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Article #60, June 2002
By Bill Cook

     The warm season is when people seem to pay more attention to trees and the forest.  Indeed, outdoor activity peaks during the spring and summer.  Insects and diseases play a huge role in this biological drama. 

     Of the thousands of insect species, only a few pesky species usually get noticed.  However, most are beneficial to forests, some even quite important to forest health for a variety of reasons.  Insects pollinate tree flowers, cull out weak trees, recycle nutrients, create habitat for wildlife, and provide a huge base for food webs.  Many insects feed on others that feed on trees.

     On the other hand, mosquitoes, black flies, junebugs, and ladybugs receive more than their fair share of attention.  Dramatic defoliations by forest tent caterpillars, budworms, and gypsy moths make news headlines.  Even “pesky” insect species play valuable roles in the ecology of forests. 

     Insect life cycles demonstrate a high degree of diversity.  Most quietly go through the standard stages of egg, larva, pupa, and then adults.  Some species may have multiple hatches per year.  Others are closely tied with specific ecological phenomena.  Some have cycles that rise and fall in terms of decades, including some of our most notorious forest damaging species. 

     The forest has an amazing array of microhabitats and insects exploit all of them.  An individual species may use different habitats for each life stage, including a mix of aquatic and terrestrial sites.  Others may live out their cycle tied to a single host species or a narrow selection of related species.  Insects have a seemingly endless number of clever survival strategies.

     Disease organisms have a similar diversity of life cycles, appearances, hosts, and environmental requirements.  Every species, living or dead, will eventually serve as a host for fungi, bacteria, viruses, or more obscure decay taxa.  Diseases that attack live trees often specialize in leaves, buds, roots, fruits, or other tree parts.  Initial invasion can sometimes spread to the entire tree, causing growth loss or death.

     Insects and diseases may work together, such as the recently discovered beech bark disease.  White pine blister rust and spruce needle rusts need alternate hosts nearby.  There are also relationships with animals, such as birds or mammals.  Factors such as tree age, tree species, soil quality, moisture, disturbances, proximity to primary hosts, stand conditions, weather, etc. all weigh-in to determine the presence, spread, and degree of damage to a particular tree. 

     For most forest owners, identification of tree problems is difficult.  Sometimes, correct identification of the tree species presents a problem, too.  A particular symptom may be indicative of several or many damaging agents.  For instance, woodpecker holes aren’t really the problem.  It’s the poor tree health that has attracted bark beetles, which attract the woodpeckers.  Or, ants will sometimes invade tree tissues weakened by fungi, which in turn, became established after a drought or after the septic truck drove over the roots.  Tree health is impacted by a complex of factors.

     Diseases can be especially difficult to identify.  Often times, fruiting bodies are present only during a short period of time in the spring or fall.  Sometimes a tissue culture must be grown and any number of fungi might provide a choice of what the real problem is.  In the end, there is usually little a forest owner can do anyway.

     Another dimension of insects and diseases is the introduction of exotic species.  Historically, Europe has provided problems such as chestnut blight, Dutch elm disease, and the gypsy moth.  China and east Asia will likely become a larger source.  Imagine the impacts if maples would succumb to a new threat!  Maples comprise one-third of the forest volume in the U.P.

     The United States spends millions of dollars every year to combat forest insects and diseases and prevent the introduction of new species.  Our forests are resilient enough to tolerate most damaging agents, even the exotic species.  Controls are sometimes unnecessary, especially for our native species, even the infamous forest tent caterpillar.  However, one must consider the cumulative affects over time and, of course, the chances of what the next “big one” might be.

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Bill Cook is an MSU Extension forester providing educational programming for the entire Upper Peninsula. His office is located at the MSU Upper Peninsula Tree Improvement Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu or 906-786-1575.

Prepared by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005 J Road, Escanaba, MI  49829
906-786-1575 (voice),  906-786-9370 (fax),  e-mail:  cookwi@msu.edu

Use / reprinting of these articles is encouraged. Please notify Bill Cook.
By-line should read "Bill Cook, MSU Extension" Please use the article trailer whenever possible.

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