Integrated Pest Management Manual
BIOLOGY AND IDENTIFICATION OF THE GYPSY MOTH
Lymantria dispar (L). The adult female moth is dirty-to- creamy white, with dark bands across the forewings. Adult females have a wingspan of about 2" but can only fly short distances. The female's body is stout and densely covered with hairs, and her antennae appear thread-like. The male is much darker and smaller than the female; the wings are dark brown with black bands across the forewings. The wingspan is about 1/4", and the antennae are feathery. The abdomen is narrower than the female's.
Eggs are globular, whitish, and about 1/32" in diameter. They are laid in oval masses of 75-1,000 (averaging 400-500) and are covered with buff-colored hairs from the female's abdomen. Egg masses may be 1/2"-2" long, depending on their shape.
Newly-hatched larvae are buff colored but turn black within four hours after emergence. Younger caterpillars (first to fourth instars) are brown to black in color with long body hairs. Later instars are black with 11 pairs of colored tubercles, or bumps, along the top surface. The front five pairs are blue, and the rear six pairs are red. Each tubercle is topped by a tuft of yellow or brown hairs, which may be up to half a body-length long. A yellow line runs along the top surface from the head to the last body segment. In the fourth through sixth instars, the dark-colored head has additional yellow lines. The true legs are dark red.
Pupae are teardrop-shaped, chocolate to dark red-brown in color, and rounded in the front and tapered at the rear. Male pupae are 3/4"-1/2" long, while female pupae may be up to 2 1/2" long. A few hairs may occur on the head and each abdominal segment. Each pupa is attached to the substrate by a few strands of silk.
See McManus and Zerillo (1978) for a photographic guide to all life stages of the gypsy moth.
The gypsy moth is an exotic species that was accidentally introduced into Massachusetts in 1869. Since then, it has spread west to Ohio, south to North Carolina, and north to Montreal, Canada. An isolated population has also become established in central Michigan. Scattered infestations occur from time to time in states outside of the generally-infested area described above, but usually these are quickly detected and eradicated.
A recent analysis (Liebhold et al. 1991) indicated that the spread of the area generally infested by the gypsy moth ranged from 1/2-3 miles/year during 1900-1965. Since 1965, the rate of spread has been 3 1/2 miles/year in areas with a January mean temperature of less than 44F, and 10 miles/year in areas with a January mean temperature greater than this. Based on this analysis, the gypsy moth is expected to spread west to Wisconsin and Iowa and south to Georgia by the year 2015.
Temperate and boreal deciduous forests are the favored habitats of the gypsy moth. Defoliation occurs most frequently in forests on dry ridges and steep slopes that have shallow soils, and on sandy plains that have deep, excessively-drained soils. Outbreaks also occur frequently along interfaces between forests and urban areas (Houston and Valentine 1985). High population densities (or transport as a result of human activities) may result in migration to nearby or distant softwood forest, urban, or agricultural environments, all of which may support gypsy moth populations on available plant foliage.
The leaves of close to 500 species of trees and other plants can be eaten by gypsy moth larvae (adults do not have fully- developed mouthparts and therefore do not feed). Table 1 provides some information on host preference based on laboratory and field observations.
Actually, host preference and suitability is more complicated than implied by Table 1, as some tree species are not suitable hosts for first instars gypsy moths, but are good hosts for later instars. This is the case for many coniferous tree species. Also, some hosts are suitable for only a short period of time, after which they undergo physiological changes that reduce their suitability. For instance, beech is suitable for young larvae for less than one week (Raupp et al. 1988).
Gypsy moths attack trees under stress (e.g., from drought or natural or man-made disturbance) more readily than healthy trees. The presence of bark flaps and deep-bark fissures, which provide hiding places for larvae, are considered important in determining susceptibility of forest stands to gypsy moths.
Table 1. Host Plant Preferences of the Gypsy Moth Relative to Red Oak. (Modified from Montgomery and Wallner ).
|Laboratory rearing (a)||Defoliation level (b)||Defoliation level (c)|
(a) ++, favored host; + acceptable; --, avoided; Massachusetts (Mosher 1915). (b) 190 plots surveyed for 20 or 30 years in New England (Campbell and Sloan 1977). (c) 575 plots surveyed I year in Pennsylvania (Gansner and Herrick 1985). (d) values are the ratio of average defoliation of the indicated species to average defoliation of red oak.
Gypsy moths produce only one generation each year. Adults emerge from pupae during June in the southern parts of their range and July-August in the northern parts of its range. Emergence is accelerated under extremely high-density conditions. Males usually appear one to two days prior to females and fly in zig-zag or (less commonly) straight patterns. Vertical objects such as tree trunks where females are most likely to be found attract the males. Most males will fly less than 1/2 mile (usually fewer than 650') from their site of emergence. Females do not fly.
