Barklice are members of an order of insects called Psocoptera. Like their relatives, the booklice (Fig. 1), barklice are not true lice (which are in the order Phthiraptera) and are harmless to humans and pets. Barklice are small soft-bodied insects that occur outdoors on plants, on rocks, and in the soil and none are vertebrate parasites, unlike true lice. Many species have two pairs of well-developed wings in the adult stage, but the adults of many other species have strongly reduced wings, or none at all. Booklice adults, which often occur indoors, are a common wingless species.
Species belonging to two families of barklice are commonly encountered in Texas: Psocidae and Archipsocidae (formerly Pseudocaeciliidae).
Aggregating barklice. Common species of Psocidae that occurs in the spring is the barklouse is in the genus, Cerastipsocus (Fig. 2).
Winged adults are about 1/4 of an inch long and are black in color. They live on bark in clusters made up of many individuals in similar stages of development, initially as wingless larvae, but later as winged adults. From a distance, these clusters often look like knotholes. The aggregated clusters of barklice move slowly together, like herds of cattle on the bark.
Web-spinning barklice. The Archipsocidae are much smaller in size, duller in color and adults may or may not bear wings. The actual insects are rarely seen. The species, Archipsocus nomas Gurney, becomes most noticeable in mid-summer through fall when it makes large communal silken webs on tree trunks and branches. In extreme cases, these webs can completely wrap a tree’s trunk and extend out to the tips of branches.
Biology and Habits
Psocid eggs are laid singly or in clusters and are sometimes covered with silk or debris. Larvae hatching from eggs resemble tiny wingless adults. Most species develop through four to six larval stages before becoming mature aduts. Because barklice feed on lichens, molds and fungi that grow on the tree’s bark, they may could possibly be considered to be beneficial insects since they perform the service of cleaning the trunks of trees.
The barklouse, A. nomas, is a communal web spinner. Common host trees include oaks and pecans, although any tree with lichen or fungal growth on the bark can support a barklouse population. Silk-wrapped trees harboring this barklouse occur in the summer when conditions are favorable for their development. The webbing often causes concern to homeowners and landscape maintenance personnel that are not familiar with this insect.
Barklice are harmless to trees and no insecticides are recommended for their control. Webbing produced by A. nomas will not be removed with insecticide treatments. At most, high pressure water sprays can be used to dislodge the insects and some of the webbing. Left undisturbed, these insects and the webbing they may produce will slowly disappear.
The author is grateful for review of the draft of this fact sheet provided by Dr. Mike Merchant, Dr. John Oswald and Bill Ree. This fact sheet is a revision of the fact sheet, UC-010, developed by the author in 1988.
Fig. 1. A booklouse.
Fig. 2. Aggregated wingless larval (left) and winged adult (center, right) stages of aggregating barklice.
Fig. 3. Silk wrapped tree trunk and bark produced by web-spinning barklice.
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Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Chester P. Fehlis, Deputy Director, Texas Cooperative Extension, The Texas A&M University System.