Invasive insects could infest 170M Michigan hemlock trees 

Hemlock Wooly Adelgid | Wikimedia Commons
Updated, 1:26 p.m., 8/26/19

Nature conservancies and the state of Michigan have been teaming up to save Michigan’s immense hemlock tree population. 

There are 170 million eastern hemlocks in Michigan that are at risk from an invasive insect smaller than a letter on this page — less than two millimeters in length.

Hemlock Woolly Adelgid | Michael Montgomery USDA, Forest Service

Hemlock Wooly Adelgid (HWA) is an insect that feeds on the sap of eastern hemlocks and will kill the host tree in four to 10 years. When feeding, insects secrete a white wax that remains at the base of the hemlocks needles, making it far easier to identify. 

HWA is believed to have first come to Michigan in shipments of out of state hemlocks, said DNR Plant Industry Specialist Mike Bryan. It was first discovered infesting native hemlock populations in Michigan in 2015 and is present in Allegan, Ottawa, Muskegon and Ottawa counties. 

This may be less well-known than other invasive species like Asian carp, but HWA also poses serious environmental and economic concerns.

In order to protect against new infestations of the insect, hemlock shipments are required to be held in a quarantine zone and inspected prior to being held or sold elsewhere in Michigan. These quarantines have been in place since 2001, but despite these precautions, two properties in Ottawa County were treated for HWA in 2010. 

“We’re pretty sure that they came in prior to 2001, from infested areas in the Eastern United States,” Bryan said.

The current management plan is to stop the expansion of HWA in West Michigan in a natural bottleneck of hemlock growth. Due to the implementation of the Michigan Invasive Species Grant Program (MISGP), experts believe they now have the funding and tools to corral and potentially eradicate this invasive species.

The program is a joint effort started in 2014 and provides $3.6 million annually to nonprofits, government agencies and universities working to prevent, detect and control invasive species in Michigan

The MISGP created opportunities for collaboration between the Michigan Department of Natural Resources; Department of Agriculture and Rural Development; Department of Energy, Great Lakes and Energy; and local conservation programs. This collaboration has assisted in dealing with pests such as HWA. 

Map of HWA in Michigan |

MISGP funding has allowed for the expansion of Cooperative Invasive Species Management Areas (CISMAs) from 65 to all 83 counties in Michigan. There are 19 CISMAs located around Michigan working to monitor, prevent, mitigate and eradicate invasive species in their areas. 

“We have infestations in some of the state parks and the DNR is paying to have that managed,” DNR Invasive Species Program Communications Coordinator Joanne Foreman said. “But right across from a state park could be a private landholder and the state can’t treat on private land.”

Unlike state agencies, CISMAs are able to work within the surrounding community and area to ensure that an inspection and treatment is thorough.

“That’s been one of the extremely effective parts of utilizing the grant funding, so we can treat invasive species across public and private lands, without having that boundary cause areas to be re-infested,” Foreman said. 

The Ottawa Conservation District was awarded $500,000 in 2017 from the MISGP to survey, map and treat infestations of HWA in West Michigan. HWA has devastated much of the hemlock population in the eastern United States. Michigan was regionally protected from the spread of HWA until 2015 when it was first found in southern Ottawa and southern Muskegon counties. 

“My goal is to slow and contain the spread,” said Western Michigan Invasive Species Coordinator Drew Rayner.

Oceana County* is the northernmost point that HWA has been confirmed in Michigan. It also serves as a bottleneck of hemlock growth. South of the county, hemlocks grow in proximity to Lake Michigan. North of Oceana County*, hemlocks grow from the east to the west side of the state.

Once the infestation has been stopped from advancing north, Rayner believes that focusing on eradication will become feasible. 

“Prior to 2017, we received our first funding for HWA and that was a federal grant, which got us going off the ground,” Rayner said. “After that grant was completed, there have been five projects projects that have been funded by the [MISGP] to address HWA.”

Two of those grants were for treatment and survey of hemlocks on private land. Two more grants funded surveys of trees north and south of the infected areas in an attempt to stay out ahead of the infestation in western Michigan, as well as a grant to monitor the hemlock population in the Upper Peninsula. 

“Without the grants, we would not be able to be out treating trees and helping landowners,” he said.

Rayner said the primary reason for success has been the cooperation between local conservation groups and the state. 

“Forest health specialists are coming out, making sure we have the knowledge and experience to do these treatments. We’re helping them with their surveys. … Without all the people that are present right now, we would not be successful.”

An initial mailer was sent to households in Osceola County, which included information about HWA and a request for permission to treat on their land. Rayner said now most individuals in the county are not only aware of the work being done, but they have welcomed the protection of hemlocks on their property.

“We understand that the best way to manage invasive species is to prevent them from even getting here. And the only way we can do that is by letting the average person know what they can do to prevent invasive species movement,” Foreman said. “It’s the most cost-effective thing we can do.”

There are a variety of pathways that have been identified for how invasive species are introduced. Using quarantines and working with Michigan tree nurseries has been an important, but imperfect method of stopping new HWA infestations.

National HWA map | USDA

In the HWA quarantine area of Allegan, Ottawa, Muskegon and Oceana counties, more than 20 firms have enrolled in a special HWA Nursery Program using enhanced record-keeping, inspections, scouting and treatments by trained employees, so that pest-free hemlock nursery stock can be moved through quarantine boundaries.

“It’s rare when we find [HWA],” said Bryan. “Where there have been a few incidences of infested trees is when they’re shipped by someone in another state who doesn’t know about our quarantine.”

Hemlocks are generally inspected by suppliers prior to shipping and the quarantine only finds infested hemlocks every two to three years. Nurseries growing Eastern Hemlock also go through a certification program. 

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Using a certified treatment program is generally required for shipment to other states and Canada and ensures that the trees are not hosts to HWA.

“The program involves the firm scouting their trees,” said Bryan. “It also involves an annual inspection by our staff and a mandatory treatment with an approved pesticide.”

* Correction: Oceana County is the northernmost area in Michigan HWA has been found. The story originally had an incorrect county.


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