Bark lice, scientifically classified in the order Psocoptera, are small insects often found on the bark of trees and shrubs. Despite their common name, they are not true lice and do not feed on the tree itself or on the blood of animals. These insects are generally harmless to both trees and people, primarily serving an ecological role as surface scavengers. Their presence indicates a suitable environment of moisture and surface growth rather than an infestation causing damage.
Identifying Psocids and Their Role
Bark lice are soft-bodied insects, typically less than a quarter-inch long, that may resemble aphids to the casual observer. They possess long, noticeable antennae and, in their adult stage, often have two pairs of wings held in a tent-like fashion over their bodies. Some species are nicknamed “tree cattle” because they move in conspicuous, dense clusters or “herds” across the bark surface.
These insects are not parasitic, and their chewing mouthparts are adapted for consuming microscopic organic material on the bark. Their diet consists of fungi, mold, algae, lichens, pollen, and fragments of dead insects found on the tree’s exterior.
Assessing Impact on Tree Health
The concern that bark lice damage trees is generally unfounded because their diet is limited to surface materials. They do not penetrate the bark to consume living plant tissue, sap, or wood, meaning they pose no threat to the structural integrity or health of the tree.
Certain species of psocids create thin, silken webbing that can wrap around branches and tree trunks, especially during periods of high humidity. This webbing is solely for protection against predators and the elements, not a sign of tree damage. The fragile, silvery webbing is cosmetic and usually disappears on its own within a few weeks due to weathering and natural breakdown.
Risks to People and Management
Bark lice pose no known health risks to humans or pets, as they do not bite, sting, or transmit diseases. Outdoor bark lice are distinct from their relatives, booklice, which sometimes infest homes to feed on starchy materials and mold. Bark lice remain on the exterior of trees and are not indoor pests.
Since these insects are harmless to the tree and people, intervention is rarely necessary, and control measures are not typically recommended. If the webbing or large clusters of psocids are cosmetically undesirable, a strong jet of water from a garden hose can be used to dislodge them. Chemical pesticides are unwarranted, as removing bark lice offers no health benefit to the tree.