Lymantria dispar (insect)

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General Impact The gypsy moth Lymantria dispar is a defoliator of mainly deciduous trees. In most areas and in most years, gypsy moths remain at low densities and cause no discernible damage. Occasionally, however populations reach high densities and these outbreak populations may completely defoliate host trees. Most impacts of gypsy moth are associated with the physiological stress in trees caused by defoliation, especially if it occurs several years in a row or in conjunction with drought. These effects include reduction in tree growth, crown dieback and tree mortality. Tree mortality is usually associated with other insects (wood borers) and pathogenic fungi that attack stressed trees. In extreme situations, nearly 100% tree mortality may occur over large areas. The most important impacts occur in urban/suburban settings. Defoliation and tree mortality may be very serious if impacted trees are valuable shade or street trees in urban settings. Outbreaks typically last 1 to 5 years. Outbreak populations then decline because of starvation and increased disease. Small mammal predators are considered the most important source of mortality in low density populations and may keep sparse gypsy populations in check for several years before the next outbreak occurs. Gypsy moth defoliation can reduce tree seed production and root sprouting, resulting in poor regeneration. The natural diversity and species composition of native insects, birds, and other animals may be altered during outbreaks through reductions in shelter, food supplies, and other benefits provided by host trees. In its caterpillar stage, the gypsy moth can feed on more than 500 different species of trees and shrubs. In North America the long list of preferred hosts includes oak, cherry white birch, maple, alder, willow, elm and trembling aspen. The Asian race also does well on coniferous trees such as larch, and has a broader host range than that of the European race (Humble and Stewart 1994). It is estimated that more than 30 million hectares of forest in the United States have been defoliated since 1970 (de Beurs and Townsend 2008). The potential area that is climatically suitable for the gypsy moth is estimated to be 595 million hectares (Gray 2004 in de Beurs and Townsend 2008). In Canada spread of the gypsy moth to the north and west has so far been prevented by climatic barriers. However current climate change forecasts are expected to increase the area of climatic suitability for the gypsy moth (Regniere et al. 2009). Similarly in the western United States climate prediction models suggest a warming trend that will increase the area suitable for gypsy moth. Annual losses can reach millions of dollars due to lost revenues from timber harvesting, cost of hazard tree removal and loss of amenity values. Defoliation and tree mortality caused by gypsy moth may also influence property values (Humble and Stewart 1994). Also, during outbreaks caterpillars become so numerous that they become a nuisance to homeowners recreating in their yards. Some people also have allergies to the urticacious hairs on gypsy moth larvae, further compromising their recreational experience. Not all impacts of gypsy moth are negative. In North America during outbreaks, gypsy moth represents a super-abundant food source for native cuckoos. In outbreak areas cuckoo abundance increased significantly, causing a redistribution of cuckoos within their current range. Presumably this would cause a redistribution of their predation impact. The flow on effects to forest food webs is unknown, but could have important effects on insect prey and plants by reducing herbivorous insects (Barber et al. 2008). Other positive impacts of gypsy moth defoliation could include a more open canopy due to overstorey tree mortality and increased shrub growth, and may benefit some understory-nesting bird species (Bell and Whitmore 1997, 2000 in Barber et al. 2008).          Location Specific Impacts: Canada   Agricultural: Industries for oak, Douglas fir, and some fruit trees are detrimentally affected by L. dispar outbreaks. Economic/Livelihoods: Oak is the preferred food of L. dispar and is valuable to the forestry industry. Invasions are detrimental to National Parks and associated service industries because they destroy the aesthetic and recreational value of these areas. Herbivory: L. dispar defoliates trees, making them more susceptible to disease, insects, and drought. Human nuisance: Many people are allergic to the hairs which cover and fall from L. dispar egg masses. L. dispar also attacks urban landscape trees, defoliating and covering them with their waste products. Modification of fire regime: Trees that die from the secondary effects of L. dispar serve as fuel for forest fires. Modification of successional patterns: If the outbreak indirectly causes the forest to burn or to be harvested, the ecosystem must reset itself. Mongolia   Herbivory: In Mongolian and Siberian forest steppe ecotones gypsy moths often completely defoliate entire forest edges of Larix sibirica. Though trees at forest edges infested by gypsy moth larvae are often so severely defoliated that they look dead, no information is available in the literature whether such damage in L. sibirica is reversible or not. Hauck et al (2008) determined that often they are not sustainably affected, as trees were fully foliated in the subsequent year. "This suggests that single gypsy moth invasions, which are frequent in Mongolia’s forest-steppe ecotone, do not necessarily result in permanent damage of L. sibirica and, with it, not necessarily lead to local shifts of the treeline, though entire forest edges are often completely defoliated." Siberia (Russian Federation)   Herbivory: In Mongolian and Siberian forest steppe ecotones gypsy moths often completely defoliate entire forest edges of Larix sibirica. Though trees at forest edges infested by gypsy moth larvae are often so severely defoliated that they look dead, no information is available in the literature whether such damage in L. sibirica is reversible or not. Hauck et al (2008) determined that often they are not sustainably affected, as trees were fully foliated in the subsequent year. "This suggests that single gypsy moth invasions, which are frequent in Mongolia’s forest-steppe ecotone, do not necessarily result in permanent damage of L. sibirica and, with it, not necessarily lead to local shifts of the treeline, though entire forest edges are often completely defoliated." Serbia   Economic/Livelihoods: During outbreaks, this generalist pest causes economic damage due to defoliation of forests and orchards over large areas United States (USA)   Agricultural: Industries for oak, Douglas fir, and some fruit trees are detrimentally affected by L. dispar outbreaks. Economic/Livelihoods: Oak is the preferred food of L. dispar and is valuable to the forestry industry. Invasions are detrimental to National Parks and associated service industries because they destroy the aesthetic and recreational value of these areas. Herbivory: L. dispar defoliates trees, making them more susceptible to disease, insects, and drought. Human nuisance: Many people are allergic to the hairs which cover and fall from L. dispar egg masses. L. dispar also attacks urban landscape trees, defoliating and covering them with their waste products. Modification of fire regime: Trees that die from the secondary effects of L. dispar serve as fuel for forest fires. Modification of successional patterns: If the outbreak indirectly causes the forest to burn or to be harvested, the ecosystem must reset itself.