Gypsy Moth (Insects)

The gypsy moth, Lymantria dispar. is one of the world’s most damaging defoliators of hardwood forest trees. It is native to Europe and Asia. It was introduced from Europe to North America near Boston, MA in 1868 and has been spreading slowly south and west ever since. A large body of research has focused on the biology, management, and population dynamics of this species.

GEOGRAPHIC RANGE AND SPREAD

Gypsy moth occurs throughout much of the northern hemisphere. Its native range stretches from Japan, China, and Siberia across Russia to Western Europe and as far south as the Atlas Mountains of North Africa. In North America, gypsy moth has spread over much of the eastern United States and Canada. Currently, the leading edge of the infestation stretches from North Carolina to Minnesota and adjacent regions of Ontario (Fig. 1). Because female gypsy moths from Europe have wings but do not fly, the rate of spread of this insect has been extremely slow. The spread occurs when newly hatched larvae spin down on silken threads and are blown in the wind. Most of this dispersal is less than 50 m, although some larvae are carried by wind currents for greater distances. Natural spread has been augmented by inadvertent human transport of egg masses laid on vehicles or other backyard objects, often to locations well outside the region infested by gypsy moth in the northeastern United States (Fig. 1). Such new infestations have been eradicated at many sites throughout North America. The spread along the front is monitored with an extensive array of survey traps baited with the gypsy moth sex pheromone. A. M. Liebhold and colleagues have used these data to develop sophisticated models of the rate of spread and understand the various factors that affect it. Chief among these, are factors called Allee effects that limit the rate of spread and cause many incipient populations ahead of the front to go extinct without human intervention, mainly because densities are too sparse for gypsy moth females to find mates. This phenomenon has made eradication possible in many locations (Fig. 1). These studies have made important contribution to the general theory of invasion biology.


LIFE HISTORY AND HOST TREES

The gypsy moth female lays a single egg mass (Fig. 2 ), usually on the stems of trees, and she covers the eggs with her body hairs. The egg mass typically contains from 100 to 600 eggs. The eggs are laid in midsummer, but overwinter in this stage. Larvae developing within the egg enter diapause and hatch the following spring at the time of host-tree budbreak. Emerging larvae climb to the tops of trees, where many of them spin down on silken threads and are borne away by the wind. If larvae land on an acceptable host tree, they begin feeding. They develop through five larval instars for males or frequently six for females throughout May and June. Beginning in the fourth instar, larvae seek resting locations during daylight hours, either in the forest litter at the base of trees or under bark flaps on tree stems. They usually pupate in these same locations. The adults emerge after about 12 days in the pupal stage. Soon after eclosion, the female releases a
Current range of gypsy moth in the United States as of 2008 adapted from selectName=ITAXAIA. The invasion started in Massachusetts in 1868. Much of these data are generated by pheromone-baited traps, deployed over much of the country south and west of the leading edge of the generally infested area
FIGURE 1 Current range of gypsy moth in the United States as of 2008 adapted from selectName=ITAXAIA. The invasion started in Massachusetts in 1868. Much of these data are generated by pheromone-baited traps, deployed over much of the country south and west of the leading edge of the generally infested area.
sex pheromone from a gland on the tip of her abdomen. Males locate females by flying upwind when they detect the pheromone. After mating, the female lays her egg mass, often just a few cm from where she eclosed, and dies soon thereafter. There is one generation per year.
Gypsy moth larvae feed on a wide range of tree species. Favored tree species include oaks (Quercus spp.), aspen (Populus spp.), and in Japan, Japanese larch, Larix leptolepis. Gypsy moth outbreaks occur in forests that are dominated by these species. Gypsy moth will feed on many other tree species, such as maple (Acer spp.) and many conifers, but significant damage to these trees usually occurs only in gypsy moth outbreaks, when more favored hosts have already been defoliated. If defoliation is complete, most deciduous hardwood trees will put out a new set of leaves. Most trees will survive one defoliation, but if outbreaks persist for several years in a row a significant proportion of the trees may die. Trees that survive defoliation suffer growth loss in subsequent years.

