Those are barklice, Cerastipsocus venosus (Psocoptera: Psocidae). These rather bizarre bugs are called tree cattle (or bark cattle) because they occur in large colonies that move about in unison (much like a herd of cattle, I guess). They are harmless and no cause for concern.

Most species of booklice and barklice are very small, almost microscopic in size. Tree cattle are huge in comparison, reaching nearly a ¼ of inch long when mature. Psocids are easy identified by their swollen faces and long antennae. Tree cattle nymphs appear dark gray with pale yellow banding between abdominal segments. Adults have shiny black wings that are held tent-like over their abdomens. The term "lice" as part of the common name of these tree dwellers is quite misleading as these insects are neither parasitic nor louse-like in appearance. As scavengers, tree cattle perform a valuable function in consuming excess accumulations of fungi, algae, dead bark and other materials that occur on tree trunks and large limbs. Tree cattle do not eat leaves or the bark of the tree, nor do they damage the tree by boring into the bark and control measures are not recommended for these insects.

Howard

Tell us about your experience with our service by completing the brief (5 min.), anonymous MSU Plant & Pest Diagnostics Feedback Survey.