Flowers and Garden Home As with any pest, management strategies for gypsy moth are tied directly to the life cycle of the insect.  
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Browse Articles: Insects and Pests
Assessing Options for Managing Gypsy Moth
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Reference: Ohio State University Extension

egg, larvae, pupae, adult pie chart

Gypsy moth (Lymantria dispar) is the most important defoliator of trees in the eastern United States. This invasive, exotic species has invaded Ohio in the northeast, from Pennsylvania, and the northwest, from Michigan, and is rapidly colonizing the entire state. Gypsy moth can not be eradicated, but there are management approaches that effectively minimize gypsy moth defoliation and its impact. However, a number of commonly recommended management tactics are ineffective at preventing gypsy moth defoliation during outbreaks. This fact sheet discusses the pros and cons of the various approaches for managing gypsy moth.

larve congregate on tree trunk

Life Cycle and Habits of the Gypsy Moth
As with any pest, management strategies for gypsy moth are tied directly to the life cycle of the insect. Gypsy moth has only one generation per year and develops in four stages: egg, larva (caterpillar), pupa (cocoon), and adult (moth). Only the caterpillar stage damages trees. Gypsy moth overwinters as eggs, which hatch just as leaves emerge in late April to early May, or just as crabapples (Malus spp.) and redbud (Cercis canadensis) trees reach full bloom.

Newly hatched caterpillars begin feeding immediately, or disperse on silken threads that carry them on wind currents ("ballooning") to suitable hosts. The vast majority of larvae travel only short distances (less than 50 yards). Feeding continues throughout the spring. Most defoliation (80 to 90%) results from feeding by older caterpillars during early to mid-June, making it appear as if trees are stripped almost overnight. Older larvae feed at night and rest during the day on trunks and branches. Many larvae leave the tree to rest on the ground or on sides of houses, which is a major nuisance. When populations are dense, larvae feed day and night.

Mature gypsy moth larvae congregate in large numbers on tree trunk

Caterpillars complete development and enter the pupal stage between mid-June and early July. Adult moths emerge in 10 to 14 days. Adults do not feed and only live long enough to reproduce. The brownish male moth flies during the day in search of females. Whitish females do not fly, but attract males by producing a chemical attractant, or sex pheromone. Egg masses are deposited during mid- to late July and hatch the following spring, completing the life cycle. Each female produces one egg mass containing between 100 to 1,000 eggs. Egg masses are deposited on trees, fences, houses, signs, and similar objects. For this reason, gypsy moths are excellent hitchhikers. Eggs attached to items such as campers and lawn furniture, that are subsequently moved to different regions, have been responsible for many new infestations.

male Gypsy mothfemale Gypsy moth
Gypsy moth males are a mottled brown
color with broad wings.
Females have wings that are white to
cream colored with black spots.

Biological Control of Gypsy Moth
Several insect predators and parasites of gypsy moth have been released and are now well established in the United States. Native predators including insects, birds, mice, and other small mammals also feed on gypsy moth. Once gypsy moth is well established in a region, these natural enemies can help to keep populations low for a number of year's. However, for reasons that are not well understood, gypsy moth populations erupt periodically despite the presence of natural enemies. Once gypsy moth populations begin to increase, research has shown that predators and parasites are ineffective at preventing outbreaks.
gypsy moth caterpillar killed by fungus
Gypsy moth caterpillar killed by Entomophaga maimaiga fungus.

Commercially available parasitic insects, especially parasitic wasps in the genus Trichogramma, have been marketed aggressively as biocontrol agents for gypsy moth. However, research has shown that Trichogramma species do not parasitize gypsy moth eggs. Unfortunately, they do attack a number of native butterflies and moths. Consequently, their intentional release can cause environmental damage, impacting native species without affecting gypsy moth. There are no commercially available insect biocontrol agents that are effective against gypsy moth.

Diseases are very important in the population cycles of gypsy moth. The gypsy moth nuclear polyhedrosis virus (commonly known as NPV) infects caterpillars, building rapidly in dense populations. NPV usually causes caterpillar outbreaks to collapse within two or three year's. The virus can be formulated as a spray that does not affect any other species of insects. However, since the virus can be produced only from live caterpillars, supplies are extremely limited, and application of the NPV spray is generally limited to environmentally sensitive habitats, such as those containing endangered butterflies and moths.

