Spray or scrape egg masses

Spongy moth (Lymantria dispar) eggs are tough! They can survive temperatures as low as –20°F.

You can spray egg masses with a labelled horticultural oil when the temperature is over 40°F.  Oiling egg masses with horticultural spray oils labeled for spongy moth such as Golden Pest Spray Oil when temperatures are above 40 degrees Fahrenheit will suffocate the eggs so they do not hatch in April.  This can help reduce the population of spongy moth next spring.  Spray oils labelled for spongy are available for purchasing online or at some garden centers and retailers.  If you have many egg masses, consider adding food coloring to your spray oil so you can see which egg masses you’ve already treated.  Although more difficult, and you must be careful not to damage the bark of the tree, you can also scrape egg masses into a container and cover them with soapy water for 2 days.  You may then discard eggs in the trash. Don’t just scrape egg masses onto the ground or try to crush them with your shoe as many eggs may survive to hatch next spring!  Any parts of the egg mass you are unable to scrape off can also hold surviving eggs.

Since egg masses often contain 600+ eggs, every egg mass destroyed can help reduce next year’s caterpillar population.

Photo Credits: Bill McNee WI DNR; Cliff Sadoff Purdue University.

Spray or scrape egg masses and watch for treatment program

Spongy moth (Lymantria dispar) eggs are tough! They can survive temperatures as low as –20°F.

You can continue to spray egg masses with Golden Pest Spray Oil when the temperature is over 40°F. The oil kills the eggs, reducing the population of spongy moth next spring.  This spray oil is available to purchase online or at some garden centers and retailers.  If you have many egg masses, consider adding food coloring to your spray oil so you can see which egg masses you’ve already treated.  Although more difficult, and you must be careful not to damage the bark of the tree, you can also scrape egg masses into a container and cover them with soapy water for 2 days.  You may then discard eggs in the trash. Don’t leave any part of the egg mass attached and don’t allow it to fall onto the ground, they will survive to hatch next spring!

Photo Credits: Bill McNee WI DNR.

Keep caterpillars out of trees using sticky barrier bands

Late April is the time to put up sticky barrier bands around your landscape trees. Immediately after hatching, spongy moth (Lymantria dispar) caterpillars disperse on silken threads and many will fall out of the trees. Barrier bands help prevent caterpillars from returning to the trees and help reduce damage from feeding. View directions to make a sticky barrier band trap using simple household materials. Monitor these each day and sweep caterpillars trapped below the barrier band into a bucket of soapy water.

Recently, similar sticky-band trapping methods (for the invasive spotted lanternfly) have caused concerns for birds in Pennsylvania and other mid-Atlantic states.  To help reduce risks to birds, consider placing chickenwire or mesh screening over the sticky bands to prevent birds from contacting the sticky surface.  See this page for additional info about preventing bird contact.

Two important notes: 1) be aware that trees with deep furrows in the bark allow caterpillars to sneak behind the barrier band and hide.  2) Do not put the sticky material directly on your tree.  Insecticides may also be used to help control spongy moth caterpillars at this time of year.

Photo Credit: Mark Guthmiller, WI DNR.

Caterpillars begin to emerge

Depending on the temperature, spongy moth (Lymantria dispar) caterpillars start emerging in late April through mid-May in Wisconsin. Sticky barrier bands should be up by the first week of May.   Directions on how to make a sticky barrier band.  Monitor these each day and sweep caterpillars trapped below the barrier band into a bucket of soapy water. Recently, similar sticky-band trapping methods (for the invasive spotted lanternfly) have caused concerns for birds in Pennsylvania and other mid-Atlantic states.  To help reduce risks to birds, consider placing chickenwire or mesh screening over the sticky bands to prevent birds from contacting the sticky surface.  See this page for additional info about preventing bird contact.

Two important notes about sticky barriers: 1) be aware that trees with deep furrows in the bark allow caterpillars to sneak behind the barrier band and hide.  2) Do not put the sticky material directly on your tree.  Insecticides may also be used to help control spongy moth caterpillars at this time of year.

Remove sticky barriers in late May as these barriers are not as effective against larger caterpillars.

Aerial spray treatments to suppress outbreaks or slow the spread of spongy moth occur in mid to late May when caterpillars are very small and vulnerable to the bacterial insecticide Bacillus thuringiensis kurstaki (Btk).  Maps of treatment areas are available online at the Wisconsin Spongy Moth portal website.  Treatment schedules are dependent on weather.  Daily updates are available by phone at: 1-800-642-6684.

Become a super-predator–put up burlap collection bands

Spongy moth (Lymantria dispar) caterpillars will make their presence known this month! Starting in June, caterpillars will leave the canopy of trees during the day to hide in crevices on the trunks of trees, the sides of buildings and even on outdoor furniture or play equipment. They return to the canopy each evening (or on a very cloudy day) to eat leaves all night.

