By Nancy Penrose

One of the most destructive tree pests in the State of Washington is the European gypsy moth. In its caterpillar form, it has destroyed millions of acres of foliage around the country.

The moth was brought to the United States in the 1860’s near Boston, Massachusetts to cross-breed with silk worms and escaped into the environment. It is now permanently established in 20 Eastern and Midwestern states.

Gypsy moth eggs are usually laid on tree trunks and branches. Egg clusters can contain up to 1000 or more eggs. The larvae (caterpillars) appear in the spring. Mature larvae are two to three inches long and have five pair of blue spots and six pair of red spots on their back. Adult male gypsy moths are a mottled brown and adult female moths are white.

The moth was first discovered in Washington in 1974. Since then, the Washington State Department of Agriculture (WSDA) has been aggressively eradicating gypsy moths with aerial applications of Bacillus thuringiensis var. kurstaki (Btk), a natural soil bacteria. Treatments typically begin in April or May when the caterpillars start feeding on trees and shrubs.

Each year the WSDA conducts trapping programs to locate gypsy moth occurrences. Approximately 20,000 traps are set up around the state and checked from June through September. The traps are made of cardboard and placed on private and public property. Inside the trap is a string coated with synthetic pheromone, as substance females emit to attract males during mating. The inside of the trap is also coated with a sticky substance that catches insects.

Here’s how you can help prevent the gypsy moth from becoming permanently established in the State of Washington:

  • Let the WSDA place traps on your property
  • If a moth infestation is found in your area, allow the WSDA to administer treatments.

You can read more about the WSDA’s treatment program at Our big tree specialists can also answer your questions about tree pests and how to protect your foliage and big trees.

Nancy Penrose is owner of Big Trees Inc. (, (tree nursery Snohomish, WA), one of the largest Seattle tree nurserys, specializing in large trees for sale and transplanting tree service. Learn more about tree preservation at and for more tips on tree care or installation go to See our video at and connect with us on Facebook at