Street trees, like all other forms of vegetation, are subject to attacks of insects and diseases. Because of the unfavorable conditions under which they grow, spraying for biting and sucking insects and suitable treatment for borers or other burrowing insects require especially careful attention.
In addition to a number of troubles common to street trees in general, each species is liable to troubles of its own; hence, the need of competent supervision by a trained man with an efficient outfit rather than leaving; the work to individual initiative.
Because of the height which many street trees attain a powerful outfit is required to spray them properly. One capable of maintaining a pressure of 200 pounds per square inch is desirable. The type of spray required for tall trees is different from that used on fruit trees and other low plants. For low trees the ideal spray is a mist within a few feet of the nozzle, application being accomplished by having the nozzles near the foliage to be treated. For tall trees it is desirable that the liquid should leave the nozzle in a solid stream, which is broken into spray as it passes through the air. The material has to be projected with sufficient force to reach the highest trees before being entirely converted into mist, as it is impracticable to extend the nozzles into the trees to reach the farthest portions, as is done with fruit and other low trees. The spray can not be applied as uniformly as a mist, but it is impracticable to climb into the tops of shade trees to cover every part with a cloudlike spray. On the other hand, the mist spray is better for small trees, as much injury may be done to low trees or to the lower branches of high trees by the force of the stream from high-pressure outfits.
It is estimated that in practice up to 95 per cent of the attacking insects can be killed with insecticides carefully applied by the stream method under high pressure.
In addition to the mechanical problem of satisfactorily covering high trees with insecticides or fungicides there is the problem of selecting materials that will be effective against the insects and diseases and at the same time will not disfigure the paint or stone work of adjacent buildings with which the materials must inevitably come in contact in street tree spraying. It frequently happens that the most effective remedies must be rejected because of the damage they would do to buildings and that less efficient materials must be used.
Whitewashing the trunks of trees is a useless and unsightly practice—useless, as it does not prevent the attacks of insects, and unsightly, because it makes the trunks of the trees obtrusive when they should be inconspicuous.
Banding with cotton or proprietary preparations may occasionally be useful, but because such applications are so seldom helpful and because some of the preparations result in injury due to constriction of the trunks, it should not be resorted to except upon special recommendation of an entomologist familiar with the existing conditions.
Details as to enemies to be expected, methods of treatment, and materials to be used may be found in other publications[88] or may be obtained by correspondence with the nearest State agricultural experiment station or with the United States Department of Agriculture.
[88] See list on following pages.