(click on photo to make it larger)
The gypsy moth (Lymantria dispar) is native to temperate regions of Europe, southern Asia, and Africa. In its native region, it occasionally reaches outbreak numbers. It was introduced to North America from Europe in 1869 when a French naturalist attempted to cross it with a species of silk worm (Bombyx mori) to engineer a silk worm that would survive in North America (Coulson and Witter 1984). Specifically, the gypsy moth first escaped in Medford, Massachusetts. Within 12 years, the insect became a problem within this area. Residents reported defoliation of nearly every tree and entire houses covered with larvae. Because it was assumed to be just a native pest, there was no initial attempts to control the spread of the gypsy moth (Johnson and Lyon 1991). The gypsy moth used the prevailing northerly and easterly winds to spread throughout the northeastern United States and southern Canada (Coulson and Witter 1984). Because there are no native predators of the gypsy moth, the dispersing larvae were able to successfully colonized new areas, and now are found over 200,000 square miles (518,000 sq km) within these regions (Johnson and Lyon 1991). The gypsy moth continues to spread into new areas today, despite stringent quarantine regulations and control programs. Using automobiles and recreational vehicles to disperse, gypsy moth eggs and larvae have been reported as far west as Washington and Oregon (Johnson and Lyon 1991). Now that harsher chemicals such as DDT are banned, the spread appears even more rapid (Johnson and Lyon 1991). It may just be a matter of time before the gypsy moth has spread throughout all the United States and southern Canada (Johnson and Lyon 1991).
The female and male adult gypsy moth are rather large (wingspan up to 5 cm), but differ both in appearance and their ability to fly. Males are dark brown with blackish bands on the forewings and are able to fly. In contrast, the females are almost white with the blackish bands on the forewings and do not fly (Johnson and Lyon 1991). The female lays egg masses with an average of 750 eggs during optimal conditions; this number will decrease when the population starts to decline at the end of an outbreak. They are laid many different places including: the branches or trunks of trees, fences, buildings, and vehicles (Johnson and Lyon 1991).
The egg masses overwinter, and the larvae begin to hatch in early April to late May (Johnson and Lyon 1991). Larvae remain in the mass for several days before they emerge, climb, and feed on a host tree (Johnson and Lyon 1991). This first instar suspends itself from a leaf by a silken thread. If the wind is strong enough, this larvae can become airborne and be carried several hundred yards (Johnson and Lyon 1991).
Once the larva emerge, they feed from mid-May to late-June (Coulson and Witter 1984). As they mature, the larva's feeding habits change (Johnson and Lyon 1991). Young larva (first three stages) restrict feeding from after sunrise to until shortly after sunset, whereas larger larva seek shelter during the day and begin to feed just before sunset to until midnight (Coulson and Witter 1984; Johnson and Lyon 1991).
Photo
of gypsy moth larvae feeding on leaf material. Click on photo
to make it larger.
After approximately seven weeks (around early July), the mature larva look for a sheltered area and pupate. Like egg masses, pupae can occur almost anywhere suitable (e.g. trees, fallen branches, vehicles, etc.). This also can enable the gypsy moth to disperse over large distances (Coulson and Witter 1984). The moths emerge in mid-July. As you recall, the female does not fly, so in order to attract a mate, she ascends to an elevated position and secretes a pheromone (chemical that acts as a sex attractant). This chemical volatilizes into the air and can be encountered by a male up to a mile (1.6 km) away (Johnson and Lyon 1991). After mating, the female immediately lays the egg masses (Coulson and Witter 1984).
The specific aspects of the lifecycle of the gypsy moth has significantly contributed to its successful invasion of North America (Coulson and Witter 1984). For example:
Oak (Quercus spp.) is by far the most preferred species by the gypsy moth. However, the gypsy moth can infest many species of hosts (up to 300 different species) (Coulson and Witter 1984).
1. Some of the more preferred species include: apple (Morus spp.), alder (Alnus spp.), basswood (Tilia spp.), hawthorn (Crataegus spp.), some poplars (Populus spp.), and willows (Salix spp.).
