Landscape - Understanding Scale Feeding

The following is a good article on some feeding behaviors of scale insects.

Most armored scale species with broadleaf hosts will feed either on bark or leaf tissues, but usually not both (Euonymus scales are a notable exception). The crawlers of these hard scales have only 24 to 48 hours after hatching before they must insert their mouthparts into plant cells and begin feeding. Once armored scale crawlers settle they do not move again for the remainder of their lives (winged adult male scales do change positions, but cannot feed and only live for 24 hours). On the other hand, most soft scale species feed both on bark and leaf tissues during different stages of their life cycles (there are exceptions, especially with coniferous hosts). Unlike armored scales, the soft scales do not lose their legs after the crawlers settle and therefore can move to different feeding locations. On deciduous hosts, the 1st instar crawlers of most species will emerge in June or July from under adult females located on bark. They then move to settle and feed on leaves for the remainder of the summer. Before the leaves drop in autumn the nymphs will move back to bark tissue where they will over winter as 2nd instar nymphs.

Information from Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University.