Question
Is the relationship between insects called tree cattle and their host trees symbiotic? If so, what is the benefit for each?


Answer
In some sense maybe.  symbiotic relationship is a relationship between two entities which is mutually beneficial for the participants of the relationship. Thus there is a positive-sum gain from cooperation. This is a term commonly used in biology to explain the relationship between two entities that need each other to survive and prosper. The psocids do clean the tree bark and the tree does provide a structure to live on. The tree does not really "need" the psocids to survive but the psocids do need the tree.


Many people notice fine silken webbing on the trunks and branches of their trees during the warm summer months. The assumption is that this is a problem but the insects that cause this webbing are not harmful to trees. They could even be considered beneficial.

The tiny insects that make the webbing are called psocids (pronounced so-cids). They have numerous common names including tree cattle and bark lice. Basically, they feed on lichen, moss, algae, fungi, spores, pollen and possibly the remains of other insects found on the tree抯 bark. As a result are sometimes referred to as bark cleaners.

Because the webbing appears suddenly on a tree抯 trunk and/or limbs, many homeowners wonder how it got there, where it came from and if it will damage their tree. The cause for the webbing can be seen if the webbing is pulled from the tree. Underneath you抣l see brownish-black insects approximately ?inch in length with some white markings. When the webbing is removed, the insects usually move away in a group and are commonly called tree cattle because of this herding habit.   

Tree cattle do not damage trees. Some people will see the webbing as it glistens in the sun, walk to the tree and visually inspect it from top to bottom - much closer than they抳e ever inspected the tree. It was the webbing that got their attention. They might notice a dead branch or other imperfections in the tree and then wrongly blame the tree cattle. I抳e talked to homeowners that sprayed their trees with insecticides or that hired pest control businesses to treat the trees as a result of finding the webbing. I talked to one person that cut down a tree after finding the webbing assuming he was dealing with a pest that was going to move through his neighborhood and kill other trees.

Adult female psocids lay their eggs in clusters on leaves, branches and tree trunks. After hatching, the immature insects (nymphs) remain together under their silk like webbing. The web serves as a protection from weather and predators. In the adult stage, the wings of this insect are held roof-like over their body. The nymphs are wingless. Psocids usually have several generations per year in Florida .

After seeing the webbing, many people insist on spraying psocids with insecticides because of the concern that these insects are damaging their trees. But as mentioned, they are bark cleaners and do not damage trees. If the webbing is considered unsightly, a heavy stream of water from a garden hose can be used to wash insects and webbing off infested trees.

If nothing is done, the webbing usually goes away in several weeks. Psocids can be found on many rough-barked hardwood trees and palms. Most people seem to find them on oaks more often than other tree species, though.