Several hours after emerging, females release a sex pheromone in bursts from abdominal glands. This chemical attracts males, who follow the scent upwind to locate the female and begin mating. Mating may last up to 1/2 hour, and females begin depositing eggs within 24 hours (Giebultowicz et al. 1991). Multiple mating may be common among males, but is probably rare among females, since the release of pheromone is inhibited by mating. Adult moths live about one week.
Generally, gypsy moths lay their egg masses are found on tree trunks and the undersides of branches, in crevices, under loose bark, and under or on rocks, tree stumps, foliage, or vehicles. The egg stage lasts for eight to nine months. Hairs from the female's abdomen surround the eggs, providing some protection from winter temperatures and natural enemies. Larval development is completed inside the eggs about a month after laying, but the larvae enter diapause and do not emerge until the following spring. Egg hatch usually begins at about the same time that red oak buds open.
Most larvae will hatch from an egg mass within a week, but the hatch period may be up to a month in egg masses in cool, shaded, or high-altitude areas. Newly-hatched larvae are about 1/8" long and remain near their egg mass if the weather is rainy or if temperatures are below 45oF. Once they have left the egg masses, larvae are attracted to light and move upwards, spinning a thread of silk, until they reach the top of the tree or other object on which they hatched. Under some conditions, they may spin down on silk threads. If the wind is strong enough, the threads may break and carry the larvae up to 650' within the forest canopy. Rarely, larvae may disperse up to 12 miles if they are carried out of the canopy by updrafts (Montgomery and Wallner 1988, Taylor and Reling 1986).
Larvae feed first on new leaves. When not feeding, the young larvae stay on the undersides of leaves, where they form a silk mat on the leaf surface for attachment. Molting occurs at intervals of about one week, which allows the larvae to grow in size. Males usually undergo four molts and females usually undergo five, but as many as nine have been recorded. After the third molt, when population density is low to moderate, larval behavior changes dramatically. Rather than remain always in the canopy, larvae leave the foliage during daylight hours and seek hiding places on the boles of trees or on the ground. Under high-density conditions, even large larvae remain in the canopy during the day.
At the end of the larval period, each larva seeks a pupation site, surrounds itself with a sparse silk net, rests for one to two days, and then becomes a pupa. The pupa breaks out of the larval cuticle, turns dark brown, and remains in its silk net for about two weeks. When development is complete, the newly-formed adult breaks out of the pupal skin, expands its wings over a period of several hours, and begins its adult life.
Gypsy moth populations exist in four distinct phases
(Elkinton and Liebhold 1990). The innocuous phase is characterized by very low population levels. Gypsy moth life stages are often difficult to find during this phase, which may persist for several years. The release phase usually takes place over one to two years and can result in population density increases of several orders of magnitude. The outbreak phase is characterized by populations high enough to cause noticeable tree defoliation. Outbreaks are rarely sustained for more than one to two years, after which high levels of mortality, primarily from starvation and disease, bring about a rapid population crash. This is the decline phase. These population changes often occur synchronously over wide geographical regions. However, there is little evidence that gypsy moth population outbreaks occur in regularly spaced cycles in North America (Elkinton and Liebhold 1990).
Responses to Environmental Factors
Temperature: Exposure of eggs to temperatures of less than - 45oF causes high mortality. Exposure of larvae to freezing temperatures may be lethal. Larval development is accelerated up to one to two weeks under outbreak conditions, probably as a result of behavioral changes which lead to greater exposure to higher temperatures (Elkinton and Liebhold 1990).
Moisture: Heavy rainfall at hatch may result in drowning of larvae. Rainy weather during the first larval instar can delay migration and cause larvae to congregate on the undersides of leaves. The duration of this instar may increase under these conditions. Extended congregation may stress larvae and increase their susceptibility to nucleopolyhedrosis virus (also known as "wilt"). Rainfall and moisture appear to increase the transmission of the gypsy moth fungus Entomophaga maimaiga (Weseloh and Andreadis 1992).
Light: Gypsy moth larvae are attracted to light just after hatch, leading them to move upward to sites from which they can be transported by wind (McManus 1973). Young larvae (instars one through three) remain on foliage during the day, while older larvae alter this behavior, resting away from the canopy during the day and returning to feed at night. Adult emergence and male sperm release are also triggered by daily light/dark cycles (Giebultowicz et al. 1990).
