The millipedes' arrival may have forced out other species. Photo: Supplied
The "alarming" re-emergence of scaly black millipedes on Wellington's south coast has co-incided with the first scientific study of the invasive species.
Earlier this year, locals described waking to find hundreds of Portuguese millipedes around their homes, discovering them in shoes, bags and even their beds.
Biosecurity New Zealand said the invertebrate had been identified on the south coast and in Lower Hutt for about 20 years, but not much was known about it, but over the next 18 months, Victoria University ecology and biodiversity student Dan Moskovitz hopes to change that.
They're back
After a brief reprieve over winter, Owhiro Bay residents said the millipedes emerged earlier this month, alongside slightly warmer temperatures.
A Southgate resident told RNZ they were creeped out, after moving in a couple of months ago and finding their new home was "crawling with them".
An Owhiro Bay resident, who's lived in the area for several years and didn't want to be named, said they'd anticipated the millipedes' spring return and just hoped the numbers wouldn't reach autumn's level of infestation, which was "really bad".
Another local said the millipedes that appeared in early September lasted for about a week.
"I've seen a few and it was fairly alarming, actually, because they all appear all at once and you kind of go, 'Oh no, here we go'.
"They just swarm down from the hills, which are behind all our houses in this suburb."
Shedding light on invasive invertebrate
Despite extensive digging, Masters student Moskovitz said very little scientific literature existed on the Portuguese millipede and nothing at all in New Zealand, meaning his project was likely the first of its kind in the country.
Like most people, he said the species wasn't on his radar, before the news reports.
"I thought it would be an interesting project to study and it seemed like there was a problem... that I might be able to help out a little bit on."
While excited to discover new things, Moskovitz wished it didn't "come at the cost of the residents having a nice home that's not millipede infested".
Over the next 18 months, he'll undertake "creepy crawly monitoring", checking more than 100 bark chip-filled bags he's placed in gardens around the south coast.
He said the study aimed to answer two important questions - the millipedes' spread and their ecological impact.
"The data that is really valuable for people on the ground is distribution - just knowing where they are. We know they're in Owhiro Bay, but we've received reports of them being in other spots.
"Seemingly beyond having some weirdly high concentrations in Owhiro Bay, they seem to have quite a patchy distribution elsewhere."
Moskovitz said the second part of the project determining the millipedes impact on ecology - what their presence meant for other invertebrates - was slightly more academic.
He said no formalised hypothesis existed, but speculated the millipedes' tendancy to swarm would push other species, like spiders, out.
"I would imagine, on a purely spatial scale, if all of the ground was taken up by millipedes, there would be less space for [other] invertebrates."
Moskovitz said the use of a biological control, such as a parasitic nematode, would likely form another research question, and ultimately hoped the findings from the study proved useful to residents and a technical advisory group established to look at how to control the millipedes.
Response from authorities
The technical advisory group - which includes an entomologist and representatives from the Ministry for Primary Industries, Department of Conservation, and city and regional councils - held a public information meeting in July.
Residents said little had happened since then, beyond the group answering questions raised at the meeting, such as how to best dispose of the millipedes - in durable, tightly sealed plastic bags sent to landfill - and whether chickens or birds eat them - no, due to the millipedes' toxic secretions.
Parasitic nematodes used by company Bioforce to try and control the millipede population were also addressed.
Bioforce managing director Christopher Thompson suggested the nematode (Steinernema feltia) was an effective biocontrol agent against the millipede, when deployed in numbers
Thompson said a millipede host could hold tens or even hundreds of thousands of nematodes, before it burst.
"It is a bit of a horror show to be fair, but it's nature."
The advisory group said it hoped the nematode would prove effective, but currently, no evidence supported it and the group couldn't guarantee funding for the necessary research.
An Owhiro Bay resident who spoke to RNZ said they'd likely deploy the nematodes regardless and anticipated their garden could be a case study for Moskovitz.
"Now that we've heard back from the council that they're not really going to do anything about it - they don't really see it as an issue or their responsibility, which is totally fair - everyone is like, 'Oh well, we've just got to do our best to deal with it ourselves', and probably do a multi-pronged approach - nematodes, pesticides, insecticides."
The technical advisory group is due to meet again on 8 October.
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