The worldly travels pathways of forest insects and pathogens

How non-native forest pests make their way to the U.S. and to Michigan.

Forest pests that make their way to United States from other countries can cause widespread damage to our forest resources as well as our economy. This article describes how forest pests and diseases move around the world as well as which pests we should look out for in Michigan.

Non-native forest insects have been arriving in the United States for hundreds of years. Some of the arrivals, such as the gypsy moth, were intentionally introduced for research purposes (but an accidental mishap allowed them to spread). In contrast, others, such as the Chestnut blight, was accidentally introduced likely by importing infected chestnut trees into the United States. Even today, non-native forest pests continue to enter the United States through a variety of pathways.

According to the Frontiers in Ecology and the Environment, over the last century, studies have shown that each year, 2.5 previously unrecognized forest pests have entered and established themselves in the United States. Some of the pests cause little to no damage; however others that cause noticeable damage to our forest resources establish themselves at a rate of one every two years. The damage caused by these forest invaders cost tax payers billions of dollars annually in treatment, eradication, and removal of hazard trees. Costs also trickle down to individual landowners in the loss of forest resources, costs of replanting trees and decreases in property values due to the loss of stately trees.

How do these pests arrive?

Forest pests arrive in the United States through a couple of different paths. Many of the damaging insects and pathogens arrive on live imported plants. These include sap and foliar feeding insects, as well as disease pathogens, all of which are hard to detect during the inspection process. The establishment of wood boring insects has increased faster than any other insect guild since the 1980’s due to the increased volume of containerized freight and accompanying wood packing material. The Emerald Ash Borer is an example of a pest that established in Michigan via packing material.

Once in the United States, these pests and pathogens move from the dock to the forest by way of humans. Some are unknowingly moved on the live plants on which they arrived, others are transported as the packing material in which they arrived is moved overland to its final destination. Deposited in the right place, these pests and pathogens can quickly spread to nearby forest resources. The Eastern United States is now home to more forest pests and diseases than anywhere else in the Country. According to the publications “A highly aggregated geographical distribution of forest pest invasions in the USA.” this is likely due to the diversity of tree species that can act as host to different pests and pathogens as well as the high density of wooded areas interspersed with urban and rural residential areas providing easy access for establishment into the greater forest resource.

What is being done?

The United States Department of Agriculture (USDA) has been working to reduce the inadvertent pest importation since the passing of the Plant Quarantine Act of 1912, which focused on limiting the agricultural pests. In 1972, the Animal and Plant Health Inspection Service (APHIS) was created, to carry on and expand the work the USDA had been conducting for over 100 years.

Today, APHIS operates 17 plant inspection stations located at ports-of-entry throughout the United States. In addition, recent regulations have required the treatment of wood packing materials to help ensure that wood packing materials are free of live pests. Interstate regulations and quarantines focused on the movement of both live plants and wood materials have also been put in place to limit the spread of a variety of pests and diseases.

Keep your eyes on the forest

Here in Michigan, there are three damaging forests pests that have established themselves in the United States, but not yet here in Michigan, although the potential is high. Please join the “Michigan Eyes on the Forest” effort to keep these pests out of Michigan by vigilantly watching for signs of their presence. Below are the three target pests with some information on how to recognize the signs of each pest. Please contact your local Michigan State University Extension office for more information about each.

The Asian Longhorn Beetle is a large wood boring insect that readily attacks maple trees, but the list of other host trees is extensive. Similar to the emerald ash borer, look for the dead branches near the tops of maple trees, as well as their exit holes, which can be the size of a pencil, wood shavings near the base of branches or the tree as well as the beetle itself. Please note, there are look-alikes in Michigan.

The hemlock wooly adelgid has been found in Michigan, likely due to the planting of infected nursery stock. Today, the Michigan Department of Agriculture and Rural Development (MDARD) readily inspects live plants for signs of infection, so close attention should be paid to looking for sign on and around hemlock trees purchased or planted before 2004. The adelgids appear as white cottony masses at the base of hemlock needles.

Thousand cankers disease of black walnut is a beetle – fungus disease complex. The walnut twig beetle, which is very small and may be hard to see, bores into the branches of black walnut and can carry spores of the damaging Geosmithia fungus. Once established, the fungus causes branches on the black walnut tree to die, which is what is likely to be noticed. Cankers form under the bark, however cutting individual branches down for inspection can be difficult and dangerous and should be performed by a professional.

If you suspect you have seen evidence of any of the pests mentioned above, please contact Michigan State University Department of Plant Pathology or contact MDARD at 800-292-3939 or at for positive identification.

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