what about a big piece of ice ... prune out all the dead.. let us know what if anything you observe down under .. is it all one plant that is dead??? i dont see any life in there .... ken

unleess you give us your name and password.. we arent getting past the sign in page.. and i dont recommend such WAG.. ants take advantage of already dead wood.. they rarely... kill things.. IF they are ants ... and if you have all this dying.. why are you replanting the same thing??? one of the big problems.. of a vast monoculture.. like a long line of boxwood .. is that once you have a problem.. its going to work its way thru the whole ... any chance.. since they are sounding overly mature.. that these used to be in full sun .. and now they are struggling.. as the deciduous trees have cast them in shade??? and even worse.. are the trees maples???? have the ever been rejuvenated??? perhaps i missed it in your flurry of posts.. but are you new to this garden... ken

Could the answer simply be that its late in the season and its been exceptionally warm and dry? There isn't anything about the plant that would make me nervous.

Nothing really "eats" boxwood :-) They can get some - not many - insect issues but these all tend to be sucking type insects. But if the infestation is large enough, it can cause leaf drop. I'm not absolutely sure what the issue is. The webbing is somewhat evocative of spider mites however I don't see any obvious sign of mite damage like stippling and discoloration.....the webbing could just be that of regular spiders and the basal dieback the result of the shading of the lower foliage. Or perhaps some chemical damage....boxwoods are sensitive to road salt. Regardless, it is not anything that will necessarily remain present in the soil or a serious insect pest that would infest any replacements. Just hosing down the plants periodically will discourage any mites and avoid using deicing salt in the vicinity.