LdMNPV and the Management of Gypsy Moths

Gypsy moth larvae consuming leaves

William Coville – Environmental Science

Julianne Foren – Animal Science

Catherine George – Horticultural Science

John Mazzone – Turf Grass Science and Managment

 

In the late 1860’s, a French scientist brought the gypsy moth to Massachusetts from Europe in the hopes of breeding disease-resistant genes into silkworms to improve and expand the silk industry (Liebhold, 2003). Due to his incompetence, a couple of his gypsy moth subjects made their way into the New England forest and found that they could live, breed, and thrive there. The carelessness of one scientist resulted in a gypsy moth invasion that persisted over the last hundred years and encompasses various ecosystems throughout the U.S. and Canada. Lymantria dispar dispar, known as the gypsy moth, is an invasive species that acts as a major pest of hardwood trees, particularly the dominant oak and aspen (Liebhold, 2003). As an example, a red oak that lies at the entrance of Quabbin Park in Belchertown, MA has been taken down due to it being mostly dead from gypsy moth defoliation (Miner, 2018). Iconic trees in parks around the country are not spared from the damage of gypsy moths and once enough damage sets in the trees are lost from the community. Not only does the gypsy moth cause an an aesthetic decline among these once beautiful hardwood trees, but they also play the role of the small beginning in a larger catalyst effect. They cause severe defoliation among the trees they feed on and cause harm to native species as well. One scientists economic greed and thoughtless actions have resulted in ecological destruction that has lasted and will continue to last well beyond his lifetime.

One of the greatest problems gypsy moths pose to U.S. ecosystems is defoliation. Defoliation is the removal or destruction of the grass parts or leaves of a plant and massive defoliation causes a decrease in production as well as the health of the plant (Oregon State University, n.d.). In 2016 alone, of the 20 states surveyed, over 1 million acres were defoliated by the gypsy moth as reported (USDA, 2018). Since 1924, it has been reported that in 20 U.S. states, a total of 96 million acres of hardwood forests have been defoliated (USDA, 2018). This extensive defoliation is not only unpleasing to the human eye, but it also leaves the host plant susceptible to other hardwood pests, diseases, and results in ultimate tree mortality (Liebhold, 2003). The constant loss of biomass through the tree’s leaves can weaken their defenses as carbohydrate reserves are depleted trying to replace what was lost (Waller, 2013). Meaning that defoliation can be a catalyst to other problems that lead to the death of trees.

Gypsy moth larvae feed on over 300 plant species during the summer, hindering the trees ability to create food (Iowa Department of Natural Resources, 2010). When Oak trees lose leaves early during the growing season, they will abort their acorns- resulting in less food for wildlife to eat in the fall and less oak regeneration (Iowa DNR, 2010). Acorns produced by Oaks are eaten by many birds and mammals including deer, squirrels, mice, rabbits, foxes, turkey, grouse, quail, woodpeckers, and many more (Iowa DNR, 2010). During the fall and winter, acorns make up about 54% of a deer’s yearly diet (Iowa DNR, 2010). When acorns are not available as a reliable food source, the deer and other animals may have to look elsewhere for food (Iowa DNR, 2010). Acorns are also an important energy source during the winter due to the high carbohydrate concentration (Gottschalk, n.d.). Since acorns makeup over 50% of the deer’s diet, they lose a very important energy source which could hinder survival if less acorns are available. The loss of food reserves is prevalent in the populations of preferred game, deer and turkeys, and negatively impacts hunters who may make their livelihood from such activities.

As a result of gypsy moth defoliation, oak trees experienced a loss of 7.1 m²/ha in 2008 (Fajvan et al., 2008). Young oak and hickory have also seen a steady decline in health (Morin & Liebhold., 2016). This results in an inability for forests to perform one of their most important ecological roles: serving as carbon sinks that remove carbon dioxide from the atmosphere (Otsu, Pla, Vayreda & Brotons, 2018). The amount of carbon able to be removed from the atmosphere due to forests expanding and growing, when compared to losses due to defoliation, is outweighed by a ratio of 5:1 (Potter, 1999). Trees not being able to remove as much carbon dioxide from the atmosphere can lead to an increase of carbon being left in the atmosphere, which poses other problems.

