Question
I live in SE Houston, TX, and have a medium-sized oak tree (approx. 20 feet high, trunk about 15-in. diameter) whose trunk and limbs are almost completely encased in a fine web--not the leaves, just the trunk and limbs.  This doesn't look like the tent caterpillar webs I have seen in pecan trees.  About 3 years ago this tree had an infestation of medium sized (about a half inch)black insects that lined its branches and produced a sticky substance on the leaves and on the grass under the tree. The insects had small bodies and long legs.  I sprayed with a general insecticide (household Raid)and got rid of most of them.  Lower limbs of the tree were pruned by a tree company about a year ago. Don't know if the current condition is related to either previous event.  What should I do?  

Answer
These webs are harmless and are being caused by barklice rather than spiders. Barklice occur on the trunks of a number of trees where they feed on lichens, molds and fungi growing on the bark. Therefore, they may actually be considered to be beneficial insects since they perform the service of cleaning the trunks of trees.

Species belonging to two families of barklice are commonly encountered in Texas: Psocidae ant Pseudocaecilliidae. Psocidae are about 1/4 of an inch long and are black in color. They live in clusters on the bark made up of many individuals in similar stages of development. Consequently, tight clusters of wingless immatures and clusters of winged adults located on the trunks of trees such as Arizona ash are encountered at about eye level. From a distance these clusters often look like knotholes. The aggregated clusters of barklice move like herds of cattle on the bark.

The Pseudocaeciliitae are much smaller in size, duller in color and adults bear wings. The species most noticeable, Archkipsocus nomas Gurney, makes unsightly silken webs on tree trunks and branches These webs can completely wrap a tree's trunk to the tips of each branch. Underneath this protective webbing, the barklice feed on lichens and fungi. Host trees commonly include oaks and pecans, although any tree with lichen and fungal growth on the bark can support a barklouse population. Silk-wrapped trees harboring this barklouse appear to occur at specific periods of the summer (July and August) when conditions are favorable for their development. The appearance of webbing often causes concern to homeowners and landscape maintenance personnel that are not familiar with this insect.

Pseudocaeciliid barklice are harmless to trees and no insecticides are recommended for their control. Once silken webbing appears, it will not be removed by chemical treatments. If removal of the silk is desired, it may be
removed using spray of water under high pressure. Left undisturbed, these barklice apparently remove the silken webbing at the end of the summer. However, an observation has been made in pecan orchards that the use of
fungicides will kill lichens. Lichens are, of course, symbiotic associations of an algae and a fungus. One cannot survive without the other. Without lichens, barklice will have nothing on which to feed and thus populations will fail to develop.