Gypsy Moths

Gypsy moths are responsible for significant damage to many hardwood trees in the northeastern states. Damage includes needle discoloration and injury.
Gypsy Moths - Articles

Updated: November 21, 2017

Gypsy Moths

Severe gypsy moth damage on Colorado blue spruce. Courtesy of PDA

Lymantria dispar (Linnaeus)

Gypsy moths are of limited geographical distribution in the United States. Regulations are in place for shipping cut trees from areas known to be infested with gypsy moth, including Pennsylvania. Contact the appropriate state or federal regulatory agency for information before shipping cut trees out of state.

Hosts

  • Many hardwood species
  • Some conifers: pines, spruce, eastern red cedar, and occasionally firs and Douglas-fir

Damage Potential

  • Low–moderate
  • Heaviest damage frequently caused by more mature larvae that have consumed all the food supply in the area; repeated light or a single severe defoliation of conifers usually results in tree mortality

Symptoms and Signs

  • Defoliation of branches or entire tree (at first noticeable at the top of the tree)
  • Tan egg masses on the trunk or underside of large branches
  • Gypsy moth caterpillars up to 2½ inches (63–64 mm) long
  • Reddish-brown pupal cases hanging from trunk
  • Adult moths in midsummer

Causes of Similar Symptoms

  • None

Identification

The oval, convex egg masses, up to 1½ inches (40 mm) long, are a mixture of hundreds of roundish, tan eggs and buff-colored hairs from the abdomen of the female. They can be found year-round attached to any hard surface such as tree trunks or branches, fence rows, siding of homes, or anything left outside for the summer. Healthy egg masses are lighter in color; emerged egg masses are darker and have numerous holes in the surface. Holes may be from emerging caterpillars or parasitoids.

Newly emerged gypsy moth caterpillars are dark brown, ¼ inch (6 mm) long, and have longer hairs at each end, which results in the larvae appearing somewhat “I” shaped. As the larvae mature, the characteristic colored spots on the upper portion of the body become apparent. Each of the first five segments of the larva’s body has a pair of blue spots; the next six segments each have a pair of red spots. Between these spots, a thin, yellowish line runs the length of the body. The head of a mature caterpillar is black and tan mottled, and numerous long, brown hairs cover the body. Caterpillars reach 2½ inches in length (63–64 mm) at maturity.

The dark reddish-brown to black pupal cases are not surrounded by any protective silk cocoon. They are attached to the substrate by several strands of silk and the cast skin from the last instar caterpillar may be nearby. Female pupal cases are larger than male pupal cases.

The gray-brown male moths emerge first. They have a wingspan of about 1½ inches (40 mm) and are agile fliers. They can be recognized by their rapid, zigzag flight as they search for female moths. This makes close examination difficult, but the males have prominent feathered antennae, and their wings have irregular dark lines across the forewings. The larger female moths have a wingspan of 2–2½ inches (50–63 mm). Their wings are white with faint, dark markings. The heavy female’s body prevents flight, so they are usually found not far from their empty pupal cases.

Calendar of Activity

Biology and Life Cycle

Eggs overwinter in the egg masses protected somewhat by the hairs from the female’s abdomen (Figures 1 and 6). The masses may contain up to 600 eggs, which are rounded and tan. Egg masses darken with age, and hatching occurs between late April and mid-May when oak leaves are expanding. Newly hatched larvae (caterpillars) remain on the egg mass for a few days before moving to the newly developing leaves. If the population is high or the host is not suitable for development, larvae will “balloon” to new hosts by dropping down from a silken thread (Figures 2 and 3). Spring winds can carry the young caterpillars up to a mile. During this phase, gypsy moth larvae are frequently seen on clothing when working outdoors. The irritating hairs of the first instar caterpillars may cause contact dermatitis for some people.

Figure 1. Gypsy moth egg mass. Courtesy of Pennsylvania DCNR Forestry Archive, Bugwood.org (#5020037)

Figure 2. Larvae “ballooning,” or dropping down onto new hosts via silken thread. Courtesy of Brian Schildt, PDA

Figure 3. Gypsy moth larvae and silk covering Douglas-fir buds. Courtesy of Brian Schildt, PDA

Larvae feed for 6–7 weeks, gradually increasing in size and appetite. Young larvae remain in the tree’s foliage day and night, but their feeding pattern changes when they are half grown (Figure 4). The more mature larvae move down the tree to take refuge in a cool, shady place during the day and feed in the canopy at night.

Figure 4. Maturing caterpillar with red and blue spots and yellowish lines running the length of the body. Courtesy of Brian Schildt, PDA

By late June to early July, the larvae find a sheltered place to pupate (Figure 5). About 2 weeks are required before the adults begin to emerge, with the male moths appearing several days before the females. When the female moths emerge, they may crawl a short distance to an elevated spot or stay near the pupal case to emit a pheromone to attract the male moths. After mating occurs, females deposit one to two egg masses before dying (Figure 6). Neither male nor female moths feed during their short lives. Egg masses overwinter until the following spring. One generation is produced per year in Pennsylvania.

Figure 5. Gypsy moth pupae suspended from tree limbs. Courtesy of PDA

Figure 6. Female gypsy moth over egg mass. Courtesy of Hannes Lemme, Bugwood.org (#1260007)

Monitoring and Management Strategies

Plantation Establishment

  • Plant the crop away from hardwood stands such as aspen or oak or other preferred hosts of the gypsy moth.

Preseason

  • Scout year-round for gypsy moth egg masses both in the plantation and in surrounding woodlots.
  • Monitor egg masses for first signs of hatching.
  • Contact a regulatory agency or county extension office to obtain information about specific management options in the area and about shipping Christmas trees.

Growing Season

  • Growing degree days: 90–448 GDDs is the treatment window for the larvae.
  • When larvae are small, consider spraying with a microbial or other type of insecticide.
  • To help monitor populations, use a synthesized female pheromone trap to look for male moth emergence.
  • At the end of the season, evaluate results and update records.