City Response to Spongy Moth Outbreak of 2023-24

Outbreak Background

In 2023, there was a spongy moth outbreak. Every 10 years or so, there is a spike in the moth population.  Other environmental factors in 2023, notably the drought conditions throughout Wisconsin, created a perfect environment for the moth to thrive.

The City of Madison took several actions in 2023 and 2024 to help protect public trees against the caterpillar.  

We also had a wet spring in 2024. The rainfall is good for the trees, and for the fungus that is harmful to the spongy moth caterpillar.

The outbreak subsided and the population in our returned to normal levels. 

This does not mean spongy moths have gone away forever.  Spongy moths are here to stay.  As noted above, we will probably see other high population spikes again at some point in the years to come, especially if weather & climate factors create opportunities for the moth to thrive. 

Destroying Egg Mass Flyers

In the winter of 2023 and the spring of 2024, we mailed flyers to over 3,000 homes within the hardest hit area of Madison giving them directions on how to destroy egg masses on their property to protect their trees. 

Oiling Egg Masses

Urban Forestry and Parks Division crews applied horticultural oil on egg masses in the winter of 2023-2024 when temperatures allowed.  The oil can only be applied when temperatures are above 40 degrees.  Urban Forestry crews focused on trees within the right of way area around Whitney Way and Mineral Point Road, while Parks Division crews worked in parks around Madison.

Oak Tree Injections

Starting in mid-May 2024, Urban Forestry crews began treating oak trees identified by the Parks Division with Tree-age R10 (which has the active ingredient emamectin benzoate).  This is the same product used to treat ash trees from the emerald ash borer. 

Staff performing the treatment were all Pesticide Applicator Certified by the Wisconsin Department of Agriculture, Trade, and Consumer Protection.

This oak tree treatment provided control against the spongy moth caterpillar from feeding on the leaves this year.

The treatment also provided protection for the oaks from the two-lined chestnut borer. The two-lined chestnut borer is a "secondary pest" that attacks stressed oak trees.

Oak tree injections were completed in early June 2024.

Ground Spraying of Kenosha Greenway & Glen Oak Hills Park

After many weather-related delays on May 10, 2024, and concluding on May 15, 2024, the Kenosha Greenway and a southern portion of the Glen Oak Hills Park was sprayed by a contracted certified arborist with a naturally occurring bacterial spray harmful to spongy moths. 

The technical name for the bacteria is Bacillus thuringiensis kurstaki (Btk).

This area was sprayed because it was seen to have a very high concentration of spongy moth caterpillars.  Also, this particular area could be accessed with the specialized truck and high-pressure tanks needed to spray into the canopy from the ground. 

Surrounding neighbors were notified via a letter prior to the spray.  Yard signs were posted around this area as well prior to the spray. 

Spongy Moth Genetic Research

The City of Madison is participating in a genetic study of spongy moths conducted by the James Lab at the University of Toronto.

This is not a population control effort like other initiatives.  This is a study.

In 2023, a single spongy moth pheromone trap was installed on an existing tree within the known infestation area on the west side of Madison. This trap did not damage the tree.

The type of trap is not used for mass removal, but is used to collect species as part of scientific research and monitoring.

The moth collection for the study may return in 2025.