Below will address questions regarding the spongy moth caterpillar spraying operation on the west side of Madison.

If you have additional questions, please contact the Urban Forestry office at forestry@cityofmadison.com or call at 608-266-4816.

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what happened here?

Two areas on the west side, the southern portion of the Glen Oak Hills Park and the Kenosha Greenway were treated with a spray to protect these public trees from the spongy moth caterpillar. 

These areas were found to have a high concentration of egg masses in 2023, so they were identified as areas that could see another spike in the population in 2024 if environmental conditions favored another spike. Therefore, the spraying was appropriate to control the spongy moth population.

The spray was applied from the ground into the tree canopy by certified arborists. 

 

Certified arborist in a lift applying the bacterial spray into the canopy on May 15, 2024

 

After a series of delays, the application began on the west side of the greenway from Regent to S. Hill Drive on Friday May 10, 2024.  Due to weather, the spray could not be completed this day. 

The work restarted on May 15, 2024 by spraying the east side of the greenway and the southern portion of the Glen Oak Hills Park..

 

The spray has to be used in a very particular way.  You need favorable weather conditions so the wind does not blow it away or the rain does not wash it away.  You also need to be sure the caterpillars are at the correct developmental stage to ingest the bacterial spray and be destroyed by it.

Tiny spongy moth caterpillar on a leaf

 

Spray Delay Timeline

It was previously stated on this website that the tentative dates were April 25 & 26, 2024.

The spray date was pushed back to May 1 & 2 based on observations regarding spongy moth caterpillar hatching.

However, the May 1 & 2 date was cancelled due to weather forecasts calling for rain.  The spray must sit on leaves to be effective and rain would just wash it off.  A new spray date of May 3 was selected.

May 3 date was cancelled due to leaf development. The spray needs to land on leaves and be present for the caterpillars to eat.  Based on observations of the spray area, and general advice from forest health specialists in the Wisconsin DNR, it is best to wait until the trees produce more leaves to be more effective.  A new date of May 8 was selected.

The May 8 spray date has been cancelled due to rain in the forecast for this date.

The spraying began on May 10, focused on the area western side of the greenway between Regent St. and S. Hill Dr. The remainder of the spray area was treated on May 15, 2024.

 

why are these areas being sprayed?

The spongy moth caterpillar defoliates trees. This is not lethal to trees by itself, but it is a stressor for them as they try to regrow them.  All of the energy spent to grow more leaves makes them more prone to disease and pests - especially during dry and drought conditions.  This stacking of stressors can lead to tree loss.

These trees are being sprayed to stop the spongy moth caterpillars from destroying the leaves, and keep the trees safe.

pesticide application flags

The pesticide application flags that appeared along the greenway on May 10 and again on May 15 were due to state law.

Even though the spray was a naturally occurring bacteria for the spongy moth caterpillar, and the was applied into the tree canopy, law requires pesticide application notice flags in the ground.

Pesticide applicator flag in Kenosha Greenway

 

Map of the areas to be sprayed

The below map shows the approximate area where the spraying will occur in the Glen Oak Hills Park and the Kenosha Greenway.

Map showing areas of the southern portion of the Glen Oak Hills park and the Kenosha Greenway shaded in red. This signifies they will be part of the spongy moth spraying effort.

 

What is in the spray and how does it work?

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

Btk affects the ability of the spongy moth caterpillar to digest, eventually killing them.

 

Safety of the spray for people, pets, and pollinators

Btk has not been shown to be harmful to people, birds, pets or pollinators, like bees or species of butterflies native to Madison.  Also, by targeting the application at this time of year, it will limit any possible harmful effects to other insects that may ingest the bacterium.

 

More information about btk

Wisconsin Department of Agriculture, Trade, and Consumer Protection: Spongy Moth Treatments: Bacillus thuringiensis kurstaki

Wisconsin Department of Agriculture, Trade, and Consumer Protection: Btk Treatment Fact Sheet

 

Neighborhood notification letter

Notification letter that will be sent to residents near the application area can be found here. 

The spraying began on May 10, and it is concluded on May 15, 2024.

The spraying has been delayed multiple times since initially scheduled.  You will notice on the letter it gives a date in April for the application date. You can read a timeline for the causes of the delays here.

 

why were these two areas sprayed and not more places in Madison?

There are two main reasons.  1) The concentration of spongy moths seen here and 2) the equipment.

 

Concentration

This area saw a very high concentration of spongy moths in 2023.

If weather conditions are favorable for the spongy moth again this spring, then we could have another population boom of the pest in 2024.  And that means the trees here could be defoliated again like last year, which we want to avoid.

The concentration of egg masses here was quite high in this area compared to previous years and also compared to the rest of Madison.  There are spongy moths in other areas, but not in the same level of as what was observed in this particular area of public trees that can be treated. 

In 2023, the Engineering Division asked for help from residents to report areas where spongy moth was spotted.  Staff from Engineering was able to use help from residents to zero in on this particular heavily wooded stormwater greenway and city park with many oak trees as a place that had an especially high concentration of spongy moths.

The best course of action when treating spongy moth is to seek out these areas of highest concentration where they can do the most damage, and work to treat that area to minimize their threat to protect the trees. 

Equipment

The other important factor is the equipment.

The specialized truck with high-pressurized tanks that can spray Btk from the ground has limited range. 

The Kenosha Greenway and the southern portion of the Glen Oak Hills Park allows for this special equipment to be deployed since it is narrow and close to the road where the lift trucks with the high-pressure tanks can be used.

Other public lands that are equally heavily wooded as the Kenosha Greenway and the Glen Oak Hills Park, and therefore potential candidates for this kind of spray treatment, are not accessible to this equipment so it cannot be sprayed.