Invasive Exotic Species- Lymantria dispar.

                                    Invasive Exotic Species- Lymantria dispar.

Author: Carl Stuart.

Introduction.

The European gypsy moth (Lymantria dispar dispar) is a univoltine folivore thermophilic moth whose larvae devour leaves of over 400 species of hardwood. It undergoes complete metamorphosis; hence, it is classified in the order Lepidoptera. It belongs to the family Erebidae. It is indigenous to the temperate Eurasian region. It was introduced to the USA (United States of America) in 1869 by Proffessor Étienne Léopold Trouvelot who wanted to crossbreed them with silk worms. They later escaped from his residence; and by 1869 they had defoliated all trees within the locality, and had also infested the entire surrounding residential areas. By 1889, there was a full scale outbreak of Lymantria dispar dispar, and, entire portions of the eastern deciduous forests were completely defoliated thereby resulting in diseases and death among animals, and severe zoonotic infections afflicting the local populaces (McDowell 489).

The larvae of the moth has a small mass and it spins silken threads which project from the larvae, thereby increasing its surface area and allowing it to levitate and be dispersed by wind. Hence, a storm can disperse the larvae over an extensive land mass. This is a natural mode of dispersion. Artificial dispersion occurs when its eggs attach and/or adsorb themselves onto inanimate objects, such as firewood, and they are transported incidentally when such objects are transported. Control strategies must factor in both artificial and natural modes of dispersal. Also, the larvae are able to ascend and descend on tree depending on the light intensity (Weinar 892).

L.dispar is currently concentrated in the following five states: Florida, Washington, Utah, California and Oregon. However, dispersion is likely to spread the pests to the surrounding regions and even to Canada. The process of biological invasion consists of four main stages which are explained hereafter. The first stage is the introduction of L.dispar into a new location or ecosystem. This stage is facilitated by hydrologic, anthropogenic and atmospheric transport mechanisms. The second stage is the establishment of the introduced species in the environment. The process of establishment must be successful for the invasion process to progress to the third stage. The third stage is marked by spread and widespread dispersion of the species within the region. The fourth stage is the impact or effect stage during which L.dispar causes both ecological and economic damages. L.dispar outbreaks last for a period of three years before collapsing. The collapse results from predation, starvation (due to widespread defoliation of trees), viral and fungal diseases, hostile climatic conditions and parasitic attacks (Weinar 905).

The favored overstory species for the moth are the following plants: Basswood, Apple trees, Larch, Boxelder, Quacking Aspen, Sweetgum, Bigtooth Aspen, Willow trees, oaks and American Mountain Ash. Hawthorn, Alder, Sumac, Serviceberry, Hornbeam and Hazelnut are the favored understory species. In the absence of the favored tree species, the moth will feed on American beech, American elm, sugar maple and red maple. The following tree species are considered resistant to the moth: Eastern white pine, Eastern hemlock and Yellow birch. The impacts that L.dispar have on the flora is dependent on the following factors: amount of foliage consumed, number of successive defoliations, status of hosts before defoliation, host species and quantity of soil moisture present. Defoliation of 50% or less of the crown of the host tree leads to a slightly decreased radial growth; while defoliation of over 50% of the crown leads to a subsequent follow-up refoliage after a period of about 4-5 months (McDowell 517).

Problems Associated with the Invasive Species.

Defoliation caused by L.dispar does not kill the host per se, but it does compromise the salubrious integrity and functional status of the tree, thus leading to infestation by secondary pests such as armillaria root rot and twolined chestnut borer which ultimately kills the host. Moreover extensive defoliation of forests that causes over 45% tree mortality leads to a change in the landscape, thus diminishing the overall scenic beauty. However, defoliation that causes the death of about a third of the forest tree population has been found to increase scenic beauty. The reason for this peculiar observation is that the tree mortality causes an increase in sunlight penetration to the underlying flowering species, such as mountain laurel (Kalmia latifolia) in the forest; and, this result in a growth spurt among these plants (Fabel 195).

 Defoliation reduces the quality of the lumber, and these low-quality woods do fetch a low-price in the market thus negatively impacting on the financial status of the lumbers who harvest trees from such forests. Studies done have shown that there has been a significant decrease in lumber activity after an area has been infested with L.dispar (Fabel 198).

L.dispar infestation of a particular area affects the people living in that area in several ways as outlined hereafter. To start with, there is extensive infestation of human habitation by caterpillars. Secondly, there is copious existence of frass within the area. Removal of the pests from the area and subsequent cleaning activity do consume a lot of time and financial resources. Infestation of commercial propetirs by the caterpillars causes the property to lose value. Thirdly, the extensive tree defoliation causes leaf fragmentation and barren trees, thus leading to reduced aesthetic quality of the area. This reduces the commercial appeal of the area, thus necessitating the need for beautification. Beautification of the area will require commitment of financial resources. Finally, the hairs of L.dispar can act as allergens and hence it causes allergic reactions (manifested as asthma, eye soreness and skin rashes) in the exposed population. Increased health expenditure due to the need to treat these allergic reactions leaves some of the families economically drained (Fabel 209).

Extensive forest defoliation by L.dispar adversely impacts the quantity and quality of water within the region. This results from transpiration losses and reduced interception caused by diminished forest canopies. Reduced interception has three main effects: increased soil moisture, increased stream flow and increased nutrient leaching. Nutrient transfer during the defoliation process leads to an increase in the soil nutrient content, hence leading to a rapid growth of the vegetation covering the ground. Some of this vegetation is made up of weed, and weed overgrowth and dispersion of its seeds to viable commercial farms has led to huge economic losses to the farmers (Fabel 217).

Life history of L.dispar.

