Bark Louse

Bark lice, also called barklice, belong to the order Psocoptera, a group of small, soft-bodied insects related to book lice and, more distantly, to true lice. Unlike parasitic lice, bark lice are free-living scavengers that feed on organic films on outdoor surfaces and have no parasitic relationship to blood-feeding insects despite the shared common name.

Many bark louse species, such as those in the genus Cerastipsocus (often nicknamed "tree cattle"), are gregarious, forming dense aggregations of dozens to hundreds of individuals on a single tree trunk that move in a coordinated, herd-like fashion when disturbed.

Ecologically, bark lice are decomposer-adjacent grazers, helping to keep bark surfaces clear of algae, lichen, fungal spores, and other organic debris, making them a quiet but beneficial presence in forests, orchards, and shade trees.

• Small, soft-bodied insects with a broad head, bulging eyes, and long, thread-like antennae.
• Body color varies by species from pale tan to dark brown or blackish, sometimes with a mottled or banded pattern on the wings.
• Winged adults hold their wings roof-like over the body when at rest; nymphs are wingless and often darker.
• Frequently seen in tightly packed, moving clusters on tree bark, fence posts, or stone walls.
• Lookalikes include aphids and small leafhoppers, but bark lice are distinguished by their gregarious herding behavior and grazing (not sap-sucking) mouthparts.

Bark lice are found on the trunks and larger branches of deciduous trees, as well as on fences, rocks, and outdoor walls, wherever a thin film of algae, lichen, or fungal material accumulates. They occur across temperate and subtropical regions and are most noticeable during warm months when populations build up into visible aggregations. They shelter in bark crevices and leaf litter during cooler periods.

Bark lice feed by scraping algae, lichen, fungal spores, and other organic residues from bark and similar surfaces with chewing mouthparts. Many species are gregarious, and when disturbed, an entire cluster may scatter and re-form together in a synchronized, herd-like retreat. They are grazers with no biting or stinging behavior toward larger animals, and they play a minor decomposer role by cycling organic material from tree surfaces.

Bark lice undergo incomplete metamorphosis (egg, nymph, adult). Eggs are laid in clusters on bark, sometimes covered with a loose silken webbing spun by the nymphs for protection. Nymphs resemble small, wingless adults and pass through several molts while grazing communally, developing wing pads as they mature. Multiple generations can occur per year in warm climates, and populations persist as overwintering eggs or sheltered nymphs in cooler regions.