The spongy moth is a native insect of Europe, North Africa, and Asia. There are two varieties of spongy moth (Lepidoptera:Erebidae), the European and the Asian. The European variety is established in the northeastern and midwestern United States, having expanded from the original introduction near Boston. European moths are smaller than the Asian variety; however, they are both members of the same species. Unlike the flightless European females, Asian female moths can fly long distances to lay their eggs. The caterpillars feed on several hundred species of trees and shrubs. Preferred hosts include oak, apple, alder, aspen, basswood, willow, birch, cottonwood, and plum; coniferous species such as Douglas fir, pine, and western hemlock are suitable hosts as well.
In 1868, scientist Leopold Trouvelot brought European spongy moths from France to Massachusetts to breed a silk-producing caterpillars or "silkworms." The experiment failed and some of his caterpillars escaped. They found the environment here in the United States conducive with their feeding requirements, with abundant food and few natural enemies.
Spongy moth spread naturally by “ballooning”. Newly hatched larvae hang from long silken threads from the end of branches. Wind breaks the threads and cares the larvaeon air currents. Distance traveled is usually short, often to the next tree. The majority are moved less than a kilometer. Long distance spread is assisted by man through the artificial transport of life stages. Typically this is by the movement of outdoor household items.
The spongy moth goes through four life stages: egg, larva (caterpillar), pupa, and adult.
Eggs hatch around the time of oak bud break. Early stage larvae will tend to stay in the lower forest canopy, but under high populations, larvae will move higher in the crown to disperse through ballooning. Larvae or caterpillars go through 5 stages or instars for males and 6 for females. Each caterpillar is capable of eating a square meter of foliage, before spinning a resting area and pupating. Moths emerge in early summer. Females release scent or pheromone to attract a mate. This pheromone has been synthesized for used in detection traps as well as for control.
In early summer, females lay eggs on trees as well as a variety of outdoor items. Each egg mass can have several hundred eggs. Spongy moths overwinter in the egg stage, and hatch in April or May. The young caterpillars are black and hairy, later becoming mottled gray with tufts of bristlelike hairs, and blue and red spots on the back. There is one generation per year. Adults differ in appearance, males being brown with a fine, darker brown pattern on the wings. Females are nearly white, with a few dark markings on the wings. European females do not fly. Caterpillars climb trees and feed mostly at night. They are capable of reaping foliage from trees, and this activity will kill many trees if repeated over a few years. Trees also become weakened and more susceptible to diseases and wood boring insects.
Like those of other defoliators, spongy moth outbreaks usually last two years. In oak-dominated stands, spongy moth outbreaks usually persist two or three years, before collapsing. Populations can remain low for up to ten years before rebounding again.
Effects of spongy moth defoliation are usually most severe during the initial outbreak in a newly infested area. If more than 60 percent of the tree’s canopy is consumed by spongy moth caterpillars, trees typically refoliate, producing a second set of leaves later in summer. Low rates of diameter growth are common during years of heavy defoliation and often for one or more years after defoliation ceases.
The amount of tree mortality and topkill sustained during a spongy moth outbreak depends on the severity and frequency of defoliation, as well as tree health. Defoliated trees often become more susceptible to attack by other insects and disease.
Outbreak populations naturally decline due viral disease, fungi, natural enemies, and starvation. Low spongy moth populations may be held in check by birds, insect predators, and small mammals for several years, before the next outbreak occurs.
Successful management of spongy moth requires an integrated approach that includes several strategies. When population densities are high or in outbreak situations, the most effective approach for preventing widespread defoliation and reduction of the population is aerial applications of a biologically derived compound known as Bacillus thuringiensis kurstaki, or Btk. Bt sprays are only effective during the caterpillar life stage, and otherwise harmless to other animals, including birds, humans, and fish.
A fungal disease of spongy moth called Entomophaga maimaiga is a promising control of spongy moth. The fungus was probably imported from Japan to areas near Boston, Massachusetts around 1910. This attempt to establish the fungus seemed to fail since extensive surveys did not reveal the pathogen. E. maimaiga was not observed in North America again until June, 1989 when dead caterpillars found clinging to trees in the northeastern U.S. revealed its presence.
Mating disruption (MD) is used primarily in the Slow-the-Spread Zone. MD involves broadcasting pheromone flakes over a specific treatment area. Pheromones are chemicals produced by insects to communicate. To control spongy moths, the pheromone is incorporated into plastic laminated flakes that are mixed with a sticking agent and released from the air. The sticking agent is not strong, but will ensure the pheromone is distributed at all levels in the forest canopy where spongy moths may live.
Once the flakes are released, they saturate the air in the treated area with the pheromone for two to three months, disrupting the communication between the female and male spongy moth. The pheromone flake release in early to mid-June is timed to correspond with the male spongy moth's flight, preventing the male from finding the flightless female and mating.