Damage

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.

The ecological and economic impact of Gypsy moth is a serious concern. Gypsy moth defoliation can change the complexity of understory growth thus resulting in an increase or decrease of certain fauna or flora. Consecutive defoliation can result in plant stress and possible death. Gypsy moth defoliation may predispose trees to attack by opportunistic insects or diseases. For example, Gypsy moth feeding can increase a tree's susceptibility to the attack by the shoestring fungus, Armillariella mellea and the two-lined chestnut borer, Agrilus bilineatus. In forested neighborhoods and urban parks, dead trees are a safety hazard. Large numbers of caterpillars are a nuisance and the hairs may cause skin and/or respiratory allergies. In addition, the fecal droppings can cover large areas and make it difficult to enjoy outdoor activities such as barbecues, swimming, and picnics. In fact, reduced attendance in recreational areas and/or resorts may occur during outbreaks.