'Horrid' find in Melbourne backyard after rainfall: 'What is going on?'

The Melbourne man grew concerned after noticing a 'mass' invasion on his property.

When a Melbourne man discovered "masses" of black insects in his garden, he wondered "what was going on". Thousands of tiny, wriggly critters had moved onto his property following a bout of rain, invading his outdoor pavers.

Desperate for answers, the Melburnian shared photos on Facebook claiming they had been there for over a week.

"We found a mass of millipedes on and around these slabs. They are alive but barely moving," he said on Sunday, adding the "magpies are ignoring them".

Invasion of Portugese millipedes in Melbourne backyard.
The Melbourne man found an invasion of tiny black millipedes in his garden. Source: Facebook

'Well known' critters identified

Photos of the insects show hundreds of them piled up in the cracks between his pavers and spread out across the concrete. While "barely moving" the man said, "individuals were very active when put in the sun".

The critters in question are likely Portuguese millipedes — an invasive species mostly found in southern Australia – Dr Owen Seeman, Collection Manager for Arachnida at Queensland Museum, said. “The photos aren’t good enough to see the key feature – a little spine on their “bum” [the last segment]," he said. "They’re well-known for moving en masse after good rainfall and they’re often active in the winter."

Are Portuguese millipedes a problem?

While not harmful to animals or humans, "they can be a significant domestic nuisance when they invade homes and gardens in their thousands" Dr Seeman explained — and as a result, they're not very well liked.

"I can't stand those things" one said in the comments. "I get so many of these in my house. I hate them," another said. A third said they're "horrid smelly things".

Reads 'What on Earth' 'There are over 1900 species threatened with extinction in Australia' a collage of animals and a map of Australia
There are over 1900 species threatened with extinction in Australia.

They can "sometimes damage seedlings of crops" Professor Ary Hoffmann from Melbourne University previously told Yahoo.

Why do birds avoid them?

Another explained they "aren't very palatable to birds," which is probably why magpies were avoiding them. This is because Portuguese millipedes secrete a pungent odour when agitated.

According to WA's Department of Agriculture, this secretion is composed of organic chemicals called quinones, which make the millipedes distasteful to predators such as birds. Professor Hoffman said cats and dogs tend to avoid them too.

'Mass spawning' in Aussie backyard

On the Facebook post, one person suggested the millipedes were likely "mass spawning" but Dr Seeman doesn't think this is the case. "Unless the observers actually see them mating then it’s not a mass spawning," he explained.

Portuguese millipedes travel in their thousands and are an invasive species found in southern Australia. Source: WA's Department of Agriculture
Portuguese millipedes travel in their thousands and are an invasive species found in southern Australia. Source: WA's Department of Agriculture

"Mating and egg laying is typically an autumn activity for these millipedes. It’s more like mass migration – the breeding is finished, so when the conditions are right, they emerge from the mulch and move."

Responding to the poster's claim about the millipedes "barely moving", Dr Seeman said it's likely because it's winter and "extremely cold". "Once they get a bit of sunshine of them they ought to be up and away again," he explained.

How to get rid of them

Prof Hoffman said you could "probably just leave them" if you notice an invasion at your home, adding "they will disappear quite soon". But Portuguese millipedes are attracted to lights at night so it's advised to turn them off.

In 2021, a millipede invasion was blamed for a train crash in Western Australia.

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