LANSING — After a 10-year hiatus from high gypsy moth infestations, populations began to increase in 2019.

This year, Michigan is seeing infestations that meet or exceed the height of infestation before 2009.

Gypsy moth outbreaks began to occur in the lower peninsula of Michigan in the mid-1980s, causing great strain to Michigan’s trees. By the late 1990s and early 2000s, efforts to suppress the population were successful in keeping numbers low and at a manageable level. Within the past few years, gypsy moth populations have exploded throughout Michigan, and counties throughout the state once again are taking efforts to suppress the population.

To help residents understand how suppression works and to learn more about the history, life cycle, identification and management strategies to help save their trees, Michigan State University Extension is offering a webinar, “Gypsy Moth in Your Neighborhood.” The webinar will be via Zoom from 1 to 2 p.m. Eastern time Tuesday. This free program is open to everyone, but pre-registration is required. Register at: https://events.anr.msu.edu/gypsymoth2021/.

The gypsy moth or Lymantria dispar is a foreign pest with few native predators to keep populations in check here in the United States. First introduced in Massachusetts in 1869, it has spread across the much of the northeast. Caterpillars feed on tree leaves, preferring those of oak, aspen, poplar and birch but will feed on more than 500 types of plants throughout the summer.

Large populations can defoliate entire wooded areas. Caterpillars in large numbers and their waste, also known as frass, are a nuisance in residential areas. Gypsy moths cannot be eradicated, but they can be suppressed to tolerable levels.

Gypsy moth caterpillars feed on tree leaves, creating “Swiss cheese”-type holes. They do not cause pre-mature leaf drop, browning or curling of leaves. They do not make a web or tent in trees.

In addition to damage to the trees, gypsy moth caterpillars can be a nuisance if populations are high enough. Caterpillars and their frass can drop down from trees onto sidewalks, driveways, yards, porches and vehicles. The hairs on the caterpillars can cause irritation or an allergic reaction to bare skin. Frass can stain surfaces, especially if it is rained on or becomes wet.

Tree defoliation by the caterpillars can have a significant negative impact on tree health. Trees defoliated more than 40% become stressed by using next year’s energy reserves to grow new leaves during the same season.

Healthy trees may withstand several years of defoliation before succumbing to the stress. Evergreens are unable to replace their needles and may die or have a ragged appearance when defoliated. Trees affected by gypsy moth should be kept watered and fertilized to reduce any stress.

The product used to control gypsy moth during outbreaks is made from a naturally occurring bacterium strain known as Btk, or Bacillus thuringiensis var. kurstaki HD-1. Btk has been used for gypsy moth control in the northeastern U.S. since 1961 and in Michigan since 1985.

Btk specifically targets only caterpillars of a certain size. It is applied when the gypsy moth caterpillars are young — usually in May — to ensure the greatest impact in reducing numbers. Because gypsy moths are not a native pest, they usually hatch before native caterpillar species. Alternative mechanical techniques, such as tree banding, egg mass scraping and hormone traps, can help reduce populations, which homeowners are encouraged to do when they notice an infestation.

This article was published by Michigan State University Extension. For more information, go to https://extension.msu.edu.

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