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Gypsy moth caterpillars are easy to identify, because they possess characteristics not found on other leaf-feeding caterpillars. They have five pairs of blue dots followed by six pairs of red dots lining the back. In addition, they are dark-colored and covered with hairs. Young caterpillars primarily feed during the day whereas the older caterpillars feed at night. When present in large numbers, the older caterpillars feed day and night. Young caterpillars spread to new locations by crawling to the tops of trees, where they spin a silken thread and are caught on wind currents. Older caterpillars are approximately 1.5 to 2.0 inches long. Gypsy moth caterpillars do not produce a web, which distinguishes it from web-making caterpillars such as the Eastern tent caterpillar, Malacosoma americanum and the fall webworm, Hyphantria cunea. The Gypsy moth larval stage lasts approximately seven weeks.
Gypsy moth undergoes four developmental life stages; these are the egg, larva (caterpillar), pupa, and adult. Gypsy moth females lay between 500 to 1,000 eggs in sheltered areas such as underneath the bark of trees. The eggs are covered with a dense mass of tan or buff-colored hairs. The egg mass is approximately 1.5 inches long and 0.75 inches wide. The eggs are the overwintering stage of the insect. Eggs are attached to trees, houses, or any outdoor objects. The eggs hatch in spring (April) into caterpillars.
In early summer (June to early July), Gypsy moth caterpillars enter a pupal or transitional stage. The pupae are dark brown, shell-like cases approximately two inches long and covered with hairs. They are primarily located in sheltered areas such as tree bark crevices or leaf litter. Adult Gypsy moths emerge from the pupae in 10 to 14 days. They are present from July into August. Females have white to cream-colored wings, a tan body, and a two-inch wingspan. Female Gypsy moths cannot fly. Males, which are smaller than females, with a 1.5-inch wingspan, are dark-brown and have feathery antennae. Both the adult female and male can be identified by the inverted V-shape that points to a dot on the wings.
Gypsy moth has only one generation per year. Gypsy moth populations will go through cycles in which the populations will increase for several years then decline, and then increase again. Area-wide outbreaks can occur for up to ten years, but generally population densities in localized areas remain high for two to three years.
The larva or caterpillar is the damaging stage as it eats the leaves of trees in the spring. They can consume tremendous amounts of leaf material. For example, Gypsy moth larvae can consume as much as one square foot of leaves per day. As a result, they produce a large amount of fecal (frass) material. When populations reach outbreak proportions, the caterpillars can completely defoliate host trees over a wide geographic area. Consistent or repeated defoliation over several years can have devastating effects, often leading to tree stress and death.
Gypsy moths have a wide host-range, which includes oak (Quercus sp.), crabapple (Malus sp.), linden (Tilia sp.), poplar (Populus sp.), beech (Fagus sp.), willow (Salix sp.), birch (Betula sp.), sweetgum (Liquidambar styraciflua), serviceberry (Amelanchier sp.), and hawthorn (Crataegus sp.). Trees less susceptible to attack by Gypsy moth are ash (Fraxinus sp.), sycamore (Platanus sp.), Indian bean (Catalpa sp.), honeylocust (Gleditsia sp.), dogwood (Cornus sp.), junipers (Juniperus sp.), yew (Taxus sp.), lilac (Syringa sp.), arborvitae (Thuja sp.), arrowwood (Viburnum sp.), and tulip tree (Liriodendron tulipifera).
Conifers are more susceptible to death than evergreens because they don't produce another flush of growth once defoliated. Conifers, such as pine (Pinus sp.) and spruce (Picea sp.), are unable to produce new leaves (needles) after defoliation as compared to deciduous trees. As a result, conifers can die after one severe defoliation.