who were just getting used to the good weather!). Butterflies become a common sight on warm sunny days and queen bumblebees start gathering pollen and nectar at willow and Red Maple flowers. But it is during warm, humid nights in late May that the giants of our local insects begin to emerge: our Giant Silk Moths.

The Giant Silk Moths (family Saturniidae) are among our most familiar moths to the average person. Iconic species like the Luna Moth and Rosy Maple Moth are familiar to many. But due to their nocturnal lifestyle and their short adult lives (most species only live for a few days to a week), our diversity of Giant Silk Moths often goes underappreciated. Nine species occur within The Land Between, ranging in size from the Io Moth to the Cecropia Moth, considered to have the largest wingspan of any moth in North America. 

I faithfully wait for the emergence of Cecropia Moths each spring.  On warm, humid evenings during the last two weeks of May I setup a LepiLED, made to attract nocturnal insects, against a white sheet on the edge of a woodland and I wait. Some nights I see nothing, but eventually it happens. The insects arrive en masse and swarm the light! Non-biting midges and mayflies show up first in the early hours of the evening. Next come the brachonid wasps, then the first moths such as Eastern Tent Caterpillars, the inchworm moths, and steadily more and more insects until the white sheet is covered in dark specks. I get out my phone camera and start taking pictures of the visitors to submit later to iNaturalist.ca. Just as the traffic starts to slow, suddenly I hear a loud thud, followed by frantic flapping. 

A Cecropia Moth has just been attracted to the light! With a wingspan of up to 6 inches, it presents more like a small bat or confused bird rather than any sort of insect, especially not a resident of our cold northern woods. But this individual quickly settles down, and rests on the white sheet set-up to make viewing easier. It is indeed an insect, and a harmless one. 

In fact, Giant Silk Moths are the epitome of harmless insects – not only do they not possess stingers or pincers, they don’t even have mouths! They do not need them because they do not feed as adults, instead relying on fat stores accumulated during the larval stage the previous summer to sustain them until they mate and reproduce. Cecropia caterpillars are  polyphagous - they feed as caterpillars on a variety of native woody host plants including cherries, maples, and oaks, and even non-native lilacs. A look-alike, the Columbia Moth, feeds only on Tamarack in The Land Between. Cecropias can reach huge sizes, with the largest individuals reaching up to 4 inches long before spinning a silken cocoon to overwinter in late summer. 

These large silken cocoons (where the namesake ‘silk moths’ comes from) seem to come in two forms – a compact form with tight silk, and a baggy form where the silk forms a sort of large bag or outer layer around a tighter spun core. I have witnessed both forms in the field by locating cocoons on shrubs during winter and by rearing caterpillars to adulthood. 

One reason for these forms could be protection from fire. Cecropia Moths are most common in edge habitat and open areas, areas where there is an abundance of their favorite host plants – native cherries/ plums and small maples, oaks, and other shrubs. Open areas and young forests favored by Cecropias are maintained and renewed by natural wildfires. In fact, many of our forests are fire dependant ecosystems that require periodic fire (albeit only every century or more in the case of our pine forests). Lepidopterist Bill Reynolds has noted that in areas of the southeast United States that are maintained by annual prescribed burning seem to have a higher number of Cecropia Moth cocoons with the baggy form and that these cocoons are sometimes found partially burned with unharmed pupae inside (1) . Could the cocoons of Cecropia Moths be providing an evolutionary advantage against wildfires? There is still much to learn about our Giant Silk Moths.

Being among our largest and most well-known moths, it may be surprising to learn that some species of our Giant Silk Moths are still being classified, let alone studied in deep detail regarding their fire-resistant cocoons. Taxonomy for several Ontario species is still in flux. One species, the Pine or Canadian Imperial Moth, has recently received a new classification as a distinct species. Once thought to be a subspecies of the more southern Imperial Moth (Eacles imperialis), the Pine Imperial Moth (Eacles pini) received its new classification as a distinct species in 2023 (2).

The range of the Pine Imperial Moth, compared to species like the Cecropia Moth, is small. Outside of Ontario it is only known from Michigan, upper New York, and Vermont. In Ontario, they range from Sault St. Marie in the west to Cornwall in the east and north to North Bay. They seem to avoid the high elevation of the Algonquin dome; they are present in the low elevation of Georgian Bay and the Petawawa Rivershed but absent from Algonquin Park (with the exception of the low elevation east-side near Achray) and most of the Haliburton Highlands north of Dysart. They range to Oshawa in the south, with a recent record in Toronto in 2018 representing a possible southern range expansion. 

Although records viewed on The Ontario Moth Atlas indicate that Pine Imperial Moths seem to be expanding both south and east in the province, it is considered a vulnerable (s3?) species in Ontario due to its globally restricted range and vulnerability to drastic and rapid population decline (2). Let me explain: if you were to look up observations of the Pine Imperial Moth on The Ontario Moth Atlas, you would see several red squares indicating historical records (19030’s-81) of this species throughout the Carolinian Zone, from Windsor to Toronto and concentrated on the north shore of Lake Eerie (3). These records, in fact, represent the nominative subspecies, the ‘Imperial Moth’, Eacles imperialis, not Pine Imperial Moths, Eacles imperialis pini, now Eacles pini.

Imperial Moths suffered a massive range contraction throughout the North East beginning and were last sighted in Ontario in around 1981-84 based on Ontario Moth Atlas records and specimens that I viewed at the Canadian National Collection of Insects and Nematodes in Ottawa in 2022. With the recent reclassification of the Pine Imperial Moth as its own species, the absence for over 40 years of the Imperial Moth in Ontario could represent a species extirpation. 

Why Imperial Moths declined is not fully understood, but evidence suggests that the culprit is a combination of land use changes, light pollution, and most of all an introduced fly that was once used a s a biocontrol for invasive Spongey Moths (Lymantria dispar). This fly, scientific name Compsilura concinnata (Tachinidae), lays its eggs on over 200 documented species of native lepidoptera – from swallowtail butterflies to Cecropia Moths. It targets large caterpillars feeding on edges of leaves in a forest setting where it lays eggs on the caterpillars skin that hatch and burrow inside, feeding on the caterpillar while it is alive before exiting to pupate in the soil. Because its target host, the Spongey Moth, has only one generation per year and the flies have 4 generations a year, this adaptable parasitoid found other hosts to compensate. It has been documented as absent from sandy coastal areas where vegetation is too sparse to support a succession of host species for this fly to feed on throughout the summer (4).

The decline of the Imperial Moth is a cautionary tale about how quickly our native insects can disappear in the presence of a novel threat, such as an introduced predator. Time will tell if our Pine Imperial Moth, with its very restricted global range, will meet the same fate and disappear not just from the province, but from the planet.  Although the global population of the Pine Imperial Moth is vulnerable to extinction, this is currently a very common moth throughout its range in Ontario, occurring by the hundreds on rock barrens dominated by its favorite host plant, Eastern White Pine. Perhaps these dry, barren areas provide ideal habitat for these moths while offering less ideal conditions for Compsilura? 

I can think of many nights in early July exploring pine dominated rock barrens throughout southern Haliburton County and Northumberland where I have encountered these large yellow moths flying through the open pine woodland by the light of my headlamp. The large yellow moths are easy to spot, and like all Giant Silk Moths, make an immediate impression upon the observer. The dragon-like caterpillars, up to 6 inches long, come in both purple and green forms, and are most easily seen in late August when they search for overwintering sites in loose soil, often being found crossing roads or campsites (Pine Imperial Moths burrow underground for winter and do not spin cocoons). I hope we can continue marvelling at surprise giant caterpillars like this for decades to come.