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 (formerly known as gypsy 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.