A disease caused by the fungus Entomophaga maimaiga also can significantly impact gypsy moth populations, even when populations are low. The fungus, which infects the caterpillar stage, is easy to distribute and spreads rapidly when environmental conditions are favorable. However, the effectiveness of Entomophaga is not predictable, being highly dependent on the occurrence of wet weather at critical times during the spring. Although Entomophaga can cause gypsy moth populations to decline dramatically in wet year's, caterpillar outbreaks will still occur where Entomophaga is established, especially during dry springs.

Managing Gypsy Moth With Bt
When populations are high, aerial application of insecticide formulations derived from the naturally occurring bacterium, Bacillus thuringiensis (commonly known as Bt) is the least toxic and most effective tool for preventing widespread defoliation. Bt sprays used for gypsy moth affect only Lepidoptera larvae (caterpillar stage of butterflies and moths) and are harmless to other animals, including bees and other insects, birds, pets, and humans. Individual trees can also be sprayed with Bt, which can be purchased by homeowners and professional landscape managers.

Bt is very effective at preventing defoliation when sprays are timed accurately. Applications should be made when the majority of larvae are second instars, generally during early to mid-May. Young larvae are more susceptible to Bt, and coverage is better when aerial applications are made before leaves fully expand, so the spray can still penetrate the canopy. Optimal timing occurs as black cherry (Prunus serotina) and Vanhoutte Spirea (Spiraea x vanhouttei) are in full bloom, or just after full bloom of common lilac (Syringa vulgaris).

There is some concern that Bt sprays can prolong outbreaks by interfering with natural enemies. However, this does not seem to be the case. Research has shown that aerial applications of Bt have little overall impact on the effectiveness of gypsy moth parasites, predators, or disease organisms. Similarly, research has shown that Bt sprays have little effect on the natural cycles of gypsy moth, the populations of which tend to increase or decrease independent of whether they had been sprayed with Bt in previous year's. This suggests that goals and expectations of gypsy moth suppression programs should focus on protecting trees from defoliation during gypsy moth outbreaks, rather than eradication or long-term reduction of gypsy moth populations.

Environmental Impacts of Gypsy Moth Suppression Programs
Aerial applications of Bt are often opposed because of toxic effects on the caterpillar stage of nontarget butterflies and moths (Bt has no effect on other insects). Studies have confirmed that Bt sprays can decrease native butterfly and moth populations, but the effects documented so far are smaller than natural variation in the populations of these insects, caused by factors such as weather. It has also been suggested that Bt applications may impact predators of butterflies and moths by decreasing their food supply. However, studies published to date have found that Bt has little effect on predators such as spiders, birds, and bats.
gypsy moth outbreak defoliation
Gyspsy moth outbreaks can almost completely defoliate forests.

Potential effects of Bt on threatened and endangered butterflies and moths are of particular concern. The short residual activity of Bt in the field (about one week) limits the harmful effects to species that feed as caterpillars during mid-May. One such species is the Karner blue butterfly, which had become extinct in Ohio but has been reintroduced to its oak savanna habitat in northwestern Ohio. Habitats where such endangered, or sensitive, species are known to occur must be avoided when Bt or other insecticides are sprayed to suppress gypsy moth.

Aerial applications of diflubenzuron (Dimilin) are also used in gypsy moth suppression programs. Diflubenzuron is a growth-regulating insecticide that, when ingested, interferes with exoskeleton formation of immature insects during the molting process. Although diflubenzuron is considered more effective than Bt, it is not used as widely because it has a much greater impact on nontarget species. Diflubenzuron affects a much broader diversity of insects than does Bt, and its effects persist much longer, lasting throughout the growing season. Because it also impacts aquatic arthropods, it cannot be applied over open water, such as streams and ponds.

Environmental Benefits of Gypsy Moth Suppression
Gypsy moth is an exotic, invasive species that substantially disturbs forested ecosystems. The severe defoliation that results when gypsy moth outbreaks are
gypsy moth eggs
Gypsy moth egg masses are often visible, but many others are difficult to find.
left unchecked has environmental impacts that should be balanced against the impact of suppression programs on native Lepidoptera. Research has shown that gypsy moth defoliation can also decrease populations of native Lepidoptera, with the magnitude of effect similar to that of Bt sprays. Gypsy moths probably out-compete native caterpillars for available foliage. Furthermore, a recent study has shown that a gypsy moth parasite is responsible for substantial declines in populations of native silk moths. Populations of this generalist parasite increase during gypsy moth outbreaks.

