I had to do this paper as part of independent research for BIOL 1004 (Biology II) at Carleton this summer. As I’ve handed it in (two days ago), I am posting it here as well.
Department of Biology
Introductory Biology II
Summer Term 2006
Ecological Impact of Gypsy Moth
Date Due: 20060720
â€œThis paper is the sole work of the undersigned, does not contain unattributed material from any source and compiles with the Academic Regulations section 14.1-4 (Instructional Offences) of the Carleton University Calendar.â€ (Biology Department, 2006, p10).
Stanislav N. Vardomskiy
In North America, gypsy moth is a serious pest of agriculture and deciduous forests that causes significant economical and environmental damage.
Gypsy moth (Lymantria dispar) is an insect native of Asia and Europe with very few natural predators in North America (Chaplin III, 2000). Asian and European races of Lymantria dispar differ by size, flight characteristics and host preferences. Asian gypsy moth is larger then itâ€™s European counterpart and is known to prefer over 500 tree species. In addition, both genders of Asian gypsy moth are strong fliers, compared to only males of European gypsy moth (Humble and Stewart, 1994).
Until recently, most of attention to gypsy moth in North America centered around European gypsy moth, however in 1991 a race of Asian gypsy moth was discovered in Vancouver, BC and in the states of Washington, Oregon and Ohio (Humble and Stewart 1994 and APHIS 2003).
European Gypsy Moth
In late 1860s, Etienne Leopold Trouvelote, an amateur entomologist, imported a gypsy moth egg cluster from France in hopes of cross-breeding disease-resistant gypsy moth and local varieties. He cultured some of these eggs in the trees of his suburban Boston home, when some of the larvae escaped and infected nearby trees â€“ first on his street, and soon in the neighborhood of Boston. (Leibhold, 2003)
Trouvelote realized the significance of escaped larvae, and notified local entomologiests, however for close to 20 years problem was largely ignored (Leibhold, 2003). Gradually more and more trees in the vicinity got infected.
First outbreak of moth occurred on his street in 1882, just as he left the country, but at the time very little was done. First attempt at containment and eradication of gypsy moth larvae was organized by Massachusetts State Board of Agriculture in 1889. At the time efforts consisted of manual removal of egg clusters, application of early insecticide, and burning of infected trees. A lot of money and effort was spent, however infestation continued to spread. Eradication methods in Massachusetts were abandoned by 1900 (Leibhold, 2003).
In Canada European gypsy moth is well established in the provinces of Quebec and Ontario and threatens parts of New Brunswick and Nova Scotia (Humble and Stewart, 1994).
Asian Gypsy Moth
Asian race of gypsy moth was accidentally introduced to Vancouver in 1991, when larvae hatched on ships in harbor was blown ashore by the wind. Male moths were trapped, and application of insecticide Btk eradicated the problem. Currently egg masses are increasingly detected on the ships, and since 1991 infected ships have been banned from inshore areas during periods of egg hatch and larval development (Humble and Stewart, 1994).
Asian gypsy moth is not established in Canada, however egg masses have been intercepted in shipments as early as in 1911, and have been intercepted almost yearly since 1982 (Humble and Stewart, 1994). In United States individual infestations occurred in Washington and Oregon 1991 and in North Carolina in 1997. In 2000 Asian gypsy moth were again discovered in Portland, OR. In all cases infestations were eradicated through aggressive trapping and spraying (APHIS 2003).
Gypsy Moth Life Cycle
Life cycle of gypsy moths consists of four stages: eggs, larva, pupae and adult moths. Adult moths generally lay egg clusters on tree trunks and branches, however any sheltered location can be used. Egg clusters are laid in August and the embryos develop over the warm days of summer. In about a month larvae is fully formed, and ready to hatch, however, instead larvae shuts down metabolic activities, and goes into diapause, becoming insensitive to cold. In the spring, as the temperature increase, larvae inside the eggs becomes more and more active. In mid-May larvae chews through the egg shells, and emerges (Duvall, 2006)
Before commencing feeding, larvae spreads through the forest by a behavior called ballooning. The larvae climbs to the top of the tree on which it hatched, and proceeds to dangle in the air on a silk thread. At this point larvae is still very light, so when wind catches larvae and breaks the thread, larvae is carried on the wind. Silk thread and long body hairs slow larvaeâ€™s descent. Most larvae land within 100 meters of where whey hatched (Durvall 2006), however some travel as far as a kilometer away from the hatch site (Sharov 1997).
