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Issues Of The Environment: 2018 Gypsy Moth Caterpillar Outbreak In Washtenaw County

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Gypsy moth caterpillars have completely stripped the leaves of trees in portions of Chelsea and caused significant defoliation in portions of Ann Arbor.  In this week's "Issues of the Environment," WEMU's David Fair talks about the infestation with Bob Bricault,  horticulture educator for Michigan State University Extension in Washtenaw County.  


  • This summer, Washtenaw County has experienced a severe gypsy moth caterpillar infestation that has residents concerned about the health and well-being of their trees.
  • Gypsy moths are an introduced pest from Europe and Asia, that appeared in the late 1800’s.  They feed on most types of trees, preferring oak and aspen.  According to the U.S. Forestry Service, over 20 insect parasitoids and predators from other continents were introduced over the last 100 years. Small mammals are a significant source of control.  In addition, a viral and a fungal infection that devastates gypsy moths seems to follow severe outbreaks, keeping population explosions in check. Spraying with the pesticide Bt has been widely used in the past, but it is not considered necessary or a best practice today because is also kills native butterflies and moths.
  • According to local experts, although the gypsy moth outbreak is more severe than it has been in years, the damage done by the caterpillars is not likely to permanently damage defoliated trees, and it is unlikely that repeat events of this severity will follow in successive years.
  • The City of Ann Arbor has established a study protocol to monitor the gypsy moth situation, but the city will not be doing any spraying.  Damage has been severe on the north end of town.  The University of Michigan North Campus has also been extensively infested, and the University also has a monitoring program.
  • Bob Bricault, Michigan State University Extension - Horticulture Educator for Washtenaw County, will explain why most landscapes in our area have not experienced problems with gypsy moth since the 1990’s, and why is it back.  He notes that the webs people might see on trees in the fall are fall webworms, not gypsy moths.

Gyspy Moth Facts from the U.S. Forestry Service

The gypsy moth, Lymantria dispar, is one of North America's most devastating forest pests.  The species originally evolved in Europe and Asia and has existed there for thousands of years. In either 1868 or 1869, the gypsy moth was accidentally introduced near Boston, MA by E. Leopold Trouvelot.  About 10 years after this introduction, the first outbreaks began in Trouvelot's neighborhood and in 1890 the State and Federal Government began their attempts to eradicate the gypsy moth.  These attempts ultimately failed and since that time, the range of gypsy moth has continued to spread.  Every year, isolated populations are discovered beyond the contiguous range of the gypsy moth but these populations are eradicated or they disappear without intervention.  It is inevitable that gypsy moth will continue to expand its range in the future.

The gypsy moth is known to feed on on the foliage of hundreds of species of plants in North America but its most common hosts are oaks and aspen.  Gypsy moth hosts are located through most of the coterminous US but the highest concentrations of host trees are in the southern Appalachian Mtns., the Ozark Mtns., and in the northern Lake States.

Gypsy moth populations are typically eruptive in North America; in any forest stand densities may fluctuate from near 1 egg mass per ha to over 1,000 per ha.  When densities reach very high levels, trees may become completely defoliated.  Several successive years of defoliation , along with contributions by other biotic and abiotic stress factors, may ultimately result in tree mortality.  In most northeastern forests, less than 20% of the trees in a forest will die but occasionally tree mortality may be very heavy.

Forest Effects:

Despite over 100 years of presence in North America, researchers are still at a loss to explain and predict the extent of the changes in forest vegetation likely to take place through gypsy moth disturbance.  A major concern is the potential loss of economically critical and ecologically dominant oak species (Quercus, spp.).  Most studies of forest compositional changes with gypsy moth defoliation indicate that less susceptible species will dominate the forest, so in effect, forests may have fewer gypsy moth problems in the future.

Natural Enemies:

A variety of natural agents are known to kill gypsy moths in nature.  These agents include over 20 insect parasitoids and predators that were introduced over the last 100 years from Asia and Europe.  Small mammals are perhaps the most important gypsy moth predator, especially at low population densities. Birds are also known to prey on gypsy moths but at least in North America this does not substantially affect populations.  A nucleopolyhedrosis virus usually causes the collapse of outbreak populations and recently an entomopathogenic fungus species has caused considerable mortality of populations in North America.


Over the last 20 years, several millions of acres of forest land have been aerially sprayed with pesticides in order to suppress outbreak gypsy moth populations.  Though some areas are treated by private companies under contract with land owners, most areas are sprayed under joint programs of state governments and the USDA Forest Service.  Your local extension service can provide more detailed information about programs in your area.

The USDA, State and local governments also jointly participate in programs to locate and eradicate new gypsy moth populations in currently uninfested areas.  Most of these projects focus on populations of European origin, but recently several Asian populations have been discovered and eradicated in the US and Canada.

In 1992, the USDA Forest Service began a pilot program to test the feasibility of slowing the spread (STS) of the gypsy moth in North America.  STS pilot programs currently exist in North Carolina, Virginia, West Virginia, and Michigan.


The gypsy moth has been intensively studied over the last 100 years in North America.  Currently there are numerous groups around the country investigating various aspects of the biology, ecology, and management of the gypsy moth.  This work is funded by the USDA Forest Service, the USDA Agricultural Research Service, the USDA Cooperative State Research Service, the USDA Animal and Plant Health and Inspection Service, and numerous state and private Universities.

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— David Fair is the WEMU News Director and host of Morning Edition on WEMU.  You can contact David at734.487.3363, on twitter @DavidFairWEMU, or email him at dfair@emich.edu

Contact David: dfair@emich.edu
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