Spongy Moth Suppression Program

What Are They Spraying Overhead?

The main defense against a Spongy Moth (formerly Gypsy Moth) infestation is an aerial application of Bacillus thuringiensis var. kurstaki (Btk). Btk is used to reduce high populations of spongy moth caterpillars at sites that meet specific guidelines for treatment. 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. Learn more about the Macomb County MSU Extension Spongy moth Suppression Program below.

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Macomb County MSU Extension Spongy Moth Suppression ProgramSpongy Moth Caterpillar

The Macomb County Spongy Moth Suppression program is a joint effort between the county and impacted communities, recreation areas and special use areas. The program is administered by the Macomb County MSU Extension.

The suppression program’s ultimate purpose is to assist with the aerial application of Bt (Bacillus thuringiensis) to qualifying areas in an effort to reduce intolerably high populations of gypsy moth caterpillars. The program also provides information about additional techniques to help reduce caterpillar numbers when levels are not high enough to qualify for aerial treatment. Prospective treatment sites are determined using MDARD guidelines. Regardless of whether an area qualifies for aerial spray, a combination of methods can and should be employed to combat the increase and spread of infestation.

The goals of this program are to:

  • Reduce the nuisance of high caterpillar populations to tolerable levels in high treed residential, recreational and special use areas
  • Reduce the possibility of tree loss through the preservation of at least 60% of tree foliage. This prevents late-season re-foliation, which puts trees under severe stress.
  • Reduce the indiscriminate use of chemical controls
  • Provide educational information

Spongy Moth Background

The spongy moth (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. Spongy moth outbreaks began to occur in the lower peninsula of Michigan in the mid-1980’s. Caterpillars feed on tree leaves, preferring those of oak, aspen, poplar, and birch but will feed on over 500 types 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. Gypsy moths cannot be eradicated, but they can be suppressed to tolerable levels.

Bt Information and Usage

The main defense is an aerial application of Bacillus thuringiensis var. kurstaki (Btk), used to reduce high populations of spongy moth caterpillars at sites that meet specific guidelines for treatment. 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. Most Bt varieties are insect pathogens that cause disease in specific groups of insects, and several are registered with the U.S. Environmental Protection Agency (EPA) as microbial insecticides. Though Bt products can be highly effective in controlling specific insects, they have little impact on other animals. Therefore, sprays made with Bt pose significantly less risk of affecting non-target organisms than conventional chemical insecticide sprays. Btk is commonly used by organic gardeners and farmers, as well as some conventional farmers, to control caterpillar pests of fruits and vegetables.

The Bt products used to control spongy moth during outbreaks are made from a strain known as Bacillus thuringiensis var. kurstaki HD-1, or more simply, as Btk. Btk has been used for gypsy moth control in the northeastern U.S. since 1961 and in Michigan since 1985.

Btk specifically targets only caterpillars of a certain age. It is applied when the caterpillars are young (usually in May) to insure the greatest impact in reducing numbers. Alternative mechanical techniques, such as tree banding, egg mass scraping, and hormone traps, are also options help reduce populations. The Suppression Program recommends the use of a combination of methods.

Caterpillar Defoliation Activity

Spongy moth 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, spongy moth 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. Evergreens are unable to replace their needles and may die when defoliated. Keep trees watered and fertilized to reduce any stress.

When Btk grows, it produces spores and non-living protein crystals. When gypsy moth caterpillars eat leaves that have been sprayed with Btk, the protein crystals dissolve in their digestive system and become toxic. This can occur only in caterpillars because of the many unique conditions present in their digestive system. For example, caterpillars have an alkaline digestive system, while humans and many other animals have acidic digestive systems.

Soon after caterpillars feed on leaves sprayed with Btk, they stop feeding. If the caterpillars consume enough Btk, they die after a few days from a combination of starvation, damage to their digestive system and bacterial growth within their bodies.

Community Follow Up

It is important to communicate all spongy moth infestations to the MSU Extension Spongy Moth program staff. Egg mass surveys are done in late fall to determine the level of infestation and whether an area qualifies for the program.

Contact the MSUE Spongy Moth program by calling 586-469-6432 or emailing Terry Gibb at gibb@msu.edu 

More information about Spongy Moth life cycle, Bt and other suppression activities by visiting: