Each type of insect affects very specific tree parts. Insect damage is categorized here by the tree part affected by the insects. Strategies for minimizing insect damage are discussed in Chapter 6: Managing Important Forest Types for the more serious pests.
Defoliating insects remove all or part of a treefs foliage. They weaken the tree by lowering its capacity to respire and to produce starch and sugars. Foliage damage takes many forms. Insects that remove only soft leaf tissue and leave the network of veins are called skeletonizers. Leaf miners bore into and eat the tissue between the upper and lower surfaces of the leaf. Window feeders eat one leaf surface, leaving the other intact. Case bearers and bag makers construct and live inside individual movable cases that are made of webbing and foliage parts. Needle tiers and leaf rollers encase, fold, roll, or tie adjacent leaves and needles together with webbing. Webworms or tent caterpillars make and live in conspicuous webbed tents. Other insects are free-feeders that eat the entire leaf or needle.
Free-feeders are the most destructive defoliating insects. Populations go through boom and bust cycles. When these insects are abundant and defoliate trees in two or more consecutive years, they can kill trees. Figure 7-7. Red oak defoliated by Gypsy moth in June.
Sapsucking insects injure trees by removing tree fluids. They usually are not serious pests in woodlands. However, heavy attacks lower a treefs energy reserves and may lead to a secondary pest problem. The general symptoms of sapsucking injury are loss of vigor, deformed leaves or plant parts, yellowed leaves, or dead branches. Galls (abnormal tissue growths) also may form.
The destructive stage of the insect usually is required for precise identification, but sometimes the presence of feeding punctures, sooty mold, eggs, fine webbing, and other signs suffice. The Saratoga spittlebug has been a serious pest in red pine plantations. Several scale insects, such as the pine tortoise scale and the pine needle scale, are important in Midwestern forests. Aphids and mites are other sapsucking organisms that may affect your trees.
Bud, Twig, and Seedling Damaging Insects
Most of these insects deform but do not kill trees. The white pine weevil is one of the most damaging insects in conifer plantations in the Lake States. White pine is their favorite host, they also attack other pines and spruces. The weevil larvae feed just below the terminal bud and cause forking and crooking, especially in open-grown trees from 2 to 20 feet tall. Open-grown trees are widely spaced with no crown competition between trees. The new growth elongates slightly before dying because of the larval borings. Weevils usually do not attack the current yearfs shoot, but it commonly wilts into a gshepherdfs crook.h Another damaging weevil, the pales weevil, eats the bark on young seedlings.
The succulent and nutritious inner bark on tree stems and large branches attracts many insects, most notably the bark beetles. Adult bark beetles deposit eggs beneath the bark. The emerging larvae feed on the cambium and phloem, preventing stem growth and the normal movement of sugar and water. They hasten the death of weakened trees, attack apparently healthy trees during population explosions and drought, and lower lumber value. They also can introduce disease organisms such as Dutch elm disease fungus and blue stain fungus.
The pine engraver is the most common bark beetle in Lake States pine stands. It attacks healthy trees en masse during drought. Flat-headed inner bark borers such as the bronze birch borer, two-lined chestnut borer, some weevils, and roundheaded borers feed on the inner bark. They rarely kill or damage more than a few trees unless the trees are severely stressed by drought or defoliation. Bark borers are difficult to control with contact insecticides because they are sheltered beneath the bark. Systemic insecticides also have little effect because these insects disrupt water movement in the tree. Cultural practices such as thinning that maintain tree vigor provide good protection. Figure 7-8. Bark beetles killed these drought-weakened red pines.
Wood-boring insects attack very low vigor or recently killed trees and rarely are a problem in vigorous stands. While common, they rarely cause tree death. They feed for several weeks in the bark before boring into the wood. Flat-headed wood-borers, round-headed borers, horntails, powder-post beetles, ambrosia beetles, and ants are wood-boring insects of common concern. Problems with chemical controls are the same as with bark borers.
Root-feeding insects are mostly a problem in nurseries or in young plantations where sod is well established. They disrupt the absorption and movement of water and nutrients. Root maggots, cutworms, root bark beetles, white grubs, and root-collar weevils are examples of root-feeding insects. In plantations, kill or remove the sod before planting trees. Killing sod after planting may cause these insects to concentrate their attacks on tree roots.
Cone and Seed Destroying Insects
Beetles, weevils, moths, and wasps may destroy cones and seeds. Usually they deposit eggs in a seed or cone. The developing larvae then eat and destroy the seed. The red pine and white pine cone beetles, red pine cone worm, spruce cone worm, acorn weevil, and walnut weevil are some of the common seed-destroying insects. Insecticides can help control these insects; however, their use is justified only in woodlands used as seed production areas. Few insecticides are registered for this use. Most are very toxic and require application by a licensed commercial applicator.