DISEASES OF GYPSY MOTH

As with most insects, gypsy moths are host to a suite of natural enemies and these play a pivotal role in the dynamics of the gypsy moth populations. There are two major diseases: a nuclear poly-hedrosis virus and a fungal pathogen. The virus, LdMNPV, causes epizootics that are largely responsible for the collapse of gypsy moth outbreaks. Similar viruses terminate the outbreaks of many defoliating Lepidoptera. High mortality from these viral diseases only occurs in dense populations, because transmission of the virus takes place when larvae feed on leaves contaminated by cadavers of larvae that have previously died from the disease. Encounters with cadavers are only likely in dense populations. Transmission of LdMNPV from one generation of gypsy moths to the next occurs primarily by way of external contamination of the egg mass; larvae become infected as they emerge from the mass in the spring. It is not entirely clear how the virus persists at low density, but it does survive in the forest litter for several decades.
The fungal pathogen, Entomophaga maimaiga was, until recently, known only from the Far East, especially Japan. In 1989, a dramatic epizootic of E. maimaiga occurred throughout the northeastern United States from Pennsylvania to Maine. In subsequent years, the fungus spread across the mid-Atlantic States and was introduced intentionally by researchers to Virginia and Michigan. It is now established throughout the region infested by gypsy moth in North America. E. maimaiga produces two kinds of spores: conidia and resting spores. The conidia are released from cadavers and are carried by wind currents to unin-fected larvae, which they infect by penetrating the cuticle; these conidia are responsible for the rapid spread of E. maimaiga in North America. Late-instar gypsy moths produce resting spores that overwinter in the forest litter, where they persist for up to 10 years before
Life stages of gypsy moth: (1,2) adult female; (3,4) adult male, (5) pupae; (6,7) larvae; (8) egg mass; (9,10) individual eggs.
FIGURE 2 Life stages of gypsy moth: (1,2) adult female; (3,4) adult male, (5) pupae; (6,7) larvae; (8) egg mass; (9,10) individual eggs.
germinating to infect new gypsy moths. A. Hajek and colleagues analyzed the DNA of E. maimaiga and showed that the pathogen in North America is identical to E. maimaiga in Japan. How E. maimaiga was introduced to North America is unknown. Since 1989, it has continued to cause high levels of mortality in gypsy moth populations, particularly in years with high rainfall in May and June. A key difference from LdMNPV is that E. maimaiga causes substantial mortality in low-density as well as high-density populations of gypsy moth. This means that E. maimaiga can prevent outbreaks from occurring, whereas LdMNPV can only cause the collapse of outbreak populations.

PARASITOIDS

As with most insects, various parasitoid species attack the different life stages of gypsy moth. In North America, efforts to introduce parasitoids of gypsy moth from Europe and Asia began around 1905, and ten species have been established. The egg parasitoid Ooencyrtus kuvanae (Encyrtidae) from Japan is frequently observed on gypsy moth egg masses in late summer and may cause as much as 30% mortality. Larval parasitoids include Cotesia melanoscela (Braconidae) and the tachinids Blepharipa pratensis, Compsilura concinnata, and Parasetigena silvestris. The most common pupal parasitoid is Brachymeria intermedia (Chalcididae). The impact of these parasi-toids on gypsy moth populations remains equivocal. Total mortality caused by parasitoids in North America is typically below 50% and is not consistently density dependent, so that their ability to regulate gypsy moth populations is in doubt. In Europe, on the other hand, parasitism of gypsy moth is often much higher than that observed in North America. European gypsy moths are attacked by several parasitoid species that were never established successfully in North America. It seems likely that parasitoids are responsible for preventing gypsy moth outbreaks, which are rare in Western Europe, but more common in central and southern Europe.

PREDATORS

Compared with parasitoids, an even larger community of vertebrate and invertebrate predators feed on gypsy moth. Very little is known about the impact of most predators, because predation is extremely difficult to measure. Many bird species worldwide feed on gypsy moth, but it is generally believed that most birds dislike the hairy cuticle of gypsy moths and avoid them.
Research groups led by H. Bess in the 1940s and R. Campbell in the 1970s, concluded that predation by small mammals, particularly the white-footed mouse, Peromyscus leucopus, has a major impact on low-density gypsy moth populations. The mice feed on late-instar larvae and pupae, particularly on the forest floor. Both research groups demonstrated an increase in gypsy moth survival in forest plots from which small mammals had been removed or excluded. More recently, J. Elkinton and colleagues showed that predation on gypsy moth pupae was strongly correlated to density of mice, and that gypsy moth densities increased when mouse densities declined. The density of mice, in turn, was correlated to the abundance of acorns, which in oak-dominated forests are their principal overwintering food. Indeed, there are many studies that link forest-dwelling mice to abundance of acorns crops. Poor acorn crops, which occur on a regional scale and are caused by a variety of weather events, could thus be the ultimate trigger of gypsy moth outbreaks. C. Jones and colleagues provided further experimental proof of these ideas. They removed mice from experimental plots and observed an increase in gypsy moth and they augmented food in other plots and observed an increase in mice.
There are a number of invertebrate predators of gypsy moth. One of these is the introduced ground beetle, Calosoma sychophanta (Carabidae). R. Weseloh has shown that this insect becomes quite abundant in outbreak populations of gypsy moth and may cause substantial mortality. Several researchers have documented predation by ants as a significant source of mortality in low-density populations, but it is usually much less than predation by mice.

