Isopoda : Crustacea
Slaters are not insects. They are crustaceans and so are more closely related to shrimps than they are to insects. There are several hundred species of native slaters, but the most common slaters found in urban gardens are introduced from Europe. Slaters have several other common names including woodlice, pillbugs, and ‘butcher boys’.
Slaters thrive in damp conditions and are usually active at night. Slaters have become more of a pest in some gardens, especially in heavily mulched gardens, over the last decade or so. Slaters are usually scavengers that mostly feed on decaying vegetable matter, but they also feed on living plants.
The key to managing slaters centres on their need for moisture. Slaters don’t have a waterproof wax layer on their exoskeleton, which makes slaters very prone to desiccation (extreme dryness) and is why slaters seek out moist environments.
When we mulch our gardens heavily with straw to control weeds and reduce moisture loss, we are creating the perfect environment for slaters. Of course, we can’t remove all that mulch because then we have a weed and moisture-loss problem – it’s a delicate balance.
Slaters are terrific at breaking down organic matter and returning nutrients to the soil, but it’s another story when they start feeding on our seedlings. If they are causing damage you will need to do something about them.
It is very difficult to completely eradicate any pest, and the introduced species of slaters are here to stay. The best you can hope for is to suppress them (i.e., ‘manage’ them) and that is why prevention treatments are so important. There are many things you can do around the garden to minimise the impact of slaters (see How to Prevent Slaters Appearing).
It pays to be persistent. What you need to do is to suppress their numbers, so that any damage slaters cause is minor or not noticeable.
Slaters belong to the order Isopoda, which is part of the arthropod class Crustacea. This class mostly contains marine and freshwater aquatic animals such as shrimps, crabs and crayfish. This explains why slaters are such different looking animals to other garden invertebrates such as insects and spiders.
There are about 300 species of native slaters found in Australia and they usually occur in high rainfall areas, and are found under logs, rocks and leaf litter in bushland. Introduced species are common in urban and rural areas in the higher rainfall areas of the southern half of Australia. In gardens they are usually found under mulch or in compost bins.
In their native habitats, slaters are usually scavengers that feed on decaying organic matter. In that sense they are an important part of the decomposition cycle, meaning they help return nutrients to the soil. This explains why slaters are so common in compost bins, unfortunately pest slaters also feed on living plants.
Common pest slaters include the Garden Slater or ‘woodlouse’ (Porcellio scaber) and the ‘pillbug’ or ‘butcher boy’ (Armadillidium vulgare).
Slaters have seven pairs of legs, two pairs of antennae (one pair is obvious, and the other pair is hidden), and eyes on each side of the head.
Adult Garden Slaters (Porcellio scaber) are grey-brown in colour, flattened, and up to 17 mm long. A pair of appendages known as uropods are usually seen extending from the rear. Garden Slaters feign death when threatened.
The Pillbug (Armadillidium vulgare) is dark grey or black, up to about 18 mm long, and has the appearance of being armour-plated. Pillbugs can roll into a tight ball for protection like an armadillo - Armadillidium literally means ‘armadillo-like’. In some countries Pillbugs are called ‘roly- polys’ for obvious reasons. Pillbugs don’t have visible uropods.
Juvenile slaters are initially translucent white with only 6 pairs of legs, but later in the life cycle grow the seventh pair and look like miniature adults.
Females lay eggs into a brood pouch, known as a ‘marsupium’, under their bodies. Females normally produce about 40 eggs per brood and lay 1 to 3 broods per year depending on the conditions. Eggs hatch within the marsupium and the young remain there for about 2 weeks. When the young emerge from the marsupium they pass through two moults before growing their seventh pair of legs.
The young continue to moult (and grow) every week or so for the next 4 to 5 months before reaching maturity. When slaters moult they don’t shed their entire skin in one go. The rear half of the body sheds first, and the front half sheds a couple of days later. Adult slaters can live for two to five years.