Dear Readers, on a visit to Creemore in Ontario, Canada, earlier this week, I found these empty chrysalises (chrysali???) on the bark of a maple tree. As you might expect, I was intrigued, and, with the help of the Insects and Arachnids of Ontario Facebook group, discovered that the remains belonged to the Spongy Moth (Lymantria dispar). And herein lies a tale, because the Spongy Moth can be found in both the UK (where it’s known as the Gypsy Moth) and in North America, but its behaviour is very different.
The Latin name ‘Lymantria’ actually means ‘destroyer’, and the moth is listed as one of the top 100 invasive species worldwide. It comes originally from Eurasia, and arrived in North America in 1869 – it was imported by French artist and entomologist Étienne Léopold Trouvelot, who intended to crossbreed them with silk moths in order to establish an American silk industry. Alas, the moths escaped from his house in Medford, Massachusetts. In one version of the story, Trouvelot contacted local officials to warn them of the danger, but no one was prepared to hunt down the moths. The larvae of the moth can eat over 100 species of plant, and are particularly fond of the leaves of oak and maple trees, which they can completely defoliate. In 1889 there was the first serious outbreak, with all the trees losing their leaves, and with caterpillars raining down on the local residents. Since then the moth, spreading at a rate of about 13 miles a year, has reached the Pacific North West, and as far north as Ontario.
How does the moth spread so widely? The caterpillars are tiny and hairy, and, like grass seeds, are transported by the wind – they dangle from silken threads just waiting for a breeze to carry them away, like tiny Mary Poppins.
It’s all the more important that the caterpillars can travel, because the adult females cannot – although they have fully-formed wings, they don’t fly. When they emerge, they start to produce a pheromone that calls in males from far and wide. The females cannot feed, and live for about a week, but the potency of the pheromone is such that nearly all females will reproduce. They will then lay their eggs on any plant material in the vicinity, and then die.
The egg-masses look like sponges, hence the North American common name of Spongy Moth. It was considered that calling these invasive nuisance insects ‘Gypsy Moths’ was derogatory to the Romani people, and so the common name was changed. In the UK the species is still known as the Gypsy Moth, but as it is a very rare migrant here (it’s hard for even the most intrepid air-borne caterpillar to cross the Channel), no one seems to have considered it necessary to change the name.
Now, it’s true that the Spongy Moth is a nuisance (and worse) across most of its native range, but it doesn’t seem to have reached the plague proportions that it sometimes does in the countries to which it’s been introduced. There are the usual reasons for this. Firstly, a great number of parasitic wasps favour laying their eggs on the caterpillars, which is great, but when one species of wasp was introduced to North America in the hope that it would reduce the Spongy Moth population, it was found to be rather more wide-ranging in its tastes, and many other butterfly and moth species also suffered. A wide variety of mice, chipmunks and birds will also consume all life stages of the moth, but the animals in North American are probably not quite as attuned to the habits of the insect as the Eurasian ones are.
The Spongy Moths are also incredibly hardy – their native range includes not only North Africa (nice and warm) but Northern Russia (which is a bit on the chilly side) and so they happily survived everything that the North American climate could throw at them. Like many species, they are probably also benefitting from climate change, which means that they can move even further north.
Alas, the European Spongy Moth (Lysmantria dispar dispar) is increasingly being joined by the Asian Spongy Moth (Lysmantria dispar asiatica), which arrives as egg masses on crates etc imported from Asia, and is a particular pest in Washington State, USA. The downside to the Asian Spongy Moth is that the females can fly up to twenty miles. Furthermore, the caterpillars are even less fussy about what they eat, with up to 500 species of plant recorded as having been munched upon, including coniferous trees which are largely left alone by the European Spongy moth. In our globalised world it’s all too easy for insects, plants and fungi to be spread about the planet randomly, and for them to have a devastating effect on local ecosystems. So far, most Asian Spongy Moth populations have been spotted and eradicated, usually with shedloads of toxic chemicals which will also kill off all manner of other species. Eventually, most ecosystems will adjust to new members, but often it takes hundreds of years and may involve the extinction of some existing species, so there is often no choice. We need to be much more careful about the biological security of habitats, so that we can notice and nip these uninvited visitors in the bud. Sadly, I don’t see much sign of it happening any time soon.
Photo One By Didier Descouens – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34634022
Photo Two By Opuntia – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2580261
Photo Three By Didier Descouens – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=27011413