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).

New egg masses are firm to the touch. Old ones are pale in color, will crush easily, often look tattered and are not of concern.

Examples of a new, fresh egg mass (left) that caterpillars will hatch from next spring and an old egg mass from last year that is already empty (right)

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!

Egg masses hidden on the underside of a picnic table
Egg masses on the underside of branches
Photo Credit: Bob Queen, WI DNR.