Once larvae lands, it proceeds to feed. Depending on sex, larvae will feed for five to six weeks. Females feed longer, in order to collect fat necessary for laying eggs. Approximately once a week larvae grow too big for itâ€™s exoskeleton, and molts. Molts separate the larval periods into stages called instars. In the first three instars larvae feeds during the day, however by fourth instar they start to feed at night and hide during the day in order to avoid predators (Duvall 2006). Approximately 90% of total leaf mass will be consumed by larvae in the last two instars (Herms and Shutlar 2000).
In five or six weeks, larva grows to the size of 4 to 6 cm. By mid-June – early July, larva reaches maturity, and starts looking for a safe place to pupulate. Once a safe spot is found, larva sheds itsâ€™ skin, and itâ€™s new skin hardens into a brown shell. In process larva can hide on vehicles and spread further during pupitation. Pupae is immobile during most of this stage, as itsâ€™ body is transformed into that of a winged insect. After one to two week pupation, adult moth breaks free of a pupal shell and emerges (Duvall, 2006).
Adult gypsy moth females are about 4 cm long, and are white with black stripe on their forewings. Females of European race can not fly, and will fall to the ground if disturbed, while Asian race females will fly away. Male gypsy moths are larger then females, have large feathery antennae, and a mottled grey and brown in color, giving them similarity to native moth species. Male gypsy moths search for females in late afternoons, that allows to distinguish them from native species that search for mates at night (Duvall, 2006).
In the adult stage gypsy moths can not feed, and have about 2 weeks in which to mate. Females release pheromones that assist males in finding them. Male searches for pheromone trace, and flies up wind until finds a suitable female. Once a male and female moths find each other and mate, female lays all her eggs in a single tear-dropped shape and camouflages them with itâ€™s own yellowish hair. Depending on how well female larvae fed in the last two instars, female can lay between 50 and 1000 eggs (Duvall, 2006).
In the larval stage of the lifecycle, gypsy moth consumes tree foliage. European race is known to favor approximately 300 plant species, while Asian race is known to consume foliate of approximately 500 plant species (Humble and Stewart, 1994). During the first three instars, gypsy moths prefer foliage of a limited selection of trees (apple, aspen, birch, larch, oak, willow, alder, hazel, etc), however once larvae gets to approximately 2 cm in size (third instar), it starts to consume foliage of many more trees, such as spruce, pine, chestnut and hemlock (Ravlin and Stein 2001).
As majority of foliage is consumed by larvae in the last two instars, very wide variety of trees can be affected.
Ravlin and Stein did work on tree classification that permits to statistically analyze forest composition, and predict the defoliation effects of an infestation. Generally forests that have a high composition of ash, balsam and Fraser fir, juniper, maple, mulberry, red cedar or sycamore are significantly less affected then forests that primarily consist of oak and birch (Ravlin and Stein, 2001).
Approximately once every 5 to 10 years a very severe infestation, termed outbreak occurs. In case of gypsy moth, early theories postulated that in low density infestation small mammal predators, such as deer mice, regulate the population, keeping equilibrium. At some point natural population of predators drops because of random failure in some other food source, and moth population rapidly jumps to a higher equilibrium level. As the density of moth population increases, various pathogens rapidly infect the population, causing the collapse of the outbreak. Current theories suggest that this is only part of a story, and involve induce-defence hypothesis, that postulates that decrease in available foliage causes a decrease in moth population (Stone 2004) â€“ in other words, moths consume all available food and starve out.
Furthermore, Jones demonstrated that while in the northeastern United States large population of the white-footed mice control outbreaks of gypsy moth, white-footed mice also spread Lyme disease, whereas small population of the mice decrease incidence of Lyme disease but allow gypsy moth to breed (Jones 1998). Relationships such as these make theoretic explanations of outbreaks extremely complicated.
Depending on the severity of infestation, up to 100% of the tree foliage can get destroyed. Normally a healthy tree would survive such an event, and generate a second generation of trees by end of July, however any strained tree would be further stressed. In turn, stressed trees are more susceptible to fungus and diseases, and do not grow as much as unaffected trees.
