There are two groups of scales that are a concern on ornamentals: armoured scales (Family: Diaspididae) and soft scales (Family: Coccidae). Armoured scales are the most common scale found on trees and shrubs. The females are small, soft bodied and are concealed under a scale covering that is usually free from the body of the insect beneath. The scale is formed of wax, secreted by the insect, along with cast skins of its early instars. Armoured scales vary in length and diameter from 1-3mm. The body shapes vary with species - circular, elongate or pear shaped.

Overwintering females of armoured scales lay their eggs under their scale. The first instar larvae or "crawlers" which hatch from these eggs, are active and can travel quite a distance without any food. This is how the scale insect disperses itself throughout a tree. "Crawlers" can also be windblown to adjacent trees. Once a suitable location is found, the crawlers settle down, insert their mouthparts into the host plant to feed, and begin to produce their scale covering. The females remain sessile for the remainder of their life, whereas the males are winged.

Unarmoured or soft scales have a life cycle similar to that of the armoured scale described above. The difference between the two scales is that the unarmoured female scale is flattened, elongate and oval, that can be exposed or enclosed in a waxy or cotton-like secretion.

Source : http://www.uoguelph.ca/pdc/Factsheets/Insect/ScaleInsectsOutdoorOrnamentals.htm

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