The life history of L.dispar is described hereafter. There are four phases in its life history: eggs, larvae (also called caterpillar), pupa (also called cocoon) and moth phase. The entire life history occurs within a period of a year. Eggs attached to stationary masses, such as trees and rocks can overwinter for a period of nine months. A single egg mass measure about 4cm by 2cm; and smaller masses indicate a declining moth population. There are about 1000 eggs covered by setae (which are derived from the abdominal contents of the female L.dispar) within the egg mass. The egg stage is characterized by two stages: embryonation and dormancy (a period of diapauses in development mediated by the prevailing ambient temperatures) (Maeda 489).

Coincidentally, the eggs hatch and release larvae during the same time period that leaves bud in the trees. The larva feeds on tree leaves for a period of 6-8 weeks. The larva is identified by the following three prominent features: hairy body, 5 pairs of raised blue spots and 6 pairs of raised red spots on the back. The larva matures into an adult through a series of mottling which are separated by an intervening instar period. A discernable increase in the size of the larva occurs during each stage of molting. Male have 5 instar periods while females have 6 instar periods prior to pupation (Maeda 505).

The larvae live within the crown of the host trees during the first three instars. During this period, they perforate the leaves as they feed, and the feeding commences on the outer edges of the leaves and then gradually progresses towards the center. The tree is extensively defoliated during the fourth instar stage when the high population of larvae consumes the leaves continuously during the day and night. About 85% of total leaf consumption occurs during the final instar stage, and this is the period when complete defoliation occurs (Maeda 511).

The pupal stage lasts for a period of 10-14 days where the cocoon encases itself with dark-brown cases. Upon completion of the pupal stage, the moth emerges with grayish-brown wings, and the female moth immediately emits pheromones to attract the male; and they mate and produce eggs and the cycle begins once again (Maeda 517).

Control Mechanisms.

Due to the extensive damage caused by L.dispar, there is a need to control and manage its spread. The five main agents that are used to control the populations of L.dispar are the natural control agents, natural predators, chemical control agents, laboratory-reared populations and silviculture. An emphasis will be placed on the use of Bacillus thuringiensis to control these pests (Jensen 653).

Natural control agents (also called biological agents) are natural enemies of L.dispar dispar. They include NPV (Nucleopolyhedrosis virus), bacteria, fungi and other entomopathogens which cause highly morbid infections in L.dispar species. NPV are known to wipe out entire populations of L.dispar (Jensen 655).

The bacterium Bacillus thuringiensis is known to cause lethal infections in organisms belonging to the order Lepidoptera; and it has thus been used as a microbial insect control. The proteinaceous crystalline inclusions produced during the process of sporulation in the bacterium have been shown to have insecticidal actions. The crystalline inclusions is composed of three highly toxic chemicals: δ-endotoxins , crystal proteins and cry proteins. The crystal protein exhibit high specificity for plant pests such as L.dispar, hence, they are safe to use in agricultural settings. Its mode of action is outlined hereafter. The crystal proteins is ingested by the larvae, and the toxin is activated in the gut by enzymatic cleavage(using the intestinal enzymes) to yield the active form of the toxin which then polymerizes to form pores in the cell membranes of cells lining the intestines; thus leading to osmotic cell lyses. The main advantages of using the crystal proteins to destroy L.dispar are: the crystal protein is highly lipid soluble, and hence it is absorbed by the external surface of the pest; it is lethal to the pest, and the protein can be purified to pure concentrated aerosol forms that can be used in insect sprays. The main disadvantages are outlined hereafter. First of all, the protein cannot be sprayed equally to all parts of the pest, hence leading to reduced effective absorption of the protein by the larvae. Secondly, it is impossible to deliver to it to L.dispar burrowed inside plant tissues. Finally, it is rapidly inactivated by ultraviolet light (Jensen 669).

Chemical control agents are pesticides which are used in ground and aerial applications.  The pesticides include: orthene, sevin(whose active ingredient is carbamyl) and difluenzuron. They are stomach and contact poisons. Natural predation of L.dispar by predators such as Peromyscus leucopus and Calosoma sycophanta can control the pest population. Unfortunately, these predators are opportunistic predators and only feed on L.dispar when no other pray is available. Laboratory-reared populations of L.dispar are sterile and their autocidal nature suppresses the pest population (Jensen 678).

Conclusion.

Population outbreaks of L.dispar are a recurrent phenomenon and it is due to unavailability of an effective control strategy that can eradicate the pest. Several control strategies such as use of chemical pesticides, natural predation and laboratory-reared populations are used to kill off the pests. However, these strategies are ineffective because of their inability to adequately suppress L.dispar populations. Currently, large-scale application of Bacillus thuringiensis has the highest success rate in suppressing L.dispar populations. It is also the safest and effective pest control strategy. Hence, it is recommended that the Bacillus thuringiensis technique be constantly applied and reassessed. Moreover, it is recommended that the public be educated on how to utilize the Bacillus thuringiensis technique in eradication of L.disapar pests.

Works cited.

Weiner, Simon. "The dispersion and dispersal of L. dispar." Applied Environmental

            Microbiology 89.4 (2011): 887-913. Print.

McDowell, Richard. “The Gypsy Moth: A Gross Overview.” Journal of Entomology 45.8

            (2011): 486-523. Print.

Fabel, Sara. “Effects of Lymantria dispar, the Gypsy Moth.” Restoration and Reclamation

            Review 77.8 (2011): 189-231. Print.

Maeda, Silas. “The Life History of Lymantria dispar” Journal of Invertebrate Pathology 87.9

            (2011): 542-588. Print.

Jensen, Gilbert. “New Methods of Controlling Lymantria dispar population” Biotechniques 45.5

            (2011): 648-678. Print.