Source: Radio New Zealand
A Portuguese millipede. RNZ/Mary Argue
Hopes have been dashed that a parasitic worm will effectively kill hordes of invasive millipedes plaguing Wellingtonians.
South coast residents sounded the alarm about infestations of the Portuguese millipede almost a year ago, with stories of finding hundreds of the scaly black creatures around their homes and inside bags, shoes, and even beds.
Since then, the Ministry for Primary Industries (MPI) had identified the species in New Plymouth and Nelson – and while it is considered a nuisance, it is not classified as a pest species and does not meet the threshold for official control measures.
Last month, website iNaturalist reported a possible sighting of the millipede in Christchurch.
Biosecurity New Zealand manager pest management programmes Cath Duthie said MPI was aware of the iNaturalist post.
She said given the species had been established in New Zealand for at least 20 years, there would not be any formal response to its spread and the population was not tracked.
“However, we will provide advice to local authorities where required. We encourage people to report these insect pests when they are detected on iNaturalist.
“While they can be a nuisance, Portuguese millipedes pose no risk to human health and are not known to have any negative effects on the environment,” she said.
Victoria University entomology professor Phil Lester had not seen the Christchurch specimen in person but said he and others on the technical advisory group (set up in response to Wellington’s millipede infestation) thought it was highly likely to be a Portuguese millipede.
Victoria University entomology professor Phil Lester. Veronika Meduna / RNZ
He said it was impossible to be certain how far the population had spread based on photos.
“But I would say that it’s pretty widespread by the sound of things.”
Earlier this year, some residents in Wellington’s Ōwhiro Bay had deployed parasitic nematodes (microscopic worms) in a bid to suppress millipede numbers – which swell in spring and autumn.
The nematodes residents planned to use, Steinernema feltiae and Heterorhabditis bacteriophora, are available as biocontrol agents in New Zealand.
But just last week, one of Lester’s students informed residents that laboratory trials showed Steinernema feltiae was ineffective against the Portuguese millipede.
Lester told RNZ the results from weeks of trials were conclusive – that while the recommended nematode dose did kill other insects, it did not kill the millipedes.
“Even within a couple of days after applying these nematodes to moth species they will die – and it’s just amazing how quickly they’ll die – but the millipedes live through this and are fine.
“We’ve tried increasing the dose and we can get some mortality … if we have 100-fold the concentration that is recommended.”
Lester thought a bacteria released by Steinernema feltiae was the deadly component, acting like septicaemia inside the insect host, killing it.
“Then the nematode feeds on the decomposing body.”
He said the laboratory trials aligned with what they saw in the field in Ōwhiro Bay – that millipedes were unaffected by Steinernema feltiae.
He said like pesticides, the nematode would affect non-target species and biodiversity, but believed residents had given up on deploying them after they failed to dent millipede numbers.
Lester said while disappointed by the results, residents who had tried their own nematode experiments were not surprised.
He said the Portuguese millipede was a major problem for those with infestations and it could be a much “bigger problem” in the future.
He was hopeful further research would yield a solution.
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand