PROTECTING SHADE TREES FROM INSECTS

R. A. ST. GEORGE.

Many kinds of insects attack shade trees. Some of the worst infest the trunk or the branches or the leaves. Some merely mar the appearance of the part attacked. Others cause severe injury. But of all of them it can be said that they have specific habits whereby they and their damage can be identified, assessed, and used to determine the need for applying control measures. It is convenient to separate the more important insect enemies of shade trees into two groups, those that attack weakened and dying trees and those that infest the more healthy ones.

THE FIRST GROUP includes many species of bark-infesting and wood-boring beetles. They can detect trees that have reached a decadent stage long before a man can see the changes associated with decadence.

The ambrosia beetles frequently are among the first insects to attack weakened trees. Their presence can be detected by the strings or piles of white, powdery frass that they push to the bark surface as they extend their tunnels deep into the wood. It is a positive indication that the tree is dying. If the infestation is confined to a small area on one side of the trunk, the tree might be saved by taking measures to revitalize it fertilizing and watering, and by applying a protective chemical spray to the stem of the tree. But if the attack extends entirely around the trunk, the processes of decadence are likely to have progressed so far that the tree will die, and the expenditure of large sums of money to save it is questionable. Often it is more practical to dispose of such a tree than to try to save it.

Certain of the roundheaded beetles attack dying trees. The females of some species of the roundheaded beetle make slits or pits in the bark and deposit their eggs in them. After the larvae have worked beneath the bark and into the wood their presence can be detected by the noise they make while cutting their tunnels and also by the coarse, shredded wood fibers that are pushed to the bark surface.

Many bark beetles attack weakened trees. They work between the bark and the wood. They make small shot holes in the bark and push their granular borings to the surface. Their color, which is similar to that of the bark, helps one to distinguish between the bark borers and wood borers. Certain species, known as turpentine beetles, are much larger than the rest of the bark beetles and confine their attacks to the bases of trees. A large, reddish pitch tube is formed at each point of entry. In the eastern and southern sections of the country, their attacks are mostly unsuccessful, but in the western pine regions turpentine beetles can sometimes kill slow-growing and injured trees and cause considerable concern to owners of mountain homes.

The obvious way to combat these insects is to keep the trees healthy to remove the factors responsible for the weakening of shade trees. Among the factors causing the most damage are prolonged droughts; earth fills; sun-scald and whipping of the stems, due to severe thinnings around trees left for natural shade; mechanical injury to the trunks and roots of trees made by heavy equipment while cutting roads and grading the soil around new homes; poor drainage; transplanting at the wrong time of year; and not using due caution in handling the trees or caring for them sufficiently until they are well established.

THE SECOND GROUP includes those insects that attack healthy trees. All parts of the tree are subject to infestation.

The stem borers include many kinds of beetles and moths. Their larvae cause injury by tunneling into the sapwood and heartwood of the trunks. The locust borer, which attacks black locust, is a good example of this group.

The twig borers and girdlers consist principally of certain roundheaded beetles, the larvae of which mine or girdle the terminal shoots. Sometimes the girdled terminals are not entirely broken off by the wind; then dangling dead branches become conspicuous, especially on hickory and oak trees.

The white-pine weevil attacks and kills the leaders of white pines.

A moth causes similar injury to the terminal shoots of the red and Scotch pines.

The elm bark beetles feed in the crotches of the smaller branches of the elm tree and, in doing so, transmit the spores of the destructive Dutch elm disease.

Certain sucking insects, known as chermids, attack the terminal shoots of white pines and frequently cause a marked drooping of the branches or their death.

The buds of several species of pines are subject to attack by tip moths.

The gall-making insects consist for the most part of tiny flies, certain plant lice, small wasps, and some mites. Most of them are relatively unimportant.

The leaf feeders include nearly all types of insects and their close relatives, the mites; the chewing insects destroy the foliage and the sucking insects remove the juices. Some mine the leaves; others work on the surface.

The elm leaf beetle and the Japanese beetle are good examples of the leaf-eating type. They skeletonize the foliage and cause the leaves to turn brown and drop to the ground. Repeated defoliations weaken elms and may cause their death.

Of these two insects, the Japanese beetle is by far the more important economically. Unchecked by its natural enemies and supplied with an abundance of its natural food plants, it soon spread over much of the eastern United States. Serious infestations now occur from Connecticut to North Carolina. The beetles appear during June and remain active until the latter part of August in the vicinity of Washington, D. C. They cause most extensive injury during the first 2 or 3 weeks, when they attack the upper and outer parts of trees and shrubs exposed to sunlight. The beetles also can seriously injure flowers, fruits, vegetables, and the grubs destroy the roots of grass and other plants.

The catalpa worm, or sphinx, is an example of the kind of insect that eats the entire leaf and frequently all the leaves of a tree.

The bagworms attack many kinds of trees. Their favorite host is arborvitae. Their presence can be detected by the cases or bags on the trees.

The locust leaf miner is a small, brownish beetle that deposits its eggs on the leaf surface. The new-hatched larva penetrates the leaf and mines the interior. Severe injury disfigures the leaves and may kill them.

Spider mites and such insects as the aphids, chermids, and scales suck the juices from the foliage of many kinds of shade trees. One leafhopper that feeds on elm leaves has been found to transmit the elm virus disease, which kills the trees more quickly than the Dutch elm disease.

GENERAL PRECAUTIONARY MEASURES can do much to prevent such insect damage to shade trees. Some, which do not require the use of chemicals, are aimed at safeguarding the trees from the weakening influences to which they are frequently subjected during and following new construction.

Trees that are being left to provide shade about new residences, after thinnings have been made among the remaining trees, should have their trunks wrapped with burlap or other suitable material to prevent sunscald in hot weather.

Isolated tall trees of small diameter should be anchored by guide wires to keep them from being whipped by the wind.

Trees cut in thinnings made during the fall months should be removed from the property before spring to avoid attracting insects; if they are cut in summer, they should be disposed of at once.

The lower part of the trunks should be boxed to protect the bark from mechanical injury while heavy equipment is being used about the property.