Eggs: The Overwintering Stage

Eggs are laid in August and the embryo immediately starts developing during the warm days of summer. Within a month, the tiny larva is fully formed. Rather than hatching, the larvae enter an inactive state (diapause)—shutting down metabolic activities and adapting to the cold. Spongy moth passes through the fall and winter months in this stage. As temperatures increase in early spring, the larva inside the egg slowly becomes more active. In late April or May, just as the leaves of trees are expanding, the larva chews through the egg shell and hatches.

Photo Credit: B. Queen, WI Dept Natural Resources

Larva (caterpillar): A Feeding Machine

Before starting to feed, larvae, or caterpillars as they are commonly known, disperse throughout the forest in a behavior called ballooning. The larva climbs to the top of the tree on which it hatched, and dangles in the air on a silk thread. When a breeze catches the larva, the thread breaks and it is carried on the wind for up to 150 yards.

After ballooning, caterpillars begins to feed. Larvae will feed for five to six weeks. Females feed for an extra week to put on the fat necessary to produce eggs. About once a week the larva will grow too large for its exoskeleton and it will molt.  The leftover exoskeletons can aggravate allergies in some people.  The molts separate the larval period into five or six stages, called instars. Early in larval development (instars one through three), larvae feed during the day. Once they reach the fourth instar, however, they start to feed at night and climb down to hide under rough bark or in leaf litter during the day, likely to avoid predation. The larvae take on their characteristic appearance with the blue and red dots down their back at about the end of the first instar.  Before that, they look quite black.  About 90% of the leaves consumed by a larva will be eaten in the last two instars. This is the reason it sometimes seems like trees are defoliated overnight!

Photo Credit: Bill McNee, Wisconsin Department of Natural Resources

Pupa: The Transformation Stage

When the larva is fully grown (about 1 1/2 inches for males and 2+ inches for females), it will wander, looking for a place to pupate. They look for a protected spot offering safety from natural enemies such as mice, birds, and parasitic wasps. Typical hiding places include between ridges of bark, under flaps of loose bark, or in cracks of rock outcroppings. They can also be found in crevices on the undersides of vehicles, in firewood piles, on the underside of tree branches, or on outdoor furniture and play sets.

Once the larva has found a safe spot, it molts and transforms in appearance to a brown, pod-like structure. Spongy moth is generally immobile during the pupal stage while it transforms into the adult form. After a week or two, the adult moths will emerge.

Photo Credit: Milan Zubrik, Forest Research Institute – Slovakia, Image 1370017. Forestryimages.org

Adult Moths: Focused on Mating

Adult spongy moth males and females look very different from each other. Females are 1 1/2 inches long and are white with a black “v” shaped marking on their forewings. Female moths cannot fly and will fall to the ground if picked up. Male spongy moths moths are mottled brown and gray and have large feathery antennae. They can resemble many native moths. However, the males are unique in that they fly and seek out females in the afternoon, while most other moths are active after dark.

The key function of the adult stage is to reproduce. Unlike many other moths and butterflies, adult spongy moths cannot feed. They have roughly 2 weeks to find a mate before they die. Once the male has found the female, they mate and the female lays all her eggs in a single teardrop-shaped mass 1 – 1.5 inches long and covers it with yellowish-tan colored hairs from her own body. Each female lays only one egg mass and each egg mass can hold up to 500-1000 eggs.

Photo Credit: USDA APHIS PPQ Archive. Image 2652079. ForestryImages.org.