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Table of Contents >> Forest Health: Additional Reading << 5.1 Insects and Diseases >> 5.2 Invasive Plants



Endemic populations of native insects and pathogens are important in healthy forest ecosystems. However, introduced non-native exotics can cause excessive damage.

Insects are prey items at the very bottom of the food chain. Along with other decay organisms, some insects transform dead and dying plant material (including trees) into nutrients that feed new plants. Insects and diseases become problems when populations reach out-of-balance, epidemic levels. Tree-growth loss and mortality can occur and the economic impact can be severe. The most devastating insect and disease outbreaks often occur when non-native pests are introduced into locations where they have no natural enemies. Throughout North America, exotic insects such as balsam woolly adelgid, gypsy moth, pear thrips, Asian longhorned beetle, and emerald ash borer have all caused growth loss and mortality. Exotic diseases such as Dutch elm disease, chestnut blight, and butternut canker have virtually eliminated their host species.


Reduce undesired mortality and growth loss from native pests, limit introductions of exotic pests, and eradicate new introductions as they are detected.




Defoliators feed on leaves or needles during the growing season. Common examples include spruce budworm and large aspen tortrix in the north, saddled prominent and forest tent caterpillar in central New Hampshire, and gypsy moth and hemlock looper in the southern part of the state. A forest diversified in both age structure and species composition limits susceptibility to defoliators. Most insects are host-specific and prefer one or two species of a particular age group (e.g., a large, uniform area of sugar maple is highly susceptible to forest tent caterpillars and a large area of mature fir to spruce budworm).

Spruce Budworm: Increasing the proportion of spruce to fir and developing a mix of forest types and ages over several thousand acres will minimize spruce budworm. Consider the forest structure within a broader landscape rather than focusing on a single, small property.

Piercing-Sucking Insects

These insects are more chronic than the defoliators. Once infested, a stand remains infested for a long time. Hemlock woolly adelgid, balsam woolly adelgid, and elongate hemlock scale are non-native, exotic piercing-sucking insects.

Beech Bark Disease: Managing to reduce or eliminate beech bark disease will take several generations of silviculture. Some beech trees, recognized by their clean, smooth boles with a minimal presence of the white woolly scale, are resistant to the beech scale insect that precedes infection by the Nectria fungus.

Wood Borers and Bark Beetles

Sugar maple borer, oak borer, Ips beetles and Dendroctonus beetles are native New Hampshire borers and beetles. They tend not to grow past endemic levels and only attack stressed, dying, and dead trees. White pine weevil is a native borer that does attack healthy trees. The number of non-native, invasive wood boring insects in North America such as Emerald ash borer, Asian longhorned beetle, and sirex woodwasp is growing. Infestations continue to spread closer to New Hampshire each year. Once infested with these exotic pests, trees rarely survive more than a few years.

White Pine Weevil: To avoid excessive white pine weevil injury in the regeneration, maintain partial overstory shade.

Root Diseases

Root diseases result from a large group of fungi that cause decay, stress, reduced growth, and death. Common examples are Armillaria, and Heterobasidion annosum (formerly Fomes annosus). Root-attacking fungi such as Armillaria are present in almost all forest soils. Damaging root-fungi attacks require favorable moisture and oxygen conditions, a point of entry into the host tree, and low tree vigor, which combine to make it difficult for the tree to defend itself.

Stem-Canker Diseases

Stem-canker diseases are fungi that attack the stem, shoots and branches and cause lesions or dead areas on the stem. Common examples include Nectria canker, Caliciopsis, blister rust, chestnut blight, and Eutypella cankers.

Foliage Diseases

Foliage diseases result from organisms that attack needles and leaves. Common examples include Anthracnose, needlecast fungi, tar spot, and sooty mold.

Heart Rots

Heart rots are the decay fungi that penetrate to the center of a tree and rot the core from the inside out. There are white rots that feed on lignin and cellulose and red rots that feed just on cellulose. The red rots leave a brown or red brittle material, while the white rots leave a white coloring where lignin was removed. Fruiting structures of these diseases are often shelf-like conks attached to the sides of the tree.

Other Diseases

Other diseases are viruses, mycoplasma-like organism (MLO), and bacteria. Ash yellows is an MLO. The microbe is thought to be carried from tree to tree by leaf hoppers. These insects spend a period of time in open grassy areas which may explain why ash yellows is more common in urban settings than in the deep forest.


2.1 New Hampshire Forest Types; 2.2 Forest Structure; 2.3 Regeneration Methods; 2.4 Managing for High-Value Trees; 6.2 Cavity Trees, Dens and Snags; 6.3 Dead and Down Woody Material.


Forestry Images. http://www.forestryimages.org/insects.cfm Accessed March 7, 2010.

N.H. Division of Forests and Lands. Forest Health homepage. http://www.nhdfl.org/forest-health/ Accessed March 7, 2010.

RSA 227-K. Forest Health. http://www.gencourt.state.nh.us/rsa/html/NHTOC/NHTOC-XIX-A-227-K.htm Accessed May 27, 2010.

UNH Cooperative Extension. Common Pests of NH Trees and Shrubs. http://extension.unh.edu/fwt/forpest/compests.cfm Accessed August 2, 2010.

University of Georgia. Center for Invasive Species and Ecosystem Health. http://www.bugwood.org/ Accessed March 7, 2010.

USDA Forest Service—Northeastern Area. Forest and Tree Health Publications and Fact Sheets. http://www.na.fs.fed.us/pubs/misc.shtm Accessed March 7, 2010.

USDA Forest Service—Forest Health Protection. Forest Insect and Disease Leaflets. http://www.fs.fed.us/r6/nr/fid/wo-fidls/ Accessed March 7, 2010.

USDA Forest Service—Forest Health Protection. Forest Health Management homepage. http://www.fs.fed.us/foresthealth/management/index.shtml Accessed March 7, 2010.

Forest Health: Additional Reading << 5.1 Insects and Diseases >> 5.2 Invasive Plants

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