Lymantria dispar populations explode in Michigan

After a 10-year hiatus from high Lymantria dispar, formerly gypsy moth, infestations, populations began to increase in 2019. This year, Michigan is seeing infestations that meet or exceed the height of infestation prior to 2009.

Lymantria dispar, formerly 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, Lymantria dispar populations have exploded throughout Michigan, and counties throughout the state are once again taking efforts to suppress the population.  

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 over 500 types of plants throughout the summer. Large populations can defoliate entire wooded areas. Caterpillars in large numbers (and their waste, frass) are a nuisance in residential areas. Lymantria dispar cannot be eradicated, but they can be suppressed to tolerable levels.

Lymantria dispar 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, Lymantria dispar caterpillars can be a nuisance if populations are high enough. Caterpillars and their frass (feces) can drop down from trees on to 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. Keep trees affected by Lymantria dispar watered and fertilized to reduce any stress.

The product used to control Lymantria dispar 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 Lymantria dispar 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 Lymantria dispar caterpillars are young (usually in May) to ensure the greatest impact in reducing numbers. Because Lymantria dispar are not a native pest, they usually hatch prior to 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.

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