2. Other, less preferred trees, constitute: elm (Ulmus spp.), black gum (Nyssa sylvatica), hickory (Carya spp.), maple (Acer spp.), and sassafras (Sassafras albidum), and in a dense population, larva may feed upon beech (Fagus grandifolia), hemlock (Tsuga spp.), pines (Pinus spp.), and spruce (Picea spp.).
3. A few species are rarely, if ever, are infested. These consist of: ash (Fraxinus spp.), balsam fir (Abies balsamea), butternut (Juglans cinera), black walnut (Juglans nigra), catalpa (Catalpa spp.), redcedar (Juniperus spp.), holly (Ilex spp.), sycamore (Platanus occidentalis), locust (Gleditsia spp.; Robinia spp.), and tuliptree (Liriodendron tulipifera). ~(Johnson and Lyons 1991).
The gypsy moth is a ferocious defoliator and can kill a large number of trees, especially when it first moves into an area (Coulson and Witter 1984). Deciduous trees can survive 2-3 years of defoliation before they severely decline or die, but conifers can not endure one complete defoliation (Johnson and Lyons 1991). The most susceptible trees, and those first to die, are suppressed trees in the understory and trees living on poor sites or under droughty conditions. Not only can gypsy moths kill a tree outright, but severe defoliation can increase a tree's predisposition to secondary infestations by the shoestring fungus (Armillariella mellea) and/or the twolined chestnut borer (Agrilus bilineatus). Together with the defoliation by the gypsy moth, secondary infestations by the fungus and borer affect the photosynthesis, growth regulators, and water and nutrient relations within the crown and stem of the infected tree (borer) and belowground in the roots (fungus). This interaction of the primary and secondary organisms leads to an increase in the probability of tree mortality ~Coulson and Witter 1984.
The gypsy moth also can contribute to an economic decline within an heavily affected area. The value of recreational areas, especially during May, June, and July when the larvae are prevalent, can be severely affected by the infestation. Likewise, the residential property values can decrease if gypsy moth is found within the area. Additionally, the timber industry, especially within the southern Appalachians, could be severely impacted by a large infestation ~Coulson and Witter 1984.
Gypsy moth is an extensive defoliator, especially of oak spp. They especially affect oak regeneration following a disturbance, such as clearcutting (Hicks et al. 1993). Similarly to both the forest tent caterpillar and the spruce budworm, this can decrease leaf area, outright kill individual trees, decrease growth, make trees more susceptible to secondary infections, change the quality of the wood, and/or contribute to a change in stand structure (Witter and Stoyenoff 1992). For example, stands that experience repeated gypsy moth infestations were found to have fewer trees per acre and a decrease in the number of oaks (Herrick and Gansner 1988). Gypsy moths also can significantly impact species in the understory and forest floor because larvae will feed on preferred species within the understory. In addition, the opening of the canopy from both defoliation and death of individual trees increases the amount of available light, nutrients, and moisture on the forest floor, and thereby increases the populations of both herb and shrub species (Witter et al. 1992). Defoliation by gypsy moths also can lead to stream-water acidification and thus, a change in the biogeochemistry within stream-water catchments (Webb et al. 1995).
In addition to the plant species within an area, the animal and insect population could be impacted as the gypsy moth moves into new areas. For example, beetle predators of the gypsy may follow infestations and invade different forest communities (MacLean and Usis 1992). Extensive defoliation by larvae that changes forest structure may indirectly affect birds (Thurber et al. 1994). Mortality in the canopy leads to a reduction in suitable nesting sites for canopy-nesting birds and to an increase in the amount of interior edge. This could augment nest parasitism and predation (Thurber et al. 1994). However, the increase in shrub and herbaceous species after defoliation of the canopy also can lead to an increase in shrub- and ground-nesting bird species (Thurber et al. 1994).
