St. Clair County Spongy Moth Program

Spongy moths are an invasive forest pest that feeds on more than 300 species of deciduous trees and shrubs, including aspen, oak, and birch. Repeated removal of leaves, or defoliation, stresses trees and can leave them vulnerable to disease or other pest infestations that can kill trees. Once it becomes established in a location, spongy moth has cycles of large population outbreaks every eight to 12 years, leading to widespread defoliation and nuisance from caterpillars.Lymantria dispar, sometimes known as the gypsy moth, or spongy moth, is a species of moth in the Erebidae family.  The Entomological Society of America has assigned a new common name and Lymantria dispar is now known by the common name of “spongy moth”. 

Spongy moth is an exotic invasive insect that has been in North America since the late 1800’s. Spongy moth caterpillars can feed on the foliage of hundreds of species of trees and shrubs, preferring oaks. Egg masses are laid in late summer and fall on tree trunks, buildings, campers, and other outdoor objects, and persist through the winter. Hundreds of spongy moth caterpillars hatch from each egg mass in late spring, and feed for two months in summer. Each caterpillar can cause extensive defoliation of the trees when their numbers are high. Caterpillars form a cocoon in late summer, emerge as adults about two weeks later, mate, lay egg masses and die. Only the caterpillar life stage feeds and causes damage.

The damage from spongy moth caterpillar feeding includes stress and defoliation to trees and forested areas, and they are a messy nuisance to homeowners and outdoor enthusiasts by reducing shade from trees and by producing large amounts of frass (excrement), leaf fragments, and squirming caterpillars. The hairs from egg masses, caterpillars, and adult moths can also cause allergic reactions in sensitive individuals.

September - May:

Embryo and Diapause Stage A single egg mass contains 100-1,000+ eggs insulated in a matting of hair from the female’s body. Masses are tan colored, oblong, and range from 1 to 3 inches. Larva is fully formed and ready to hatch within a month. The larva goes into diapause, becoming insensitive to cold.

Mid- Late May:

Hatching Stage Hatching coincides with the opening of tree leaf buds. Newly hatched larvae are less than ⅛ inch long and appear black in color. They climb trees or other objects and drop on silken threads to be dispersed by the wind in a behavior called ballooning. Once landing in its host tree, the larva begins feeding. Hatching and ballooning may last for 7-10 days.

June- Early July:

Larval Feeding Stage (caterpillar) Caterpillars molt,shedding their exoskeleton (5 times for a male and 6 times for a female). Each molt is called an instar. Fourth instar caterpillars are identified by a beige head and dark marks, 5 pairs of blue dots followed by 6 pairs of red dots down their back. Larvae feed at night and generally rest during the heat of the day unless populations are very large and under stress. They continue to molt and feed until they are about 2 ½ inches long. A single caterpillar eats an average of one square meter of foliage during this stage.

Late June - Mid-July:

Pupa Stage During this stage the caterpillar looks for a protected place to pupate (change into a moth) where it will be safe from predators like mice, birds, and parasitic wasps. The caterpillar sheds its skin, and its new pupal skin is leathery and a dark reddish-brown color. It is usually attached to a tree trunk, rock, or other sheltered place by a loose net of silken threads. After about 10 days of metamorphosis the adult winged moth emerges, leaving the pupal case behind. Female pupae are larger than male pupae.

July- August:

Mating & Egg Mass Laying Stage The female moth cannot fly, and is larger and creamy white with dark chevron marks on her wings. Males are mottled brown and gray, and also have chevron wing bands. In the late afternoon they fly in zigzag patterns following the scent of female pheromones they sense with their large, feathery antennae. After mating, the female lays her eggs in a single mass she covers with hairs from her body. The adult spongy moth cannot feed; its only function is to reproduce. The moth lives about two weeks, completing a one-year life cycle.

You should report any new caterpillar activity to the St. Clair County Spongy Moth Suppression Program each year between May and July via our on-line survey ( click here to report activity ).

