Forest health: Insects

Each kind of insect affects very specific tree parts. Insect damage is categorized here by the tree part affected by the insects.

Defoliating Insects

Defoliating insects remove all or part of a tree’s foliage. They weaken the tree by lowering its capacity to produce starch and sugars.

Foliage damage takes many forms. Insects that remove only the softer 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 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 within conspicuous webbed tents. Other insects eat the entire leaf or needle.

Sapsucking Insects

Sapsucking insects injure trees by removing tree fluids. They usually are not serious pests in woodlands. However, heavy attacks lower a tree’s 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, illustration of sapsucking insecteggs, fine webbing, etc., may 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, midges, and mites are other sapsucking organisms that may affect your trees.

Bud, Twig, and Seedling Damaging Insects

Most of these insects deform trees. The white pine weevil is one of the most damaging insects in conifer plantations (notably pine) in the Midwest. White pine is the favored host, but other pines and spruces also are attacked. 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. The new growth elongates slightly before dying because of the larval borings. The current year’s shoot is not attacked, but commonly wilts into a “shepherd’s crook.” Another damaging weevil, the pales weevil, eats the bark on young seedlings.

Bark-Boring Insects

The succulent and nutritious inner bark on the main stem and large branches attracts many insects, most notably the bark beetles. They tunnel beneath the bark and can girdle a tree, thus preventing 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 the lake States pine stands. It mass-attacks healthy trees 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.

Wood-Boring Insects

Wood-boring insects attack very low-vigor or recently killed trees and rarely are a problem in vigorous stands. While common, they rarely cause a tree’s demise. 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

Root-feeding insects are mostly a problem in nurseries or in young plantations where sod is wellestablished. They disrupt 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, control the sod before planting trees. Killing sod after planting may cause these insects to concentrate their attack on tree roots.

Cone and Seed Destroying Insects

Beetles, moths, and flies 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, they are justified only in woodlands used as seed production areas or in oak stands where acorn production is critical for wildlife. Few insecticides are registered for this use. Most are very toxic and require application by a licensed commercial applicator.

Non-native invaders

Although many insects are a normal part of native woodland ecosystems, nonnative insects are a different story.  Nonnative insects and diseases, can lead to widespread tree mortality.  For instance, the emerald ash borer (EAB), which was discovered in Minnesota in 2009, rapidly invades and kills ash trees.  EAB is native to Asia.  Because our forests did not evolve in the presence of EAB, they are not adapted to survive EAB infestations.  There will therefore likely be widespread ash mortality when EAB arrives in Minnesota.

Similarly, the Gypsy moth is likely to cause widespread damage to Minnesota’s native forests.  Gypsy moths are defoliating insects.  Like other defoliators, Gypsy moths are not likely to kill trees.  However, repeated defoliation is a source of stress that can make trees more susceptible to other insects or diseases.

Mel is the Extension Forestry program leader. He’s based on the St Paul campus.

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  1. Hello : Last fall i have purchased a home home in Rogers MN.

    I have (6) white pines.. The trunks measure 6/8 inches in diameter.
    The trunks are full of holes.

    Who or what is doing this? Needles are green and we have many pine cones. Any idea as to what is causing the holes in the trunks??

    Thank you,
    Steve Zanosko

    1. Hi Steve. The bark damage could be from many things, but a likely candidate is yellow-bellied sapsucker. Are the holes in regular rows? You can see a few pictures of yellow-bellied sapsucker damage on Minnesota white pine here: picture 1, picture 2. If those don’t look right, email a photo to me at and I’ll post it. Hope this helps.

  2. Hi Steve,

    I was throwing wood into my basement for the winter (I use a wood burning stove to heat the house.) and I noticed numerous round holes in some of the wood. One piece inparticular had a bunch of little tiny “baby” bugs–and one big reddish “momma” bug??? There was a lot of powder on the wood. Is it ok to throw the wood into the house to burn over the winter–or will I have a huge infestation?