Scale insects are members of the order Hemiptera, which also includes aphids. In our climate, the most common scale insects are Diaspididae (armoured or hard scales), with hard armour; Coccidae (soft scales), with a leathery, waxy coating, but no armour; and Pseudococcidae (mealybugs), covered in waxy whitish filaments.
These insects undergo incomplete metamorphosis (hemimetabolic insects), which means that the larvae resemble small adults and go through several moults before reaching their final size. Males and females are different.
Eggs: They are tiny. A female (oviparous) may lay 400 to 1,000 eggs, depending on the species.
Larvae: Resemble adults, only smaller (0.2 mm) and translucent. Newly hatched larvae (1st instar larvae or crawler stage) have six legs and a pair of antennae.
Males: Resemble tiny flies (2 mm long), with legs and usually a pair of wings, but no mouthparts. They live for only a day or two and are rarely seen. They appear at a precise time of year and usually account for only a small percentage (1-2%) of the population.
Females: Small (1-12 mm), usually with no eyes, antennae or wings (apterous) and often no legs (apodal). The head has a long flexible tube (stylet), used for sucking sap. The body is usually covered with protective armour or waxy secretions.
In our climate, scale insects overwinter on plants as eggs, larvae or adults, depending on the species. In spring, when the buds burst (from mid-May to mid-June), the overwintering population comes to life and the eggs hatch.
After the eggs hatch, the young, mobile larvae emerge from their mother's protective armour and move into new territory. This “crawler” stage lasts 48 hours. During this short time, the young insects are more vulnerable because they do not yet have a scale covering. It takes about ten days for them to secrete their first armour or protective waxy coating.
Among sedentary species, the young female larvae lose their legs during the first moult. They then settle. They moult several times to allow their armour to expand as they grow to the adult stage. This stage lasts about two months.
In late summer, after mating, the males die and the females lay their eggs under their armour or in a sac (ovisac) at the tip of their abdomens. Egg laying can continue until late August. Females in most species die after laying their eggs, but the eggs remain sheltered under their shells until they hatch the following spring.
Depending on the species and climate, these insects may produce one to three generations per year.