It seems unjust that trees can succumb to the tiniest of problems. These mighty stalwarts of the forest survive wind, rain, snow, hail and myriad other problems and can live on for centuries. Yet almost without reason, creatures that are almost microscopic may eventually threaten some of these woody giants.
Insects are only one organism that can spell doom and gloom for woody plants. Pathogens including fungi, bacteria, mycoplasmas, viruses and nematodes are busy attacking the plants. Then add the environmental stresses of temperature extremes, water extremes, nutrient excesses and deficiencies and widely fluctuating temperatures. With all these problems it is a wonder that plants grow at all.Insects are more of a concern to most gardeners than the other prob-lems. Several reasons account for this. The pests are usually large enough to be seen, and their damage is often recognizable. Their feeding damage shows up as plant parts chewed away, as sucking damage where the plant liquids have been removed and as internal wood damage with boring insects. Although the damage is evident in many cases, it is important to distinguish between cosmetic damage and damage that may threaten the life of the trees.
Several insects are active right now that are potentially life-threatening to trees. Lilac borers attack both lilacs and ash trees. They are most destructive on the ash (Fraxinus) species. The adults are clear-winged moths that resemble wasps in appearance. Their wingspan is about 11/4-inch wide and they are black in color.
The larvae are the destructive stage, damaging the tree by feeding under the bark and into the wood for most of the year. They pupate inside the tree and emerge as adults in late April or May. The adults mate and lay their eggs in fissures in the bark or around pruning wounds. After the eggs hatch, the larvae invade the trees and feed through the following year.
Fresh pruning wounds are very attractive to the pests. Avoid pruning prior to the time the pests fly. Previously damaged wood is best removed during the late fall or early winter to avoid attracting the moths. Trees stressed for other reasons are also more severely affected. Plant the trees in good soil and make certain they are watered and cared for properly. Avoid damaging the trunks with string trimmers or lawnmowers.
Trees that are well-cared for but still suffer borer attacks are going to need some intervention to control the pests. Sprays of Dursban, Lindane or Thiodan are all effective when used according to label directions. The spray dates are a little later this year because of cooler weather during part of this spring. Apply the pesticide right now in the Salt Lake Valley. Spray the trunk and main branch areas. Unless the infestations in your area are extremely heavy, one spray is sufficient.
Birch trees are troubled with another insect pest. The bronze birch borer is a metallic-colored, wood-boring beetle. The adult beetles are seldom seen. The most common symptoms are the D-shaped exit holes created as the adult beetles emerge from beneath the bark. After the adults emerge they feed briefly on the foliage and then mate and start to lay eggs. They deposit eggs in cracks and crevices on the bark of the trunks and larger branches. The larvae tunnel into the cambium layer and feed there. This produces galleries that will slightly raise the outer bark and make it appear rough and swollen.
The susceptibility to bronze birch borer damage is greatly increased because the trees are not well adapted in our area. The borers routinely attack weakened or stressed trees. In Utah they routinely suffer from iron chlorosis and many other maladies because they are usually grown as single specimen trees in the lawn. Heat, wind and other problems take their toll and create ideal environments for beetle attack. Most of the potential problems are not easily controlled.
Besides avoiding stress on the trees, make certain they are pruned correctly. Do not prune the trees unless there are obvious problems. Pruning increases the attractiveness to potential borer infestations. Small branches that are infected with borers should be removed and disposed of before the beetles emerge in the spring.
No birch trees are immune to attacks from borers. Those with red bark such as the river or the water birches are less susceptible. Consider planting groves of birch trees. When a trunk is killed by an insect or disease it can be removed and another sprout trained up to grow in its place.
The best protection against any pest is to choose the right tree for the right place. Keep the tree healthy and avoid stresses that will weaken or damage the tree. Follow good integrated pest management principles and reserve sprays for those high-value trees that are threatened by pests.