Gypsy moth populations are high: What conifer growers need to know

There were extremely high populations of gypsy moth during 2020 and we anticipate elevated populations this year. Scout and treat spruce, fir and Douglas fir for gypsy moths.

Aerial map of Michigan with red blotches in certain areas showing high gypsy moth populations.
Photo 1. Gypsy moth aerial survey performed during 2020 showed up to one million acres were defoliated in 2020. Photo from Michigan Forest Health Highlights 2020 Report.

Gypsy moth populations are expected to be very high again this year. According to the Department of Natural Resource’s report, “Michigan Forest Health Highlights 2020,” Michigan’s Lower Peninsula had nearly one million acres that were defoliated by gypsy moth. According to a map of gypsy moth defoliation provided in the report, the heaviest defoliation was in Montmorency, Alpena, Oscoda and Alcona counties in eastern Michigan and Manistee, Lake and Newaygo counties in western Michigan (Photo 1). Other Michigan State University Extension educators report many egg masses in more urban counties, such as Kent County, and others in southern Michigan. Michigan has not seen this much defoliation since the early 1990s when the populations of gypsy moth last exploded.

Gypsy moth spends the winter in buff-colored egg masses in sheltered areas or on trunks of trees (Photo 2). These eggs hatch in the spring and the young caterpillars (Photo 3) spread to new locations by crawling to the tops of trees where they spin a silken thread and are caught on wind currents. Once they settle, they will begin to feed and develop for about seven weeks until they are approximately 1.5 to 2 inches long. Typically, this occurs in early summer—June to early July. They then enter a pupal or transitional stage.

Egg masses.
Photo 2. Gypsy moth overwinter in buff-colored egg masses. Photo by Milan Zubrik, Forest Research Institute - Slovakia, Bugwood.org.
Gypsy moth caterpillar
Photo 3. Gypsy moth caterpillar. Photo by Jon Yuschock, Bugwood.org.

Implication for spruce, fir and Douglas-fir growers

Nursery and Christmas tree growers of spruce, fir and Douglas-fir should scout fields and any woodlots or fence rows around fields for gypsy moth activity (Photo 4). Gypsy moths can cause severe defoliation of these species and reduce the aesthetic quality of the trees (Photo 5). These species are at high risk for gypsy moth because they are known hosts of gypsy moth, and the branches and needles are so dense that it is almost impossible to find 100% of the egg masses laid in the tree (Photo 6).

Nurseries that ship trees and shrubs out of Michigan must have the stock certified for the USDA Gypsy Moth Quarantine if it is shipped to non-infested areas. It is a significant pest throughout the U.S. and many counties within states and entire states have quarantines due to its presence. How do you know if the location you are shipping to has a quarantine? Check out the quarantine list on the USDA APHIS website.

Fence line.
Photo 4. Scout your fence lines and adjacent woodlots for signs of gypsy moth. Photo by Jill O’Donnell, MSU Extension.
Defoliation on Christmas tree.
Photo 5. Gypsy moth can cause severe feeding damage on spruce, fir and Douglas-fir Christmas trees. Photo by Jill O’Donnell, MSU Extension.
Gypsy moth egg masses.
Photo 6. Gypsy moth egg masses are often hard to find on conifers due to the dense foliage.

Managing gypsy moth

In order to scout conifers for gypsy moth, check out “Gypsy Moth Caterpillars -Scout For Them Now” by Cornell University’s Tree Integrated Pest Management Program. Examine the trunk of trees to look for gypsy moth egg masses. The foliage is very dense, so it will be hard to find all egg masses. In addition, remember that a chemical treatment is required to ship into non-quarantined areas.

Scout neighboring woodlots or fence lines for egg masses to determine if there is a very high threat to the neighboring crop. Also, cover hoophouses with netting to prevent the spread of gypsy moth into the crop during production (Photo 7) and cover trailers with shade cloth prior to shipment (Photo 8).

Trees with covering over them.
Photos 7-8. Cover susceptible crops with shade cloth or netting during production (left) and prior to shipment (right). Photos by Jill O’Donnell, MSU Extension.

The warm weather is moving gypsy moth development along and we are beginning to see eggs hatching in areas of Michigan's southern Lower Peninsula. As of April 14, 2021, the counties bordering Indiana had growing degree day (GDD) totals up to 175. GDD is calculated based on the amount of heat accumulated over time (based on 50 degrees Fahrenheit) (Photo 9).

GDD totals in Michigan.
Photo 9. The growing degree day totals range from as low as 34 to 175 throughout Michigan as of April 14, 2021. Photo from MSU Enviroweather.

The Michigan Department of Agriculture and Rural Development (MDARD) and MSU have developed spray windows to help growers time their gypsy moth control. The spray window targets the time between when they disperse, develop and just before they begin to pupate. The first window to open is for the use of Dimilin. Dimilin is an insect growth regulator, disrupting the normal molting process of the insect larvae. This window opens at 150 GDD calculated from base 50 F.

The second window used in Michigan is for insecticides with a more conventional mode of action, referred to as the “all other compounds” window, which opens at 200 GDD. Because of the different modes of action, the Dimilin spray window is closed slightly earlier (700 GDD) than all other compounds (800 GDD).

Currently, the gypsy moth spray window for Dimilin is now open in our southern-most counties and will open for the use of other compounds as the areas reach 200 GDD (Photo 10). As a reminder, once the spray window opens, the trees cannot be shipped to non-quarantined areas until a preventive treatment is applied to control the larvae which have hatched.

Since gypsy moth development will vary by location and the weather, you will need to monitor development around your farm. Weather conditions will affect caterpillar development and the end date of the spray window. Remember, we have the gypsy moth treatment window tool on MSU’s EnviroweatherThis program allows you to track the opening and closing dates of the gypsy moth spray window by county.

Spray window in Michigan.
Photo 10. Currently, the spray window is now open for Dimilin in the southern-most counties of lower Michigan to control gypsy moths. Photo from MSU Enviroweather.

All species of cut Christmas trees shipped from Michigan, into or through areas that are not infested with gypsy moth, i.e., not part of the area regulated under the Federal Gypsy Moth Quarantine, must meet the following requirements:

  • Spruce, fir and Douglas-fir must be treated with an approved pesticide during the required spray window period.
  • Christmas tree fields of any species must be inspected by MDARD.
  • The Christmas trees must come from a field or block that has been designated by MDARD as being free of viable gypsy moth egg masses.

MDARD annually reviews the list of compounds that are acceptable to use as regulatory treatments for gypsy moth. The list of approved compounds has been reviewed and will be included in a mailing to all growers enrolled in the previous year’s Christmas tree certification program. The list of approved compounds is also available online.

Growers should note that the required treatments for spruce, fir and Douglas fir also applies to those species of trees when shipped as nursery stock including balled and burlapped stock, bare root stock and containerized stock. All other species of trees do not necessarily require a treatment, however will have to pass inspection by MDARD as being free of gypsy moth if shipped outside of the gypsy moth regulated area.

Contact your local MDARD plant health inspector regarding any questions about inspection and certification of cut Christmas trees and nursery stock. Inquiries may also be sent to MDARD Plant Health at MDARD-NurseryCE@michigan.gov.

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