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THE PORTUGUESE PEST PROBLEM IN SOUTH AUSTRALIA

The Portuguese millipede is a nuisance pest that is easily controlled despite its tendency to appear in large numbers that can be alarming for homeowners.

Over the past 60 years, millipedes have become an increasingly serious nuisance pest species, especially in the southern regions of Australia. The main pest species – the Portuguese millipede (Ommatoiulus moreletii) – was first reported in Port Lincoln SA in 1953. Since then, its presence has been recorded in WA, ACT, Victoria and Tasmania and in a number of cities and towns, including Adelaide. Although they are a nuisance pest and present little threat to health or homes, their presence can be quite distressing due to their ability to invade in their thousands.

Millipede biology

Millipedes belong to the class Diplopoda, which references their having two pairs of jointed legs on most body segments (in reality, this arises from two single body segments being fused together). Their double pairs of legs distinguish them from centipedes, which have only one pair per visible segment. Millipedes have rounded bodies and a hard outer casing.

Morphologically, the Portuguese millipede is known as a julid millipede, capable of burrowing strongly in soil, enabling it to survive harsh conditions. They generally live outdoors in damp areas with good coverage, making well-watered garden beds a favoured spot. On occasions where they are found indoors, it will typically be in an area with high moisture, such as a damp subfloor.

Typically they are saprophages feeding on decomposing plant material, mainly leaves and decayed wood. This protein-poor diet is a major constraint, though occasionally, they are found feeding on higher quality food, such as fallen fruits, mammalian faeces and even dead invertebrates, which are very rich in nutrients.

Although moist environments are preferred, they easily cope with normal, annual periods of dryness using both behavioural and physiological mechanisms. They are not usually active during dry periods. Their basic behaviour is to take refuge in cavities or to burrow into the soil. Physiological adaptations allow millipedes to survive dry conditions for weeks or months, and dry periods are often spent moulting.

Females lay their eggs in underground nests created by excreting soil and using their anal folds to shape it as required. Typically a few hundred eggs are laid, depending on the size and condition of the female. Young millipedes hatch inside the nest and remain there. Usually within 12 hours of hatching they moult into their first stadia (instar). They remain in the nest for the first three moults, and gain legs rapidly with each moult.

They also display periodomorphosis whereby the males extend their life by moulting from a sexually mature individual into an intercalary stadium (intermediate form), and later moult again into the sexually active stage.

Millipedes as pests

Changing weather patterns may be a reason for the insurgence of millipedes in recent years. Elevated temperatures and changing summer rainfall patterns increase breeding and maturation. Typically they are thought to reproduce in March or April, laying most of their eggs in April and May, which might explain why they appear so mobile after the first autumn rains.

The Portuguese millipede displays a positive phototactic response, meaning it is attracted to light. Homes with garden beds close to the property walls are most at risk of infestation, as millipedes will be drawn to the lights from inside. An infestation may only become apparent when large numbers suddenly appear in the house!

Although millipedes do not bite (unlike centipedes) and are not known to carry disease, significant damage can be caused through the stains they leave behind. Defensive glands, which run along both sides of their body, emit a foul-tasting yellow fluid containing quinolones, which can damage carpets, soft furnishings, curtains and bedding. Their sheer numbers can cause other unforeseen problems – a millipede plague in Perth gained international infamy when it was blamed for a train crash in 2013, after large numbers of millipedes congregating on the tracks were crushed by trains, making the line slippery!

Portuguese millipede, Ommatoiulus moreletii

Ensystex regional director, Steve Broadbent, advises not to despair when faced with millipede infestations. “Proofing is a good way to start – blocking up external cracks and crevices with sealant will put a halt to their movement into houses. Novithor Flexigel is a sanitary grade, mould-resistant and pest-resistant sealant, that is also termite-proof, and the ideal solution for use outdoors and internally, even in wet areas.”

Pest managers should also recommend for external lights to be turned off in the evenings or curtains/blinds fitted to reduce the light being emitted, to make homes less attractive.

“Getting rid of leaf litter will also help remove their food source,” added Mr Broadbent. “Treatment with either Bithor Dual Action Insecticide (45g/L bifenthrin and 55g/L imidacloprid) or Ultrathor Water-based Termiticide and Insecticide (100g/L fipronil) is then recommended.”

Using a barrier spray around the outside of the property will kill any millipedes that are present and provide lasting protection against new arrivals. Treatment of the soil beds around a house is strongly recommended, as this is where millipedes are breeding and harbouring during the dry periods and this will break the breeding cycle.

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