Gypsy moth is an exotic species in North America, however, there are some species that will prey on the gypsy moth (e.g. black-capped chickadee, blue jays, white-footed mice, shorttail shrews, spiders, harvestmen, and ground beetles). Predation by small mammals such as the white-footed moose (Yahner and Smith 1991) and some species of ants (Weseloh 1994) has been known to keep small populations of gypsy moths innocuous within limited areas, but predation rarely plays more than a minor role in controlling numbers during an outbreak (Coulson and Witter 1984).
There are also some parasites in Europe and Asia that have been successful at limiting gypsy moth numbers (Johnson and Lyon 1991). Some common examples of these include: wasp parasites, an egg parasite (Ooencyrtus kuvanae), and a fly parasite of the caterpillar (Blepharipa pratensis) (Johnson and Lyon 1991).
Climate also can be a major control of gypsy moth numbers. Eggs depend on insulation from snow to protect them from extreme winter temperatures (Coulson and Witter 1984). However, overwintering eggs will die if the temperature dips below 29oC. The survival of overwintering eggs also can be affected by their the winter conditioning and long periods of low temperatures (Coulson and Witter 1984).
Biological controls
Humans are attempting to use biological controls to maintain populations of gypsy moth innocuous within North America. Parasites and predators from North America, Europe, and Asia that have been released by humans as biological controls have had limited success at regulating numbers. One example of a North American predator that has been released against the gypsy moth is a species of ground beetle (Calosoma sycophanta). However, its effects on gypsy moth populations were restricted to limited areas on a small scale primarily because of the difficulty associated with the rearing and releasing large numbers of beetles (Weseloh 1990).
There also have been some attempts to activate a virus that is harbored within 10% of the population. This baculovirus (viruses containing double-stranded DNA and infecting primarily invertebrates) carries the egt gene (Park et al. 1993). When expressed, this gene enables the baculovirus to hinder infected hosts from molting and pupating (Park et al. 1993). Even though the larvae do not stop eating when infected with this virus, they do gain much less weight (Park et al. 1993). Baculoviruses (in particular the nucleopolyhedrosis virus; product name GYPCHEK) have been manufactured to be used as an alternative to chemical insecticides (Reardon and Podgwaite 1992). When the population density of gypsy moths increases, this virus can reach outbreak proportions (Reardon and Podgwaite 1992). Therefore it could have a potentially significant effects on that population, and keep the gypsy moth numbers under control (Reardon and Podgwaite 1992). Another biological insecticide licensed for use against the gypsy moth is the microbial insecticide Bacillus thuringiensis (Reardon and Podgwaite 1992).
There are also fungal insect pathogens that have had some success in controlling gypsy moth. One such fungus is Entomophaga aulicae (Johnson and Lyon 1991). It has been successful in controlling populations in Asia and potentially could be used as a microbial insecticide in North America (Johnson and Lyon 1991).
Humans also have tried to use pheromone traps to capture adult male gypsy moths and prevent them from reproducing. This is probably one of the most used measure to prevent outbreaks, not only of gypsy moths, but of several insects, especially those that damage crops (Taylor et al. 1991). However, it appears that as the gypsy moth density increases, the effectiveness of these traps to restrict the population decreases (Taylor et al. 1991).
Other Human Control Measures
There have been attempts to mechanically restrict gypsy moths. One such method is the use of barrier bands around the boles of trees to repel and/or trap caterpillars as they move up and down the tree (Cooperative Extension Service, Michigan State University 1991a). There are sticky band traps, which trap caterpillars, and slippery bands, which caterpillars are unable to travel across (Cooperative Extension Service, Michigan State University 1991a). Another type of band is a cloth band. As you recall, during the day, larvae will move to protected areas; thus, larvae will collect under the cloth banding for protection from direct sunlight. Once they move into these bands, people can then remove them by sweeping them into a bucket of soapy water, which kills the larvae instantly (Cooperative Extension Service, Michigan State University 1991b). These methods can be very effective in residential areas, however, they are probably not very useful on a larger scale.
Other current management techniques being used include:
1. Enforcing the Federal Domestic Quarantine to slow down the artificial spread of the gypsy moth by
2. Educate the public about the biology and spread of the gypsy moth to help prevent accidental introductions into new areas.