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Knowing Nature . . .

A Familiar Little Friend

May 27, 2008
Jocie Ingram

Sow Bug photo © Dave Ingram

My property is teeming with sow bugs, those small grey bugs with lots of legs and armoured backs that make them look like miniature armadillos. Also called wood bugs, wood lice and pill bugs, these little guys are everywhere. There are hordes under every flowerpot, and masses in the compost. They are in the crawl space under the house, and occasionally one boldly traverses my living room.

Hopelessly outnumbered by these creatures, I may as well get acquainted with them. The first thing I learned is that sow bugs are not “bugs” or insects. They are actually crustaceans, and are related to things like shrimp and crab. Most crustaceans live in the ocean, and some in fresh water, but sow bugs are the only crustaceans that are specially adapted to carry out their complete life cycle on land.

Sow bugs are oval in shape, and are somewhat convex above and flat underneath. They are typically about 15 mm long and 8 mm wide. Colour can vary from slate grey to brown and some are lighter and may be speckled (colour varies particularly in newly molted individuals).

Like all crustaceans, sow bugs have an exoskeleton, or external shell. This armour-like shell is made up of seven overlapping plates, or segments, that give the sow bug flexibility to move. Sow bugs have seven pairs of jointed legs, and two pairs of antennae (only one pair of antennae is very visible). Paired appendages at the back end, called uropods, are used to take up water.

Pill Bug photo © Dave Ingram

Pill bugs, also very common, look similar to sow bugs, but they lack uropods, and are shiny blue-grey with a more rounded back. The most reliable way to tell sows and pills apart is to gently poke one. If it rolls into a tight ball it is a pill bug, and if it doesn’t it is a sow bug. Since pill bugs have this rolling ability, they are affectionately called “roly-polies”.

Sow bugs can reproduce throughout the year, but do so mostly in the spring. Female sow bugs incubate up to 200 eggs in brood pouches under their abdomen. After six to eight weeks, the young emerge looking like miniature adults. Sow bugs molt four to five times before they reach adult size. First the back half molts, and then the front half, which may give young sow bugs a two-toned look. They live in family groups until the young are grown, and family members are identified by a specific chemical “badge”.  A sow bug may live two to five years.

Unlike most insects, sow bugs do not have a waxy, waterproof cuticle. Their exoskeletons are water permeable, which means that they can dry out easily. Instead of excreting liquid waste in the form of urine, sow bugs excrete waste in the form of gaseous ammonia, directly through their exoskeleton. Sow bugs breathe through modified gills on the underside of the abdomen. These must be kept moist; “to dry is to die” for the sow bug.

Sow bugs seek out dark damp places, such as beneath old boards, leaf litter, or along the edges of sidewalks and concrete foundations. They tend to clump together, which also helps prevent water loss. They feed primarily on dead and decaying matter, but will feed on tender young plants, and sometimes berries. Sow bugs are beneficial in that they are effective as decomposers, helping to break down organic matter.

Though sow bugs may nibble at some garden veggies and berries, they are not serious pests to most gardeners. Sow bugs do not bite or spread diseases, and are really quite harmless. Sow bugs do, for unknown reasons, sometimes invade homes. Since most of us would rather prevent this, there are some steps that can be taken to keep the sow bugs out.

1. Reduce humidity by having good ventilation in the home, including crawl spaces and attics, and have fans in bathrooms and kitchens (remember that sow bugs need moisture to survive)
2. Remove old wood and leaf debris from near the home. Put ceramic feet under flowerpots, or raise them on 1 by 2 wooden spaces to provide more air and light.
3. Weather-strip all doors, and use a caulking gun to fill cracks and crevices near ground level.
4. Pesticides are not effective when it comes to sow bugs so don’t use them. If moisture conditions remain, the sow bugs will always return.

Sow bugs originally hail from Europe, but have been spread far and wide. They thrive in coastal BC, where dark, moist conditions are readily available. Love them or hate them, sow bugs will be here long after you or I, so we may as well get to know them!