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Gypsy Moth in Virginia: An Update
E. A. Roberts
Department of Entomology, Virginia Tech
Most Virginians are aware that the gypsy moth is a serious pest of hardwoods in the state. Although this insect has maintained a low profile the past few years, there was a general resurgence in moth populations in 2000. This population increase serves as a reminder that, in areas where gypsy moth has become established, this pest is still present in the environment even when populations are too low to be noticed.
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Gypsy moth is a native of Europe, Asia, and northern Africa. It was accidentally released in the U.S. over 130 years ago by a Frenchman who wanted to cross it with native silk moths. From its original introduction near Boston, Massachusetts, this pest has spread into the mid-Atlantic and Midwestern states (Fig. 1).
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 Figure 1. Area of General Infestation of Gypsy Moth in 1998. |
Virginia experienced its first gypsy moth defoliation in 1984. Since that time, nearly 4.5 million acres have been defoliated in the state (Table 1). Of the 71,000+ acres defoliated in 2000, Shenandoah and Bath counties suffered the most damage (Table 2). Most of Virginia currently falls into the federally regulated area (Fig. 2).
 Figure 2. Gypsy moth federally regulated area in Virginia as of 1999. |
This area is considered to be generally infested and is governed by certain state and federal regulations regarding gypsy moth monitoring and control as well as the transport of articles which possibly could harbor gypsy moth life stages.
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defoliation in Virginia |
Table 2. Gypsy moth defoliation
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Life history. The gypsy moth has one generation per year. Eggs hatch in early spring at about the time oak leaves are beginning to develop. Newly hatched larvae (caterpillars) climb high in trees and can be carried several hundred meters by the wind. This is the primary means of natural dispersal. As the larvae mature they undergo a series of growth stages called instars. Although feeding activity of young larvae may not be noticed, older larvae are voracious feeders and can grow to a length of over 2.5 inches (Fig. 3). It is estimated that a gypsy moth caterpillar consumes about one square meter (over ten square feet) of foliage in its lifetime. Mature larvae form a pupal stage, from which the adults emerge one to two weeks later. The female moth does not fly; she releases a chemical attractant that the male detects and uses as an aid in locating the female for mating. A synthetic version of this sex pheromone is used as a lure in the common gypsy moth traps seen on the roadside during the summer. After mating, the female lays an egg mass containing from a few hundred to nearly one thousand eggs (Fig. 4). These eggs are the overwintering stage of the insect. It is usually the egg mass stage that is inadvertently moved by people on cars, trailers, campers, firewood, and outdoor equipment and furniture.
 Figure 3. Fifth instar gypsy moth larva |
 Figure 4. Female gypsy moth laying an egg mass |
Gypsy moth larvae are quite hairy and can be distinguished by pairs of blue and red dots along their backs (Fig. 3). They sometimes are confused with eastern tent caterpillar or fall webworm, but the gypsy moth does not form large silken webs and has entirely different behavior and seasonal development from these insects. The egg masses are buff colored and covered with hairs from the female's abdomen. Adults do not feed and live only a few days after emergence and mating.
Gypsy moth will feed on many tree species, but its preferred hosts include oak, aspen, willow, birch, apple, and basswood. Young larvae are much more discriminating than older ones, which will feed readily on less preferred species such as maple, hickory, elm, and many conifers. Usually, these less preferred species are fed upon only when insect populations are high and preferred food sources become scarce. Some trees are never fed upon, the classic example being tulip poplar.
Population cycles. Gypsy moth is an outbreak pest. It can remain at low levels for several years and then undergo large population increases in only one or two years. Although these cycles are influenced by numerous factors, the low populations in Virginia in recent years generally are believed to be the result, at least in part, of the spread of a gypsy moth disease caused by the fungus Entomophaga maimaiga. This fungus was introduced into the U.S. as a possible control agent in the early 1900s but never appeared to become established. However, it was "rediscovered" in 1989 in New England and has since been found throughout a large portion of the generally infested area, including Virginia. Dry weather in the winter and spring of 2000 perhaps was unfavorable for the fungus, and this might have contributed to the increased activity of the gypsy moth. The unusually wet summer this year should be favorable for the fungus, so it is possible that some populations in 2001 will be lower than originally expected due to mortality caused by this disease. Gypsy moth populations are also affected by a second disease, caused by a virus. The virus is the usual cause of mortality in high populations whereas Entomophaga maimaiga serves as a control agent at both low and high pest densities.