Another negative effect of gypsy moth defoliation of trees is that defoliation leads to a higher rate of tree infection by opportunistic pathogens. Opportunistic pathogens are defined as bacteria, viruses, or fungi that take advantage of an opportunity not normally available, such as a host with a weakened immune system (Faria, 2015). The gypsy moth feeds on leaves, leaving open wounds for which pathogens may readily enter the plant. Gypsy moths also have been shown to consume more plant material under drought-like conditions- adding more stress to an already drought stressed tree. For example, Amarilla root rot is a common root and butt rot pathogen that is a forager of weakened trees, killing only weakened or stressed hosts. Amarilla has been associated with the high mortality of Oak species stressed by gypsy moth defoliation (Dukes et al., 2009). Some trees may not ever be able to recover as a result of acquiring another infection on top of already being defoliated.

Defoliation also has a negative impact on birds since defoliated trees provide less canopy cover and expose nests. The exposed nests cause an increase of birds being preyed upon, or an increase in predation rates. The defoliation from the larvae typically affects the predation rate of the birds in that area, resulting in about a 20% increase to predation rates (Thurber et al., 1994). In moderate years of defoliation, it is responsible for a 10% decrease in canopy cover (Bell & Whitmore, 2000). Years that have higher abundances of gypsy moths could have higher percentages of decrease in canopy cover. Which in turn will lead to an increase in the predation rates of birds. More birds being preyed upon decreases their populations which leads to a decrease in biodiversity of the local bird populations. There is also a strong correlation seen between the decline of cavity-nesting bird populations and defoliation (Showalter & Whitmore, 2002). Cavity nesting birds include woodpeckers, swallows, wrens, and nuthatches. All of these species are important insectivores that play an important part in the insect control of the ecosystem (Scott et al., 2015). A decrease in these ecologically important species will ultimately lead to a rise in the number of insect pests seen in a forest.

Defoliation is not the only hardship gypsy moths impose on the ecosystem, the introduction of invasive species can lead to a decrease in biodiversity, or variety of species in an area. This is accomplished through competition between gypsy moths and the native caterpillars living in the area of outbreaks. Gypsy moths can increase the mortality rate of native caterpillars, such as the Northern Tiger Swallowtail. Their mortality increases to 100% when they feed on quaking aspen leaves painted with gypsy moth larvae fluids (Redman & Scriber, 2000). This increase in mortality rate can decrease the abundance of Northern Tiger Swallowtails. Another study in northern oak forests shows abundances of gypsy moths among native species average about 75% and in some areas can increase to 96% (Timms & Smith, 2011). Particularly there are decreases seen in White-Marked Tussock moth and forest tent caterpillars which are native species in the areas of this study. Due to gypsy moth’s preferred species of tree being oak, they compete with a number of other caterpillar species that feed primarily on oak. Which includes Crankworms, Linden loopers, Leafminer, Oakworms, and Webworms (Ellis and McCullough, 2001). The specific species mentioned here are just some of the caterpillars affected through competition with gypsy moths. Many other species in different areas of outbreaks are affected.

A variety of methods have been used to control the gypsy moth populations to lessen the damage they cause. Traditional pesticides could be used, but in recent times biological control methods have been more frequently employed. A bacteria known as BtK (short for its scientific name, Bacillus thuringiensis var kurstaki) that is non-toxic to mammals and birds has been used in a variety of readily available insecticide products (“Insecticides | Gypsy Moth in Virginia | Virginia Tech”, 2008). However, this method has one distinct problem: while highly targeted, the bacteria targets other organisms besides the gypsy moth (Beyond Pesticides, n.d.). This means it will impact other species of moth and butterfly. Our proposal is the use of the of a virus known as LdMNPV (short for Lymantria dispar multicapsid nuclear polyhedrosis virus). LdMNPV is a baculovirus, which is a family of viruses that target invertebrates. Even though both BtK and LdMNPV both infectious agents, LdMNPV has one distinct advantage over BtK: it is known to exclusively target gypsy moth larvae. It has been tested and found non-infectious in the cells of mammals, birds, reptiles, and even other insects (Durkin, 2004). This makes LdMNPV more ecologically responsible, as other species under order Lepidoptera will not be affected by the application of LdMNPV.
LdMNPV has been harvested and applied before in the form of a product called Gypchek. It is a powder made from infected gypsy moth larvae that have been grown in lab and is often mixed with water and molasses and deployed aerially (“Gypchek Fact Sheet,” 2018). When applied, Gypchek is incredibly lethal. Mortality rates in dense populations of gypsy moth larvae were found to be as high as 90% (USDA Forest Service, “Gypchek,” 2018). Though this practice has not been adopted as a widespread method as of yet, implementing Gypchek or a similar product has the potential to eliminate as much as 90% of gypsy moth populations in the areas where it was used. This would yield incredible results and in the end would greatly decrease the damage done by high abundances of gypsy moths.