To avoid harming native species, it is important to confirm what you are seeing is a spongy moth, an invasive species. The spongy moth caterpillar is a destructive, invasive pest, but it is often confused with Eastern tent and fall webworms, both of which are benign native species. While some find their tents and webs unsightly, Eastern tent caterpillars and fall webworms are a natural and important part of our Michigan ecosystem. They clear foliage to allow sunlight to reach smaller plants at ground level and act as a food source for native birds and other animals.

One of the primary purpose of the St. Clair County Spongy Moth Suppression Program is to monitor and track outbreak or infestations in our county. Residents are the first line of defense in the suppression program.

YOU SHOULD REPORT ANY NEW SPONGY MOTH CATERPILLAR SIGHTING FROM MAY THROUGH JULY on the County’s automated survey ( click here to access ). 

Instructions:

   Step 1 - enter your address in the location bar on the map.

Step 2- Confirm the dot's location is your address.

Step 3 - complete the questionnaire located below the map with details of your spongy moth activity. 

Step 4 - Once completed, click "Submit" button at the bottom of the questionnaire. 

Each year after the collection of survey data on the locations and quantity of caterpillars, surveillance monitoring of egg masses can begin in the fall to determine the prevalence and density of the egg masses. This surveillance work will assist in forecasting spongy moth populations for the following year, as well as any potential treatment zones. Residents are encouraged to leave egg masses in place through December in order to be counted by the surveyors.  Residents may remove and destroy egg masses from January through April. 

“ Strategies for Landowners to Manage Spongy Moth ”. 

 Spring 2023 Spongy Moth Program Update  

Based upon the results of the 2022 survey and subsequent field work, the St. Clair County Board of Commissioners authorized aerial spraying for spongy moth throughout the county. The spray blocks were a direct result of the survey and field work. For the 2023 aerial spray, the county will fully fund this effort and there will be no cost-share required for citizens nor municipalities for the authorized spray blocks. 

The map on the right outlines the areas authorized for the 2023 aerial spray. You may zoom-in on the map to see affected areas and/or you may type your address in the upper right hand corner to see if your property is included in the 2023 spray blocks. 

SPRAYING WILL OCCUR IN MAY TO EARLY JUNE DEPENDING ON CONDITIONS

All property owners will be notified by mail in late March and will have an opportunity to opt-out of spraying. The notification letters and informational packet can be found here.

 Notification Letter 

 Spray Opt-Out Information 

 Suppression Program Information Sheet 

 FAQ 

Spongy moth populations are cyclical, with large outbreaks occurring every decade or so and generally lasting two to four years before collapsing naturally or by management actions. One option for suppressing spongy moth caterpillars during a severe outbreak is an aerial spray of Bacillus thuringiensis var. kurstaki (Btk). This product is used to reduce high populations of spongy moth caterpillars at sites that meet specific guidelines for treatment. Spongy moths cannot be eradicated, but they can be suppressed to tolerable levels. Btk is a naturally occurring bacterium found in the soil and on plants and is not harmful to pets, birds, fish, plants, beneficial insects, or humans.

Thousands of Bt varieties exist in nature, each with its own unique characteristics. Though Bt products can be highly effective in controlling specific insects, they have little impact on other animals, and pose significantly less risk of affecting other organisms than conventional chemical insecticide sprays. Btk is commonly used by organic gardeners and farmers to control caterpillar pests of fruits and vegetables. Btk has been used for spongy moth control in the northeastern U.S. since 1961 and in Michigan since 1985.

Btk specifically targets only caterpillars at a young age. It is applied to the tree foliage when the caterpillars are young (usually in May), and the caterpillars die when they ingest the Btk as they feed. Alternative techniques, such as tree banding, egg mass scraping, and soapy water sprays, are also options that help reduce spongy moth populations. St. Clair County’s Suppression Program recommends the use of a combination of methods.

Detailed information on Btk can be found here. ( link to the MSDS )

If you have additional question, please do not hesitate to contact us.

St. Clair County Spongy Moth Suppression Program

Email:  gypsymoth@stclaircounty.org 

Phone: (810) 989-6317