The Economic Importance of Shade Tree Insects

Abstract

It is not so easy, in the case of shade trees, to establish the relation between a generally unsatisfactory condition and insect infestation as in the case of fruit trees. There is no difficulty in seeing the connection between a dying tree and the work of cambium borers, such as the two-lined chestnut borer, the bronze birch borer, the spotted hemlock borer and the sugar maple borer. The scale insects constitute a relatively inconspicuous group capable of causing serious injury. The oyster shell scale and the elm bark louse, in particular, are occasionally very abundant and injurious, the former frequently killing good sized ash and poplar trees. The tulip tree scale is another pernicious insect. The aphids or plant lice are occasionally extremely abundant and while the immediate results may not be as striking as in the case of severe infestations by scale insects, there is undoubtedly a lowering of vitality, through interference with the functioning of the leaf. A small series of gall insects produce appreciable injury. This is especially true of a number of the knotty oak galls. In the case of shade trees, symmetry and sightliness are important factors, consequently insects which affect the normal development of the tree by preventing growth or killing branches are undesirable. The leaf eating insects, such as canker worms and the elm leaf beetle frequently devour a considerable proportion of the foliage, and this can but have a material effect upon the vigor and the growth of the tree. The periodical outbreaks of the Pandora moth in western pine forests have resulted in the reduction in the width of the annual rings by 80%. It is more than probable that severe infestations by scale insects and plant lice materially affect the thickness of the annual rings. There are other causes which may produce this, namely deficient rainfall and thin soil or a gradual depletion in soil fertility. It is not easy to differentiate between the various causes. The mere reduction in the amount of wood produced in anyone season is not the most serious phase of the problem. Weakening of branches or leaves is usually evident first in the upper limbs and is followed by greater susceptibility to both insect attack and fungous infection. It is believed that a combination of these agencies, each of variable importance under different conditions, explains in considerable measure the many dying tips generally assoicated with what we consider a “stagheaded” condition. We need more data as to the part insects play in producing such conditions. How much of this can be traced back to earlier attacks by insects? Shade trees have a material value and it is our contention that this should be taken into account in estimating the economic importance of shade tree insects. We believe it much more economical to protect a tree from insect depredations than it is to replace such a tree with one of equal size.