Some may argue that given the short reproductive cycles of insects, that a resistance could build in the community over time such that the virus eventually becomes ineffective. This is true in the case of bacteria, that have many generations in a matter of minutes that allow them to adapt to targeted antibiotics (“Growth of Bacterial Populations”, 2018). However, the gypsy moth is not so fortunate. The life cycle of the gypsy moth spans the entirety of the summer, and they only have one generation in a year (“Gypsy Moth Biology & Life Cycle”, 2018). With the incredibly high mortality rate of Gypchek and the difficulty of survivors with an adaptation in finding a mate with lower populations, the development of a resistant form of gypsy moth larvae is unlikely at best. Resistance rising from a mutation in the population is also very unlikely since mutation rates of DNA is extremely low, being about 10⁻¹⁰/base pair per cell per gene. (Balin and Cascalho, 2009). Meaning that the chances of a resistant gene emerging in one generation is very slim. Since gypsy moths only have one generation per year, it would take many years before a mutation could cause resistance in the population. By then, ideally, the populations would already be under control and not over abundant.
The economic loss to the forest industry is far greater than the cost of producing and distributing the LdMNPV virus. In Iowa alone it is predicted that the wood products businesses loses over $22 million annually due to the gypsy moth’s activity in their forests (Iowa DNR, 2010). That is only one state’s prediction out of the many affected by gypsy moth destruction. The U.S. Department of Agriculture estimates that in Washington state the economic losses caused by the gypsy moth has averaged $30 million annually over the past 20 years, most of these losses caused by quarantines imposed on timber and other wood products (Washington State Department of Agriculture, 2011). The numbers not taken into account include the loss of income from recreational parks, loss of income from fish and wildlife associations, and the loss of game for hunters. It is estimated that foliage feeders, including gypsy moths, cause $868 million in damages in the U.S. each year- primarily to homeowners (Aukema et al., 2011). Among homeowner expenditures, the cost of tree removal, replacement, and treatment outweighed the costs of federal treatment programs (Aukema et al., 2011).

Gypchek, on the other hand, is much less costly. When treating with Gypchek, it is estimated that the treatment costs roughly $10 per acre treated (“Gypsy Moth Outbreak in Allegany State Park”, 2018). When estimating costs to treat the Allegany State Park in 2013, it was thought that treating the entire park could range in cost between $60,000 and $190,000 (“Gypsy Moth Outbreak in Allegany State Park”, 2018). However, there is potential that these costs could decrease over time. Studies have found that in-vitro strains of LdMNPV produced in labs had similar mortality rates when compared to the in-vivo versions that are typically prepared from crushed gypsy moth larvae (Podgwaite, 2013). Producing the virus in-vitro would mean to produce the strain in a lab and this would result in a lower cost of production as compared to in-vivo strains that are produced using the living organism. It would be more beneficial to adopt better control methods, like using LdMNPV, to prevent the millions of dollars gypsy moths do in damage per year.

A targeted virus has been used before in order to successfully reduce the population of an invasive species. On Marion Island, in early 1949, feral domestic cats had been introduced in order to control feral house mice. In 1977, the population of domestic cats had already reached 3409 from its original 5 cats and it was decided that the growing number of cats were having a detrimental effect on the populations of native species of birds due to their predatory tendencies (Van Rensburg et al., 1987). In order to combat this problem, several cats were infected with the feline panleucopaenia virus (Bester et al., 2002). This virus is said to be very host-specific, highly infectious and results in a high mortality rate among cats infected (Bester et al., 2002). Over the span of 18 months, the virus successfully reduces the feral domestic cat population be 54% (Bester et al., 2002). Litter size among the feral cats decreased as a result of the virus and the age structure changed drastically to show a large reduction in subadult numbers (Van Rensburg et al., 1987). Though the virus alone did not completely eradicate the cat population, it did decrease the population by over half in a year and a half.

Given the weight of the evidence on the detriments of the gypsy moth invasion in the U.S., there appears to be a moderate scientific consensus that gypsy moths will continue to pose a significant threat. By defoliating, reducing tree growth, and causing ultimate tree mortality; the gypsy moth has some direct and many cascading effects, including a change in water and air quality, nutrient cycling, climate regulation and recreation and cultural services (Aukema et al., 2011). Though our proposal to release a virus more widespread among the gypsy moth population may not completely eradicate the species from U.S. forests, it promises a noticeable decline. This viral proposal coupled with the methods of control that are already being implemented, such as traps and mating disturbance, promises a satisfactory result.

 

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