Besides these impacts on Lepidoptera, many other direct and indirect effects of gypsy moth defoliation on natural ecosystems have been documented. Gypsy moth defoliation has been shown to increase the rate of nest predation of forest songbirds, possibly by increasing nest visibility. Defoliation of oaks also dramatically decreases acorn production, which can decrease numbers of small mammals including mice (key gypsy moth predators), chipmunks, and squirrels, as well as alter the foraging patterns of bear and deer. Browsing of deer on understory vegetation may increase in response to decreased acorn availability. Gypsy moth defoliation is also thought to be one reason red maple is replacing oak as a dominant species in some previously defoliated forests. Increased temperature and sunlight on the forest floor can damage shade-adapted plants and animals, and favor invasive plants. Defoliation also disrupts natural cycling of nutrients and can harm water quality by increasing nitrate content of forest streams.

Can Alternatives to Bt Applications Prevent Gypsy Moth Defoliation?
Methods such as collection and destruction of egg masses, use of sticky bands to prevent larvae from climbing trees, removal of larvae that congregate under burlap skirts placed around trunks of trees, and pheromone traps are often recommended as alternative approaches to managing gypsy moth. However, studies have shown repeatedly that these tactics are not capable of protecting trees from defoliation during outbreaks, even when used in combination.
gypsy moths gather in burlap
Gypsy moth larvae will collect under burlap bands placed on trees.

Collection and destruction of egg masses is ineffective because most egg masses are well hidden or inaccessible. Even thorough searches by experts detect only a portion of those present. Furthermore, newly hatched, "ballooning" larvae can reinfest trees after eggs are collected. Burlap bands wrapped around the lower trunk of trees can attract large numbers of gypsy moth larvae, which hide under them during the day, when they are not feeding. This tactic can be useful for detecting the presence of gypsy moth populations when populations are low and may be useful for protecting small, isolated trees from defoliation. However, research and experience has demonstrated that trunk banding is ineffective at preventing defoliation of even moderate sized trees. The overwhelming number of larvae that must be collected and disposed daily during outbreaks represents only a small portion of the total population feeding on the tree. Furthermore, a high proportion of larvae never leave the canopy and upper branches, especially during outbreaks when larvae may feed 24 hours a day.

The use of pheromone traps to decrease gypsy moth populations is sometimes recommended but is also futile. Only males are attracted to the traps, so trapping has no effect on the females that are responsible for changes in the population. Furthermore, the traps quickly become overwhelmed with trapped males, even when populations are very low. Pheromone traps are very useful for determining the presence or absence of gypsy moth populations and are used effectively in monitoring programs.

In some cases, application of gypsy moth sex pheromone over large areas can be used to suppress low gypsy moth populations. This is currently called spreading pheromone "flakes." Wide-spread application of pheromone (usually by aircraft) saturates the environment, preventing males from detecting pheromones produced by individual females. Mating disruption is most effective when gypsy moth populations are low but starting to increase. When populations are high, the day-flying males can easily locate mates visually. Because of cost and limited supply of the pheromone, mating disruption is generally utilized in environmentally sensitive areas and at the leading edge of the gypsy moth infestation, to delay establishment in new areas.

Summary
Gypsy moth outbreaks will continue to occur despite the effects of natural enemies, including Entomophaga, the effectiveness of which is highly dependent on weather. Bt applications are the safest, most effective way to prevent widespread defoliation of trees during outbreaks, but such applications can have negative effects on native butterflies and moths. However, gypsy moth defoliation also has detrimental impacts on forest ecosystems, including effects on native Lepidoptera. The environmental impact of gypsy moth defoliation should be balanced against that of Bt when deciding whether to spray. Research has demonstrated that alternative methods, such as trunk banding, pheromone traps, and collection and destruction of egg masses, are not capable of preventing defoliation during outbreaks. Commercially available insect biocontrol agents are also not effective.

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  Data Source: Ohio State University Extension. Articles and resource may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide and it is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used.