Scale insects are members of the order Hemiptera, which also includes aphids. Most of the species in the order are exotic ones belonging to the following families: 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.
Many exotic scales do not hibernate. Some species may produce several generations a year, given suitable weather conditions. This means that you may see insects at all the different development stages (eggs, larvae and adults) at the same time.
The length of the life cycle depends primarily on temperature, while the eggs hatch faster when humidity levels are high. Warmer temperatures and higher humidity shorten the life cycle. Depending on the species, the life cycle may even extend over several months. Under optimal conditions, mealybugs complete their entire life cycle in about twenty days.
After the eggs hatch, the young, mobile larvae emerge from their mother's protective armour and move into new territory. This “crawler” stage lasts only about 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.
After mating, the males die and the females lay their eggs under their armour or in a waxy, white sac (ovisac) at the tip of their abdomens. The females in some species are able to reproduce without fertilization (parthenogenesis) or give birth to already-hatched larvae (viviparity).
The females in most species die after laying their eggs, but the eggs remain sheltered until they hatch.
Depending on the species and climate, these insects may produce 1 to 10 generations per year.