44 Disease and Death of Trees — Generalities 
plants which do not feed on the host plant, but use it simply 
as support, such as mosses and lichens, orchids and various 
climbers, like the Virginia creeper. As a rule, these are of 
little consequence; yet if in excess may become troublesome. 
The excessive development of mosses and lichens on the 
bark is a sign either of a very humid atmosphere, of very 
wet soil, or of very slow growth of the tree. In the latter 
case, the outer bark is sloughed off very slowly, thus giving 
time for the epiphyte to develop. Usually there is no 
damage to be anticipated from the epiphytes, but, if exces- 
sively dense, the cover of lichen and moss may become: 
injurious by closing the breathing pores of the bark, and 
had better be removed. 
Creepers are usually harmless, but may become objection- 
able on young trees and young parts, like wires or artificial 
ligatures, if the pressure with which they resist the expansion 
of the body of the host plant becomes so great as to retard 
or prevent the supply of formative materials to the portions 
below the constricted part. In such cases the lower portion 
will be retarded in its diameter growth. 
Insect Damage. A host of insects belonging to the orders 
of beetles, moths, butterflies, gall-flies, saw-flies, plant and 
bark-lice, are parasitic on all parts of trees, feeding on leaves, 
twigs, cambium, roots, and wood. 
Sometimes it is the imago, the beetle, that does the mis- 
chief, but mostly it is the larva, the grub or caterpillar, 
which is to be feared. Some live and feed on the exterior 
of the plant, especially those which depredate the foliage, 
others are internal feeders, living during some stage of their 
development within some part of the host plant. Such are 
the gall-insects and leaf-miners, which lay their eggs in the 
soft tissues of leaves, leaf-stalks, or fruit, where the larva 
develops and feeds. The larve of the bark-beetles feed 
