Bark lice (Psocids) are tiny insects that often gather in conspicuous numbers on tree bark, creating a thin, silken sheet that can cause alarm. Despite their name, these insects are not true parasites and do not feed on the tree’s wood or sap. They are benign, primarily feeding on the surface growth of fungi, mold, and algae that collect on the bark. This guide provides practical, non-destructive methods to remove bark lice and address the underlying conditions that attract them.
Identifying Bark Lice and Assessing the Threat
Visually confirming bark lice involves observing the insects and the characteristic webbing they produce on trunks and branches. The individual insects are small, soft-bodied, often grayish or brownish, and can appear in large, dense, slow-moving clusters, sometimes referred to as “tree cattle.” A definitive sign of their presence is the fine, silky webbing or sheeting that covers the bark, which the nymphs spin to protect the colony from predators and the environment.
Bark lice are not tree pests; their chewing mouthparts are used only to graze on surface materials like lichens and microscopic fungi. By consuming these growths, they may even be beneficial to the tree’s health, acting as natural surface cleaners. Since they do not bore into the wood or suck plant fluids, their presence indicates the availability of fungal growth, not active damage to the tree. The webbing is fragile and naturally degrades, but many choose to remove it for aesthetic reasons.
Immediate Removal Strategies
The simplest method for removing bark lice and their webbing involves a physical blast of water. Use a standard garden hose nozzle set to a strong, focused jet to dislodge the insects and tear away the silk sheets. Direct the water stream to wash the colonies off the bark, ensuring the pressure is strong enough for removal but not so forceful that it damages the tree’s outer layer. This method is highly effective because the insects are soft-bodied and easily displaced.
For smaller, localized gatherings of bark lice, manual removal can be accomplished with a soft-bristled brush or a damp cloth. Gently sweep the brush up the trunk and branches to dislodge the insects and remove the delicate webbing. This approach works well for trees with smooth bark where the colonies are easily accessible. After manual removal, rinse the area with water to wash away any remaining insects and webbing debris.
If physical methods are impractical or if the bark lice quickly return, a simple homemade horticultural soap solution can be applied. Mix one tablespoon of pure liquid castile soap, which contains the necessary fatty acids, into one quart of water. Use pure soap without degreasers, fragrances, or additives, as these can harm the tree’s bark. The solution works by dissolving the insect’s protective outer layer, causing dehydration, so direct contact with the colonies is required.
Apply the soap solution directly to the infested areas, preferably in the early morning or late evening when temperatures are cooler to prevent leaf burn. Since horticultural soap has no residual effect once dry, it only targets the insects present at the time of application, minimizing harm to beneficial insects. Always test the solution on a small, inconspicuous patch of bark first to ensure no adverse reaction occurs.
Long-Term Management and Prevention
Preventing the return of bark lice focuses primarily on reducing the environmental conditions that encourage the growth of their food source. Since the insects feed on mold, fungi, and algae, the goal is to decrease the high moisture levels and poor air circulation that favor these organisms. Improving light penetration and airflow around the tree is a highly effective long-term strategy.
Pruning dense interior branches can help increase sunlight and air movement through the tree’s canopy. This allows the bark to dry more quickly after rain or heavy dew, making it less hospitable to the fungi and mold that the bark lice consume. Ensure that any mulch around the base of the tree is kept a few inches away from the trunk flare to prevent moisture from accumulating directly against the bark.
Proper cultural care, such as ensuring the tree receives adequate water, supports overall tree health, which indirectly reduces susceptibility to fungal issues. Chemical pesticides are almost never recommended for bark lice because they are easily controlled by non-chemical means. If the infestation is massive and recurs despite environmental adjustments, consulting a certified arborist is the most appropriate next step to assess underlying issues, rather than applying broad-spectrum insecticides.