A butterfly flew through

It’s winter time here in New Zealand and sensible people are keeping their windows shut. Not me though. I keep a window open year round, at least a little, so I can hear, and count, when some of my favourite birds are singing. So it was earlier this week on the sunny late winter morning of 13 August. I was chatting with a colleague in my Lincoln University office when, to our surprise, a butterfly flew past my window, circled about, and flew inside.

I immediately realised that it wasn’t a local butterfly. I first thought it was an off-course vagrant from Australia, likely an Australian painted lady. I started to get excited at how strange it was to find such a vagrant in the winter time. I immediately photographed it, collected it in a container, and posted my photos on iNaturalist NZ.

An out-of-season Australian vagrant would have been an interesting story, especially for this age of global warming, but this turns out to be a much stranger tale. Looking more carefully, I realised that this wasn’t an Australian painted lady, or any Australian butterfly I knew. I added the comment to my iNat NZ observation, “Looking again, I don’t think this is a painted lady at all. What is this butterfly?!”. Within 20 minutes, Auckland-based entomologist Stephen Thorpe had solved the mystery and correctly identified it as the European small tortoiseshell (Aglais urticae).

That’s the really strange part. As their name suggests, European small tortoiseshells live in Europe. I looked up the species on the biodiversity database of the Global Biodiversity Information Facility (GBIF). The only other southern hemisphere observations of this butterfly were from Indonesia (four records in 2009) and one well north of Cairns, Australia, in 2003. 

The butterfly that flew through my window is the first sighting of a European small tortoiseshell in the temperate southern hemisphere! Also, all five of the tropical southern hemisphere GBIF observations are DNA barcode specimens from a European database. I’m not yet convinced that these records have correct coordinates.

I alerted Biosecurity New Zealand through their hotline (0800 80 99 66) and they initiated an incursion response. That, and a flurry of interest on iNaturalist NZ, has led to lots of people looking for more small tortoiseshells in and around Lincoln.

It seemed unlikely that the only individual of this species in Lincoln would happen to find my office and fly into my window. However, so far no more have been found, and my window would have been one of the very few open that day. The species is also known in Europe to fly through open windows, perhaps to find hidden places to overwinter. It’s seeming more likely that this was a one-off stowaway, a pupa or adult, that came to Lincoln in a recently delivered package from Europe.

Simon Fowler knows these butterflies well. Simon is an England-raised entomologist now working across the road from me at Manaaki Whenua-Landcare Research. He tells me that they are cold hardy and can overwinter as adults. Their caterpillars feed on nettles, including the small European nettle Urtica urens that is an abundant weed in Lincoln. They would likely do well here if they established.

John Marris, the curator of our university entomology museum, discovered their cold hardiness first-hand when my butterfly easily survived six hours in a freezer. When he removed it from the freezer to pin it, it warmed up and started fluttering about his office! Surviving a flight half way around the world in the a cold hold of an airplane seems quite plausible for this rugged butterfly. Overnight in a -20°C freezer eventually did the job and it’s now a specimen in the Lincoln University Entomology Museum, except for one leg, which Biosecurity NZ are using to extract its DNA.

Would it matter if European small tortoiseshells established in New Zealand? That is the type of question asked over and over again in New Zealand as more and more of the world’s insects slip through our borders. New Zealand has some of the strictest border biosecurity processes in the world, but that is offset by a massive volume of international trade and tourists.

In the case of the small tortoiseshell, it’s hard to tell what the consequences would be if it established. Biosecurity NZ quickly ruled it out as a potential pest of our primary industries. If anything, by feeding on weedy nettles it might provide some biological control for farmers, plausibly complementing the nettle feeding insects we already have in New Zealand.

However, it could also be a competitor to New Zealand’s native nettle feeders, like our kahukura/red admiral butterfly. Also, one of New Zealand’s native nettles, Urtica perconfusa, is “At Risk (Declining)” on NZ’s threat classification system and could be hard hit by an outbreak of a newly arrived herbivore. For these reasons, Biosecurity NZ was quick to alert the New Zealand Department of Conservation to my discovery.

Perhaps the biggest threat of a new insect establishing is the unknown. Each new arrival is a roll of the “unexpected consequences” dice. What insect pathogens, parasites, and viruses is it bringing with it? What would it be a host for in New Zealand? If it became abundant, as many species do in the first decades after their arrival, then how would that affect the species that it competes with or shares enemies with? Does it prefer to pollinate flowers of European plant species, which includes many New Zealand weeds? If so, will this increase their seed set relative to the native and beneficial New Zealand plants they compete with?

Ecological systems are difficult things to predict in detail, because they are complex and because they are strongly influenced by history (for example, the order with which species arrive can make a big difference). What is predictable is that adding lots of new species changes the way that ecosystems work. It’s also predictable that change tends to favour new arrivals over the resident natives well-adapted to the status quo. If we keep stirring up the world’s biodiversity like this, the risk is that nature gets simplified and homogenised.

It’s for these reasons that I’m uneasy about a European butterfly so easily crossing the globe. I’m pleased that it seems to have been a one-off stowaway.

We live in science fiction times, where a butterfly can fly (or crawl) through an open window, or door, somewhere in Europe, and a day or two later fly through a window on the opposite side of the world. We’ll know some of these jet-setting stowaways well enough to expect that they’ll be benign. Others we’ll already know are damaging pests that we should act swiftly against. The majority, however, are ecological unknowns that have never been carefully studied in their home countries.

I remain on the lookout for new species. You should be too. It’s fun getting to know what’s living around you, and that has the added benefit of sharpening your ability to notice unusual newcomers. If you do find a species you’ve not seen before, be sure to share photos of it on iNaturalist. It might have just arrived from the other side of the world.