Scientist turns detective to identify mysterious moth

It was previously thought that there were only two dogwood feeding species of Antispila moth found in Britain: A. metallella and the smaller A. petryi. That was until an apparent new species was identified from a moth trap in the Museum's Wildlife Garden.

Dr David Lees, Curator of Microlepidoptera, explains his investigation to unravel the mysterious moth's arrival.

An unexpected visitor

When an adult moth was found in the Museum's moth trap in August 2016, David could tell that it was from the family Heliozelidae. These are moths whose larvae are leaf miners, meaning they feed on a plant's leaf tissue. The adult moths in this family are small and glitter with metallic spots.

The moth in the trap bore a striking resemblance to the two British native Antispila species, but David was suspicious as they don’t usually appear so late in the year.

He explains, 'It immediately occurred to me that this was well past the adult flight period for the two known native species. These species both tend to produce adults in May to July.

'The one that I found was over a month late. Plus, as far as I know, this is the first-ever live moth from this family that has been found in South Kensington.'

His interest piqued, David contacted Heliozelidae expert Erik van Nieukerken, who suggested that the animal may in fact be the real A. treitschkiella. This species and the native A. petryi were considered synonymous since 1978 - but the former had never been recorded in Britain before.  

Leaf miners tend to be host plant-specific, with A. petryi preferring to feed on native dogwood. Erik suggested looking for evidence of the 'new' species A. treitschkiella on the leaves of Cornelian cherry trees (Cornus mas).

So began David's investigation.

Searching high and low

David searched London for Cornelian cherry trees. They are not native to Britain but have been planted ornamentally in the capital since the mid-1800s, often in botanic gardens, cemeteries and parks.

'It's not particularly common,' says David, 'but there are places in London where it is planted in parks and you can see it growing through the railings. It's conspicuous, especially when the red berries come out in the autumn.

'The search for leaf miners was on - it was great fun. I went to Kew Gardens, Wisley and Cambridge Botanic Gardens. Everywhere I looked, almost every branch bore the telltale mines with elliptic cuts where the caterpillar saws its way out of the leaf.

'It's a great thing, having these living collections. We did a survey of the botanical gardens for the Cornus species and were able to prove that the moth only feeds on Cornelian cherry.'

David found evidence of the moths from Surrey up to Cambridge. Even in the bustling centre of London, the Cornelian cherry trees along Victoria Embankment show obvious damage to its leaves, making it surprising that the species has gone unnoticed until now.

'The moth has apparently not yet spread that far westward or northward. I went to Oxford and it hasn't got there yet, so we've caught it in its spread, but we don't know when it arrived.'

A. treitschkiella is not native to Britain and its limited residence here is the mark of an invasive species. It is estimated that around 1% of all European butterfly and moth fauna is invasive.

But the intrigue doesn't end here: despite determining that Cornelian cherry is A. treitschkiella's sole host plant, this tree has not been planted anywhere in the Wildlife Garden. In fact, David could find none planted in public spaces within about two kilometres of the Museum.

'It's a mystery how this moth appeared in the Wildlife Garden. Maybe someone has Cornelian cherry planted in their garden,' he says.

A new moth species for Britain?

Names of species can change, reflecting the current scientific understanding of animals and their relationships to one another.

'There are lots of examples where the names have changed and a few cases where moths have been split up,' explains David.

Since the mid-1970s, A. treitschkiella was thought of as the same species as A. petryi, with the two being synonymised despite each being a perfectly valid species.

As adults it is almost impossible to visually tell these two apart from the upper wing pattern. It is really only as caterpillars that they can be identified: from their warts.

David explains, 'If you look at them closely, the caterpillars of the two species have a different number and size of warts on the dorsal surface of the eighth abdominal segment. It's a very clear difference.

'But Dziurzyński wrote two huge monographs on this, and yet these works have been ignored for 50 or 60 years. Instead the species have been synonymised without people looking at these original books.

'We had moth specimens DNA barcoded and it proves that the one from the trap is A. treitschkiella.'

Looking at the moth's DNA shows that the invasive species is clearly distinct from the British native A. petryi.

David says, 'If you measure the difference in the DNA barcode, it's 12%, which is a lot. So they may look similar but they are a deeply divergent species.'

Into the field

The discovery in the Museum's Wildlife Garden is a reminder that there is much left to be discovered and understood about British wildlife.

David says, 'This discovery shows how important it is for people going through a moth trap to examine carefully every small insect that they find. 

'If they didn't find that one, we probably wouldn't have discovered this invasion in Britain. It is also important to look for new damage turning up on trees like the Cornelian Cherry.

'We tend to be very biased in the Museum and working within the collection, but we need to go outside and get into the field, too.

'The field is anywhere out there - it's not just in the tropics that you get interesting species. Even in major European cities like London there are still very interesting things to resolve.'