Millipedes in the house? Why Portuguese millipedes invade

Black millipedes pouring into the house are almost always Portuguese millipedes, an introduced species that invades homes in large numbers in autumn, especially after the first rains, and again in spring. They are drawn to lights at night, which is why they end up on your walls and under your doors. They are harmless and can’t breed indoors, but they are a real nuisance. Turning off outside lights, sealing doors, clearing mulch from the walls and a barrier treatment keep them out.

We get millipede calls every year, often from people who’ve never had them before and suddenly have hundreds. Here’s why it happens and what actually works.

What Portuguese millipedes are

The Portuguese millipede (Ommatoiulus moreletii) comes from south-west Europe and is now established in South Australia, Victoria, Tasmania, the ACT and Western Australia. A WA Department of Agriculture fact sheet (distributed by SA’s Eastern Health Authority) describes adults as 20 to 45 mm long, smooth and cylindrical, slate-grey to black, with juveniles a lighter grey-brown. When disturbed they curl into a tight spiral and give off a pungent yellowish fluid. CSIRO notes the fluid can stain skin and material, and that the species often reaches plague proportions in southern Australia.

Why they invade houses

Portuguese millipedes are one of the few millipede species attracted to lights at night, and that’s the likely reason they head for houses. They breed in autumn and early winter, which is why they suddenly appear in such numbers after the first good rains. The fact sheet notes they can invade homes and gardens in their thousands each autumn and spring.

Two things are worth knowing. They don’t breed inside the house, and once inside they will probably die. And the ones in your house most likely came from eggs laid in the soil within about 100 metres of it, so what’s in your own yard matters.

What to do tonight

• Turn off outside lights near the house, especially over doors.
• Close curtains and blinds so less light spills out of the windows.
• Check door seals and weather strips. A good seal under the door stops a lot of them.
• Sweep or vacuum up the ones inside rather than squashing them, so they don't stain the carpet.

Longer-term ways to keep them out

• Reduce mulch, leaf litter and compost right against the house. Less organic cover means less food and shelter close to your walls.
• Keep plants trimmed back so they don't bridge from the garden onto the walls.
• Smooth physical barriers work because millipedes can't climb smooth vertical or rounded surfaces. They must be continuous and kept clean to work.
• A chemical barrier needs to be wide, at least a metre according to the fact sheet, and it has a limited life, so it needs reapplying for ongoing control, especially in rainy periods.

There’s some good news from history, too. In 1988 the Department of Agriculture in South Australia released a parasitic nematode that attacks only millipedes at more than 2,000 locations around the state, and the fact sheet reports the millipede’s pest status has decreased in many areas of SA since then. It hasn’t solved the problem everywhere, as plenty of Barossa and Adelaide households can tell you.

Getting the timing right

Timing is most of the job with millipedes. Our earwig and millipede control is a barrier treatment around the perimeter, door thresholds, weep holes, vents and garden-bed edges, timed for when the numbers spike. If you get millipedes every year, book ahead of autumn. If they’re already coming in, call now. A treatment still cuts the numbers sharply, and we’ll be honest about what it can and can’t do mid-swarm.

Earwigs follow a similar cool-season pattern, sheltering in mulch and pot plants and wandering inside at night, and the same barrier treatment covers both.

Call Garry on 0403 018 325 for a free quote. We cover the Barossa including Lyndoch & Williamstown, Kapunda and Nuriootpa, plus Gawler and Adelaide suburbs.