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|As with any pest, management strategies for gypsy moth are tied directly to the life cycle of the insect.
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Assessing Options for Managing Gypsy Moth
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Reference: Ohio State University Extension
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.
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.
Biological Control of Gypsy Moth
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
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
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
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?
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.
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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.|