Establishment of gypsy moth in any new habitat can causes economical damage. Any lumber, tree nursery products or natural products leaving affected area could have trading restrictions applied to them. Affected forests grow slower, with higher incidence of tree death. As larvae eats leaves of fruit trees, blueberries, strawberries and other foodcrops, gypsy moth has potential to severely affect agriculture (BCgov 2006). Asian race of gypsy moths is less picky about their food, and consumes coniferous trees, such as larch (Humble and Stewart, 1994).
During outbreaks, gypsy moth caterpillars are considered to be a nuisance in residential areas of Eastern North America. In urban environments larvae can congregate on buildings, driveways and sidewalks, as they search for food. Caterpillar hairs, shed by larvae are allergens that cause hazards to human health. (BCgov 2006).
Containment and Control
Gypsy moth is an exotic invasive species in North America, and doesnâ€™t have as many natural controls in North America as it does in Europe or Asia. In North America natural predators of gypsy moth include birds, insects, and small mammals (Herms & Shetlar, 2000) with most important being shrews (Sorex spp), deer mice (Peromyscus maniculatus) (Leibhold 2003b) and white footed mice (Peromyscus leucopus) (Jones 1998). As most small mammals are generalists, there is no strong correlation between abundance of moths and abundance of small mammals (Leibhold 2003b).
Presence of hair on larvae makes that moth lifestage unattractive to most birds, but a few species, such as yellow-billed (Coccyzus americanus) (MSU 1997) and black-billed cuckoo (Coccyzus erythropthalmus) seem to enjoy eating larvae. Overall, in North America birds do not significantly contribute to the decline of gypsy moth population (Leibhold 2003b).
It is established that gypsy moth in North America can not be eradicated (Leibhold, 2003) so current efforts are concentrated on reduction of damage and on prevention of infestation (Diss 1998).
Damage reduction consists of silvicultural (change in tree planting and harvesting) control to make forests less habitable by the moth and minimize the damage, biological control to slow the growth of population and control outbreaks, killing the caterpillars and removal of egg masses (Diss 1998).
Prevention consists of inspection and quarantine of vehicles that might transport larvae (Humble and Stewart 1994), combined with monitoring for new infestations.
Mating pheromones of gypsy moth, disparlure ((7R,8S)-7,8-Epoxy-2-methyloctadecane and cis-7,8-Epoxy-2-methyloctadecane) were synthesized in 1970s, and since then many attempts were made to manage low-level infestations by disrupting mating habits. Disparlure was found to be effective only in low density infestations (Sharov et al. 2002), or as trap bait in order to check for presence of males (Humble and Stewart, 1994).
Over 20 species of insect predators and parasites have been released in wild in order to control population of gypsy moth (Leibhold 2003a) with various degrees of success.
Natural bacteria Bacillus thuringiensis var. kurstaki is the base of a commercial available insecticide Btk that is commonly used against gypsy moth infestations (Humble and Stewart, 1994). Unfortunately Btk is extremely sensitive to timing, and is only effective for a few days after being spread. In that time slot it must be consumed by feeding larva in order for it to be effective (KC 2006). Statistics gathered by Washington State Department of Agriculture indicate that Btk based insecticides are fallible, and possibly produce effects that are not better then disparlure (WSDA 2005).
Gypsy moth is most susceptible to nucleopolyhedrosis virus (NPV), more commonly known as the â€œwiltâ€. Infection happens once the larvae consumes foliage that is contaminated with viral bodies. Once inside the larvae, NPV invades through the gut wall, and rapidly reproduces in internal tissues, disintegrating internal organs and eventually causing rapture. Once host raptures, viral oclusion bodies spread, and infect other individuals (Leibhold 2003c).
NPV particles persist in the soil, and in low density gypsy moth populations, however with fewer hosts to infect, NPV causes little mortality. During moth outbreaks, NPV rapidly propagates, and inflicts heavy casualties on the larvae population. NPV is the most common cause of the collapse of the outbreaks.
Research is being performed on development of NPV into a biological pesticide. Currently limited qualities of this material, referred to as â€œGypchekâ€ are available for control of the outbreaks, however it is costly to produce, as manufacturing process currently requires moth larvae (Leibhold 2003c).
While total eradication of gypsy moth in North America is currently not possible, containment measures consisting of infestation prevention and damage reduction are slowing down gypsy moth proliferation (Diss 1998). Leibhold indicates that only about 25% of the potential habitat of gypsy moth have in fact been infected so far (Leibhold 1992, Leibhold 2003).
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