In Summer 2021, an outbreak of spongy moth (formerly known as gypsy moth) was discovered in Dee-Devon Woods, commonly referred to as Southwest Woods. Although the impact to trees in the Forest Preserve appeared alarming early on, most of the trees “re-flushed” later in the summer (as they usually do). While severe defoliation does stress a tree, healthy trees can usually tolerate several years of mild to severe defoliation. However, trees that have been weakened by multiple years of defoliation and other stressors are at a much higher risk of mortality.

To contain the outbreak and slow the spread of spongy moth onto adjacent properties, the city performed two aerial sprays of the infested area in early-June utilizing a bacterium that, although harmless to humans, is effective in controlling the spongy moth in the caterpillar stage and is the safest and most selective method for reducing spongy moth populations. BtK is an organic, environmentally friendly alternative to chemical pesticides that has been used in the US since the 1960’s and approved by the U.S. Environmental Protection Agency, Health Canada, and the World Health Organization, among others. Aerial treatments were also performed in the 1980’s and early 2000’s and were effective in reducing the population of the pest.

This pest has cyclically impacted Park Ridge’s urban forest since the 1980’s and unfortunately is here to stay. Spongy moth populations typically have a temporary outbreak in an area every 10 years or so.

February 2023 Update
Since the 2021 outbreak, Forestry staff has been monitoring the existence of spongy moth in infested areas of the city. While evidence of caterpillars was present on several properties in 2022, summer surveys indicated very little defoliation or significant damage to trees. To help estimate levels of infestation in 2023, staff has been conducting winter egg mass surveys in targeted residential areas adjacent to the 2022 treatment area. The data collected during the survey will help determine if and where the City will apply an aerial treatment in 2023. Another update will be posted in March with more information.

This spring, we will be entering our third year of this spongy moth wave. In most cases, spongy moth populations naturally decline in 1-3 years of an infestation’s peak due to naturally occurring build up of disease (virus and fungus) and predators. In order to allow these natural diseases to build-up, the City will only consider applying treatment to areas where outbreaks are expected to significantly impact tree health (i.e. multiple years of severe defoliation of weakened trees caused by biotic and abiotic stressors).

In most cases, the mere presence of caterpillars, although a nuisance, may not warrant treatment. In such cases, residents are encouraged to work with a Certified Arborist to explore treatment options for valuable trees on their property, such as oaks.  

Forestry staff will monitor spongy moth populations in subsequent years to see if applications will be warranted in the future.

What should I do about trees on my property?

Keeping a tree healthy is the best defense against spongy moth caterpillars. Properly mulching and feeding trees along with making sure they receive enough water (1 inch per week when dry) will help keep your trees healthy and viable.  

If you have experienced moderate to severe defoliation, treatment might be a good option. Many local tree care companies deliver safe and effective treatments for trees on private property. Tree-äge® is one product used by tree care professionals to control spongy moth and it is the same product we use to treat ash trees for emerald ash borer. The product is simply injected into the tree’s vascular system in the most effective and environmentally responsible way possible. As a preventative treatment, it can protect the entire canopy for up to two years so in many cases one treatment is all that is needed since spongy moth populations naturally decline within 1-3 years. Treatment should take place in early spring.

For more information, please visit the follow resources:
Illinois Department of Agriculture (IDA) - Lymantria Dispar/Spongy Moth
Michigan State University Extension - Spongy Moth IPM
University of Wisconsin-Madison - Spongy Moth in Wisconsin
United States Department of Agriculture (USDA) - Gypsy Moth
National Slow the Spread Program

If you have any questions, or would like more detailed information, please contact the Park Ridge Forestry Division at 847-318-5231.