Damage caused by gypsy moth. Predicting tree mortality as a result of gypsy moth infestation is difficult because many factors contribute to determining tree responses to defoliation. These include site and stand characteristics, species composition, individual tree health, the susceptibility and vulnerability of the tree species, drought conditions, and the extent and frequency of defoliation. When trees are heavily defoliated, they produce a second set of smaller, lighter colored leaves. This refoliation causes additional stress on the tree. Healthy trees can withstand one or two severe defoliations, while unhealthy trees might die after one defoliation episode. On a forest stand basis, a rule of thumb is to expect 15 to 35% average mortality when gypsy moth first invades an area. Some stands might have very low mortality while others might have mortality approaching 75% or even greater. The actual cause of death in trees attacked by the gypsy moth is not the defoliation itself but is from secondary organisms which invade the weakened tree. The most common of these is the shoestring root rot fungus and the twolined chestnut borer. A guide to silvicultural control is provided by K. W. Gottschalk, Silvicultural Guidelines for Forest Stands Threatened by the Gypsy Moth, USDA Forest Service General Technical Report NE-171. To receive a copy call 740-368-0123.
Control options. Treatment against gypsy moth usually is by aerial application of insecticides applied to young larvae. While several insecticides are registered for this pest, the most commonly used treatments are a bacterium, Bacillus thuringiensis var kurstaki (Btk), and diflubenzuron (DimilinÒ ). Btk is a stomach poison and is toxic only to butterflies and moths. Aside from being the most commonly applied aerial insecticide against gypsy moth, it is also one of the options available to the homeowner. DimilinÒ , an insect growth regulator, interferes with the caterpillar's ability to develop normally and is a restricted use insecticide which can be applied only by licensed pest control operators. Some homeowners attempt to protect individual trees by removing egg masses or by killing larvae collected from beneath bands placed around trees. These methods are sometimes effective but are tedious and do little to reduce the overall pest populations. Drenching egg masses with a 1:1 soybean oil and water solution may be effective in protecting trees from gypsy moth in the following year, but this works best only on a few isolated trees.
What is being done. Gypsy moth management is conducted at the federal, state, and local levels. Federal agencies are responsible for detection and management of gypsy moth infestations outside the regulated area. The U.S. Forest Service Slow the Spread Project is a large integrated pest management project that targets low level populations in the transition zone (Fig. 1). The Virginia Department of Agriculture and Consumer Services has a cost-share program, the Virginia Cooperative Suppression Program, which assists localities by providing up to 50% of the costs of gypsy moth management. For more information on this program contact Larry Nichols, Office of Plant Protection, VDACS, 804-786-3515. The guidelines to this program are available from The Gypsy Moth Server at Virginia Tech web site.
Advice for landowners. The article mentioned above by K. Gottschalk is a good source of guidelines for landowners with forested stands. Persons concerned with yard trees will need to rely on keeping abreast of pest populations in their areas and coordinating with municipal, county, or homeowner groups for monitoring and treatment. Contact your local arborist, extension agent, or the VDACS offices in Richmond to obtain the latest information on the gypsy moth situation in your area. The VDACS cost-share document mentioned above contains information on how to assess the gypsy moth threat to your property.
Selected online resources
The Gypsy Moth Server at Virginia Tech: gypsymoth.ento.vt.edu/vagm
The Slow the Spread Project: www.ento.vt.edu/STS
Gypsy Moth in North America: www.fs.fed.us/ne/morgantown/4557/gmoth
