Millipedes have long bodies composed of numerous similar segments between a head and a tail end. Each body segment has 2 pairs of legs on each segment, although the first few segments may have only a single pair of legs. Mature Black Portuguese millipedes are smooth and cylindrical, 20-45mm long and slate-grey to black in colour. Juveniles are light brown and striped. If you disturb one of these millipedes, it will curl into a tight spiral or thrash around.
This species is native to Portugal. It was accidentally introduced into Australia (perhaps via another country), being first noticed in Port Lincoln in 1953. It has spread to other parts of South Australia, and interstate to Victoria, Tasmania, the ACT, southern New South Wales and in Western Australia around Perth. The worst outbreaks have been in Adelaide and parts of Victoria. Eventually it may occupy much of southern Australia. Grassland, woodland and suburban gardens are favoured habitats.
Portuguese millipedes hatch from eggs in the soil, and initially have only 3 pairs of legs. They moult as they grow, adding segments and legs, and reach maturity at 2 years. They are mostly active at night and are vegetarian. During hot dry weather the millipedes remain hidden in the soil. Rainy weather in spring and particularly autumn stimulates activity, often leading to outbreak numbers with thousands of millipedes on the surface.
Although Black Portuguese millipedes are not harmful to humans, they can be a serious nuisance when large numbers, sometimes in the hundreds or thousands, invade houses. In the garden they may also damage seedlings and horticultural crops. When disturbed or squashed they release a pungent yellowish secretion that stains.
Barriers - both chemical and physical - and light traps are the most practical ways of preventing millipedes from invading houses. At the landscape scale, biological control is probably the only feasible method, but no suitable agents have been found yet. A parasitic nematode has had limited effect. A suitable chemical, applied in a band wide enough to kill millipedes crossing it, can be applied to brick or cement surfaces around the house, and to doorsteps and window ledges. Prepare and apply the chemical according to directions, and reapply as necessary. Physical barriers stop and/or trap millipedes moving towards the house. A smooth, clean, vertical surface is effective, or a moat with overhanging sides. They are also attracted to light, and you can construct a millipede trap out of a length of oblong-section galvanised steel downpipe and a low voltage bulb.