POPULATION DYNAMICS

In the 1970s, R. Campbell developed the first comprehensive theory of gypsy moth dynamics, wherein populations alternate between low-density and high-density phases, each maintained by different factors and sources of mortality. At low density, predators, particularly mice, maintain gypsy moth populations indefinitely at a low-density equilibrium. The concept of an equilibrium implies that predation is density dependent, which means that it increases as gypsy moth density increases until total mortality balances fecundity of gypsy moth and the population density stops growing. The equilibrium is an unstable one, however, because the density-dependent response of most natural enemies is constrained by a variety of factors. For example, most predators or parasitoids have their own natural enemies. These constraints produce a threshold density above which gypsy moth population growth outpaces the mortality caused by natural enemies, and as a result densities of gypsy moth increase rapidly into an outbreak phase. At the much higher outbreak densities, a different set of natural enemies become predominant. These natural enemies, coupled with competition of gypsy moth for available foliage, limit further increases in gypsy moth density. The outbreak population either persists for several generations at high-density, or collapses back to the low-density phase. The model is particularly appropriate when the principal mortality factors maintaining the low-density equilibrium are generalist predators, such as mice. Unlike specialist natural enemies, whose densities often track those of their hosts, the densities of mice are not determined by gypsy moths. Instead their densities are determined by their overwintering food supply, mainly acorns. Whether this conceptual model is an accurate description of the gypsy moth system remains to be demonstrated.
Adding to the complexity of gypsy moth dynamics in North America is the recent appearance of the fungal pathogen E. maimaiga. In the northeastern United States, epizootics of this fungus have occurred nearly every year since 1989, except in years that were extremely dry. We still have much to learn about this agent, but it causes substantial mortality in both low- and high-density populations and appears to have prevented several incipient outbreaks. It appears that incidence of E. maimaiga, which is largely determined by rainfall in May and June, is now a prime determinant of whether outbreaks occur.
Gypsy moth outbreaks are spatially synchronized (Fig. 3). Outbreaks tend to occur simultaneously over a large region. A cause of this pattern may be acorns crops, which are heavy or meager over a region in response to regional weather patterns. However, as indicated by P. Moran, populations of many species are synchronized by a variety of weather-related influences. Weather may synchronize
Historical record of area defoliated by gypsy moth in each of five states in northeastern United States from 1924 to 1996.
FIGURE 3 Historical record of area defoliated by gypsy moth in each of five states in northeastern United States from 1924 to 1996.
gypsy moth populations by exerting a common influence across populations on many of the factors that affect gypsy moth growth and survival. Recent analyses by Johnson et al. indicate that there is a 10-year cycle of abundance of gypsy moth embedded in the erratic temporal pattern evident in Fig. 2. If these cycles exist, the factors that cause them remain poorly understood. However, a mathematical model of the gypsy moth system developed by Dwyer et al. that incorporates predation by mice and mortality caused by NPV exhibited periodic behavior that mimicked the behavior of real gypsy moth populations.

MANAGEMENT OF GYPSY MOTH

Management of gypsy moth in North America and elsewhere has evolved over time as different tools became available and as public attitudes toward pesticide use have changed. In the 1960s, large areas were sprayed by air with DDT. In the late 1960s, DDT was banned and was supplanted by other chemical pesticides, such as Carbaryl. In the late 1980s, Bacillus thuringiensis (Bt), a bacterial pesticide, became a viable alternative to chemical pesticides and became the material of choice in many regions. The advantage of Bt is that it is more selective than most chemical pesticides; it only affects larval Lepidoptera that feed on Bt-contaminated foliage. Bt is one of several biopesticides that have entered the market in recent years. Government agencies in the generally infested region in northeastern United States have abandoned large-scale application of pesticides against gypsy moth because it was neither ecologically acceptable nor worth the considerable cost. Most forests trees survive gypsy moth outbreaks and the outbreak populations soon collapse on their own. Control activities in these regions generally aim at foliage protection on high-value trees, rather than suppression of gypsy moth populations. Management of gypsy moth by state and federal governments in recent years has focused on the slowing the spread of gypsy moth along the invasion front that currently extends from Wisconsin to North Carolina (Fig. 1). Spread is monitored with pheromone-baited traps and new infestations that are detected at the leading edge of the advancing front are eradicated using a variety of tools that include pesticides and application of gypsy moth pherom-one to disrupt mating. Eradication has been achieved many times because once infestations fall below a threshold density they will decline to extinction due to failure of females to find mates. Other successful eradication efforts have focused on various ports where there have been occasional new introductions of gypsy moth from Asia, particularly the Russian Far East. Adult female gypsy moths from these regions are able to fly. Consequently, the rate of spread would be much faster, if they became established.

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