To check the efficacy of treatments, how do I distinguish dead vs. live scale insects?
Under magnification, dead scales will look desiccated, usually discolored and may not be closely attached to the plant. Lift adult armored scale covers to check the insect beneath. Probing with the tip of a pin or razor blade can help confirm. It may take a couple of weeks for recently killed scales to dry out when evaluating treatments. There may be outlines of the waxy leaf cuticle where dead scales have fallen off.
A good stereoscope can help and doesn’t need to be expensive. Check out the recent e-GRO article on choosing microscopes. Digital and hand-held models weren’t discussed but are also an option.
While in the field, hand lenses, Optivisors, or other magnifiers are useful; swiping a stick along an infested twig can also quickly reveal if many scales are alive, especially for pigmented species (e.g. white prunicola scale, magnolia scale).
— Dan Gilrein