You can take advantage of this behavior to reduce the number of caterpillars on your landscape trees by putting up burlap collection bands and turning yourself into a super-predator. Caterpillars find the burlap band an attractive hiding spot and will congregate there each afternoon. An easy way to eliminate caterpillars that have congregated is to scrape them into a cup of soapy water. Full instructions on how to make a burlap collection band can be found here.

NOTE: Spongy moth caterpillars are covered with fine “hairs” (setae) that can cause both skin and respiratory irritation. Avoid touching caterpillars bare-handed; disposable gloves, forceps, or other tools are recommended to knock caterpillars into a container of soapy water. Setae can remain in clothing, so fabric gloves or clothes that have contacted spongy moths should be washed separately. If skin irritation occurs, consult a physician for advice on topical medications to relieve irritation.

Insecticides may also be used to help control spongy moth caterpillars at this time of year.

Photo Credit: Mark Guthmiller & Bill McNee, WI DNR

Moths emerge at end of month

Spongy moth (Lymantria dispar) caterpillars finish feeding in July. The insects will then pupate and emerge as adult moths in mid-late July.  Male moths look like many other brown moths but they can be distinguished by the fact that they fly about looking for females in the late afternoon, unlike other moths, which wait until dark. Female spongy moths are white, 1 1/2 inches long, and although they have wings they do not fly. They are usually found laying an egg mass as in the photo below.  Both the pupa cases and moths can be crushed easily if you can find them. 

Note: Adult female spongy moths possess a long-lasting chemical pheromone that strongly attracts male moths. Getting this pheromone on your skin, clothing, or objects such as gardening tools can make contaminated surfaces attractive to male moths. Using a disposable item (stick, etc.) or washable object to crush adult females can be helpful in this regard.

Mating disruption is a management technique used to decrease reproduction of spongy moths by preventing male moths from finding females.  This technique works by applying the chemical pheromone (attractant) of the female to an area to mask the natural pheromone levels.  As a result, male moths can be overwhelmed by the scent and have a harder time locating a female to mate with.  The chemical pheromone used for mating disruption is highly specific to spongy moths, and is not harmful to humans or other animals.  There is one pheromone-based product currently registered in Wisconsin for mating disruption—SPLAT GM-O (ISCA Technologies).  This product is applied approximately two weeks before the emergence of adult moths.  

A limitation of this technique is that it is suitable only for low populations of the spongy moth over relatively large areas.  Mating disruption is used by the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) to combat spongy moth over larger areas as part of its “Slow the Spread” program, but is not an applicable technique for property homeowners.

Photo Credits: Milan Zubrik, Forest Research Institute – Slovakia, Image 1370017. Forestryimages.org; USDA APHIS PPQ Archive. Image 2652079. ForestryImages.org.

Moths lay eggs

Depending on the weather and location, adult female spongy moths (Lymantria dispar) will continue to lay egg masses through the first 2 weeks of August.  Look for egg masses in crevices and protected hiding spots on rough bark, the undersides of branches, under signs attached to trees, and even on buildings, play equipment, benches, and picnic tables.

Beginning the third week of August, look for “pinholes” on the egg masses, evidence that they have been parasitized by a tiny, non-stinging wasp called Ooencyrtus kuvanae. Wait to remove or destroy egg masses until after the first hard frost to allow this beneficial wasp an opportunity to build up its numbers.

Photo Credits: Daniel Herms, The Ohio State University, Image 1523115. Forestryimages.org; MSU.

Look for egg masses

If you live in a quarantined county, survey your property for spongy moth (Lymantria dispar) egg masses. Look for egg masses in crevices and protected hiding spots on rough bark, the undersides of branches, under signs attached to trees, and even on buildings, play equipment, benches, and picnic tables.  Conducting a thorough survey on your property can help determine if spongy moth is likely to cause significant defoliation next spring (instructions here).

Hold off treating or removing egg masses until after the first hard frost to let a natural enemy of the spongy moth—Ooencyrtus kuvanae—attack the egg masses. When temperatures remain just above 40°F, spray egg masses with a dormant horticultural oil (such as Golden Pest Spray Oil) to kill them and reduce the population of spongy moth next spring.   The spray oil kills the eggs, reducing the population of spongy moth next spring.  This spray oil is available to purchase online or at some garden centers and retailers.  If you have many egg masses, consider adding food coloring to your spray oil so you can see which egg masses you’ve already treated.  Although more difficult, and you must be careful not to damage the bark of the tree, you can also scrape egg masses into a container and cover them with soapy water for 2 days.  You may then discard eggs in the trash. Don’t leave any part of the egg mass attached and don’t allow it to fall onto the ground, they will survive to hatch next spring!

Photo Credit: Bob Queen, WI DNR.