Moth found in West London is a new species from Western Australia

A small, unassuming moth found in West London has come a long way.

In 2021 an amateur moth-er living in Ealing, West London, opened her moth trap to find something she didn’t recognise. Despite her best efforts, Barbara Mulligan couldn’t figure it out, so sent the tiny brown moth to a local expert. Little did she know that she’d just discovered a new species to science that was more than 14,500 kilometres away from home. 

Detailed analysis of the moth, including both morphology and DNA, revealed that the insect was indeed a new species to science, but one that is native to Western Australia rather than Ealing. Not only that, it also turned out that the Natural History Museum already had a specimen of the new moth that had been sitting in the collections undescribed since it was first collected in 1886.

Mark Sterling is a retired lawyer and Scientific Associate at the Natural History Museum and helped to describe the new species’ anatomy in detail, which has now been published in The Entomologist’s Record and Journal of Variation. 

‘What Barbara had found was an undescribed species,’ explains Mark. ‘She found this extraordinary moth which has somehow come over from Western Australia and established itself in three or four places in West London.’

‘And she is still the only person in the UK to have found it. This is a real coup for citizen science.’

New technology meets old techniques

In order to figure out what the little moth from Ealing was, the scientists approached the problem from two different angles.

To begin with, tiny samples of the tiny insect were analysed for their DNA. This gave the researchers what is known as a DNA barcode, which then allowed them to compare the unknown specimen to a huge database containing the barcodes of thousands of known species of moths from all around the world.

While it didn’t come up with a definitive answer, it did narrow things down. It told the scientists that the moth belonged to a group of species that includes one called Tachystola hemisema. This moth is found in New South Wales, Australia, and itself has become an invasive species in California, and probably also New Zealand and Hawaii. 

The London moth did not quite match this international interloper, but it did provide further clues. This is because T. hemisema was first described by schoolmaster and amateur entomologist Edward Meyrick in 1885, whose collection is now cared for by the Natural History Museum.

Going back and looking at this historical collection, the researchers were able to find the original specimen from which T. hemisema was named, plus an undescribed species that was collected for Meyrick in Western Australia just a few years later in 1886. 

It just so happened that at around the same time this unknown moth was doing the rounds, the Natural History Museum was trialling a state-of-the-art genetic technique known as ‘genome skimming’. This allows for a specimen’s genome to be sequenced in tiny little fragments.

‘This was the first opportunity we had at the Museum to do this, and I immediately thought of this interesting moth from Ealing,’ explains Dr David Lees, the Senior Curator for Microlepidoptera at the Natural History Museum.

‘So I got a leg of the Meyrick type specimen for Tachystola hemisema from Sydney, and I also got a leg from one of Meyrick’s specimens from Western Australia. Both of them produced sequences and by adapting existing data analytics programs we were able to read all the data.’

From this new sequencing of 135-year-old specimens, they could see that not only were the two moths genetically distinct, but that the Ealing moth matched with the undescribed specimen from Western Australia.

‘The key thing was that there’s a difference of 3.65% in the DNA barcode between the Western Australian specimen and the species described by Meyrick from Sydney, so that is the kind of level we would normally expect for a distinct species,’ says David.

The team were then able to go back to the specimens and look for subtle differences in the anatomy of the proposed species from Western Australia to confirm what they were seeing in the DNA.

‘What David describes is the twenty-first century analysis to determine whether they are the same or different species,’ says Mark. ‘The next stage in satisfying yourself as to whether it is a new species is the rather more old-fashioned nineteenth and twentieth century analysis of looking at the morphology.’

‘So we looked carefully at the anatomy, dissecting out the genitalia of the specimens. And there were small, but constant differences.’

This gave the researchers all the information they needed to name this new species as Tachystola mulliganae, after Barbara who found it. 

A long way from home

How, exactly, the moth has ended up in London is a little less clear. The likely route is probably through imported plants, but that is yet to be confirmed.

‘These moths can feed on dead leaves, so they may be able to get through in leaf litter on plant pots and that kind of thing,’ suggests David. ‘In fact, Barbara had originally noticed that there were some eucalyptus leaves around her in a neighbours garden in Ealing.’

‘We actually searched to try and find some larvae, but we’ve not yet managed to find any in Britain even though she’s managed to find something in the order of 25 adult specimens.’

What seems more certain, however, is that the arrival of the moth must have been quite recent. To date it has only been found in around four locations in West London, meaning that it can’t have been in the city for very long otherwise it would have spread further.

‘Pinpointing an event like this in time and space is quite something and demonstrates the power of modern DNA sequencing and big data analysis,’ says David.