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 \'irginia 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-fiies, 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 larvae of the bark-beetles feed 



