what are the various stages of their life?
They develop from the egg through a first instar (crawler) stage and a second instar stage before becoming adult.
Scale insect
 develop from the egg through a first instar (crawler) stage and a second instar stage before becoming adult. 
what is the scale insect? They are small insects of the order Hemiptera, suborder Sternorrhyncha. are they pests? Yes,  crop pests.
wiki:20189602
Scale insect [SEP] Life cycle
Female scale insects in more advanced families develop from the egg through a first instar (crawler) stage and a second instar stage before becoming adult. In more primitive families there is an additional instar stage. Males pass through a first and second instar stage, a pre-pupal and a pupal stage before adulthood. The first instars of most species of scale insects emerge from the egg with functional legs, and are informally called "crawlers". They immediately crawl around in search of a suitable spot to settle down and feed. In some species they delay settling down either until they are starving, or until they have been blown away by wind onto what presumably is another plant, where they may establish a new colony.