SCIBERRAS, A. (2013) Fauna regarded as domestic pests in the Maltese islands - The Woodlice

The Woodlice
By Arnold Sciberras

If there is a high level of humidity in your house then one of your residents must be the tank- like packed creature commonly known as the woodlouse. This friend of ours is a crustacean (same group which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles). The latter hosts 50,000 described species till now and Woodlice is just one suborder of this group called Oniscidea within the order Isopoda, with over 3,000 world-wide known species. Generally, woodlice are identified by their rigid, segmented, long exoskeletal and fourteen jointed limb body. The number of local species is still uncertain, but the level of interest is high as some species like Maltese Wood louse, Armadillidium schmalfussi, Hanzir l-Art ta’ Malta and Maltese Cave Dwelling Wood louse, Armadillidium aelleni, Hanzir l-Art tal-Gherien ta` Malta are endemic to our archipelago and the latter, as name implies, is confined to caves.

Woodlice need moisture because they breathe through gill like structures called pseudo-trachea, and thus are usually found in damp, dark places, such as under rocks, wood or in our basements. They are usually nocturnal (night dwellers) and are detritivores, i.e. they feed mostly on dead plant matter, though they have been known to feed on cultivated plants, such as ripening strawberries and tender seedlings. Woodlice are ecologically very important because their mode of feeding ultimately leads to a faster replenishment of nutrients back to the soil. In artificial environments such as greenhouses where it can be very moist, woodlice may become abundant and damage young plants.

As already mentioned, these creatures have a shell-like exoskeleton, which must progressively shed as it grows. The moult takes place in two stages; the back half is lost first, followed by the front half which takes two to three days to shed. This method of moulting is different from that of most arthropods, which shed their cuticle in a single process.

A female woodlouse will keep fertilized eggs in a marsupium on the underside of her body until they hatch into small, white offspring. The mother then appears to "give birth" to her offspring.

Some species of woodlice are able to roll into a ball-like form when threatened by predators, leaving only their armoured back exposed. This ability, or dominant behaviour, explains many of the woodlouse's common names. Woodlice in the genus Armadillidium can roll up into an almost perfect sphere as a defensive mechanism; hence some of the common names such as pill bug or roly-poly. Most woodlice, however, cannot do this.

Metabolic rate is temperature dependent in woodlice. In contrast to mammals and birds, invertebrates are not "self heating": the external environmental temperature relates directly to their rate of respiration. They are generally not regarded as a serious household pest, as they do not spread disease and do not damage wood or structures; however, their presence can indicate dampness problems and their number may become irritating to the house owners.

Woodlice are eaten by a wide range of insectivores, but the only animals known to prey exclusively on woodlice are spiders of the genus Dysdera.

Pill-bugs (woodlice of the family Armadillidiidae) are usually confused with pill millipedes. Although they are distantly related to one another, the two are taxonomically distinct.

Both of these groups of terrestrial segmented arthropods are about the same size. They live in very similar habitats, and they can both roll up into a ball. Pill millipedes and pill-bugs appear superficially similar to the naked eye. This is an example of convergent evolution (the acquisition of the same biological trait in unrelated lineages).

Pill millipedes can be distinguished from woodlice on the basis of having two pairs of legs per body segment instead of one pair like all isopods. Pill millipedes also have thirteen body segments, whereas the woodlouse has eleven. In addition, pill millipedes are smoother, and resemble normal millipedes in overall colouring and the shape of the segments.

Besides the Common Maltese Woodlouse which one may occasionally find in a residence, the Common Woodlouse, Armadillidium vulgare, Hanzir l-Art Komuni is the most familiar one in establishments and unfortunately the one which regarded the most as a pest. These creatures are harmless and also beneficial but are regarded as pests because they tend to congregate in large numbers. On some hose facades each year thousands cover the facades during the evening and then disperse or die out in the morning. This behaviour is still a phenomenon not understood. This species may reach a length of 18 mm, and is capable of rolling into a ball when disturbed. This ability, along with its general appearance, gives it the name pill-bug and also creates confusion with pill millipede species.

This species is able to withstand drier conditions than many other woodlouse species, and is mostly found in calcareous soils or coastal areas. It feeds chiefly on decaying plant matter, but also grazes lichens and algae from tree bark and walls.

It is able to regulate its temperature through its behaviour, preferring bright sunshine when temperatures are low, but remaining in shadow when temperatures are high; temperatures below −2 °C or above 36 °C are lethal to it. The latter is also less susceptible to cold during the night, and may enter a state of dormancy during the winter in order to survive temperatures which would otherwise be lethal.



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