Wind: Larvae disperse mainly by wind. Newly-hatched larvae trail silk as they climb to treetops or the upper surface of the objects on which they hatch. These larvae are most active during the daytime, when winds are usually strongest. When they encounter wind, they arch their bodies (to catch the wind) and extrude a silk thread which may act as a balloon or parachute. In addition, first instar larvae are covered with comparatively long hairs, which increase their buoyancy in air.
Gypsy moth feeding has been shown to decrease the nutritional value and increase the levels of toxic chemicals in the remaining foliage (Montgomery and Wallner 1988). As a result, larvae grow more slowly and gain less weight on defoliated oak trees. In some cases, these changes may contribute to the decline of high populations (Schultz and Baldwin 1982).
Under outbreak conditions, development time is reduced (by up to two weeks), sex ratios become male-biased, and smaller adults which lay fewer eggs are produced (Elkinton and Liebhold 1990). Older larvae at innocuous population densities feed in the canopy only at night and seek protected resting places during the day. Under outbreak conditions, late instars remain in the canopy and feed intermittently throughout the day and night; however, they appear to consume no more foliage than larvae from innocuous densities.
Impact of the Gypsy Moth
The gypsy moth is one of the most destructive defoliators of hard and softwood trees. Tree mortality resulting from gypsy moth defoliation is highly dependent on the interaction between tree species, tree health, environmental stresses, and the severity of defoliation. Mortality to overstory oak trees subjected to gypsy moth defoliation in Pennsylvania ranged from 13% for trees with good initial crown condition to 35% with poor initial crown condition (Herrick and Gansner 1987). Mortality averaged 67% for understory oaks. Over 84% mortality of white oaks following defoliation was recorded in a New Jersey forest (Kegg 1973). In addition to aesthetic problems and reductions of timber stand value due to defoliation, forests suffering gypsy moth attack may suffer increased risks of fires due to canopy reduction and accelerated drying of litter. Effects of defoliation on watershed output and water quality are unclear at present (Corbett and Lynch 1987). In recreation areas, unsightly defoliated areas and wandering larvae can result in decreased visitor use and revenues (Goebl 1987).
Defoliation of forest trees can lead to increased susceptibility to other pest damage, most frequently invasion by the shoestring fungus, Armillaria mellea, and the twolined chestnut borer, Agrilus bilineatus, and alteration of ecological succession at affected sites (Houston 1981). The long-term effects of tree defoliation and mortality on the forest ecosystem are not known.
Asian Gypsy Moth
The currently established North American population of gypsy moths was introduced into Massachusetts from France in 1869. Until recently, there was no evidence of subsequent introductions. In 1991, gypsy moth egg masses on a Soviet ship docked in Vancouver, British Columbia, were found to be hatching. Because it was feared that larvae may have blown onshore, steps were taken to detect and identify new gypsy moth introductions into northwestern North America. During the summer and fall of 1991, asian gypsy moth adults were found in Portland, OR, and Tacoma, WA, in the United States, and in Vancouver, British Columbia in Canada. A mitochondrial DNA sequencing technique is presently used to distinguish the asian gypsy moths from the North American gypsy moths. Eradication efforts and extensive delimitation trapping programs were initiated in 1991 in all three of these locations.
The asian gypsy moth is similar in appearance to the North American gypsy moth, except that the asian larvae vary more in color. Asian females, unlike flightless North American females, are strong fliers (>20 miles). Lights attract asian females, and they lay their eggs on foliage and on objects near lights, in addition to tree trunks and other objects. In its native range, the asian gypsy moth feeds on at least 600 plant species and appears to thrive better on marginal hosts than the North American gypsy moth.
MONITORING AND THRESHOLDS FOR GYPSY MOTHS
Several methods are available for monitoring gypsy moth populations. The choice of method should be based on the population level suspected, location of sampling site in relation to the established United States infestation area, and resources available. The U.S. Forest Service currently provides gypsy moth survey assistance to any federal agency on request, and should be consulted if you wish to have a survey conducted.
Adult male trapping: These techniques involve the use of special traps baited with a synthetic form of the sex pheromone produced by receptive female gypsy moths. The trap currently used for gypsy moth surveys by the U.S. Forest Service and the USDA Animal and Plant Health Inspection Service (APHIS) are fully described by Schwalbe (1979). Although several variations of the trap design are manufactured, the USDA-approved traps can be obtained from your regional U.S. Forest Service office.
Pheromone traps should be placed before male moths begin flying (see Life Cycle section, above). Schwalbe (1979) describes the use of pheromone traps to detect low gypsy moth populations (detection survey) and to define specific areas of infestation (delimiting survey). An effective technique only for relatively low populations, pheromone trapping is recommended for use in areas outside (or on the edges of) established infestations.
The interpretation of pheromone trapping results is subjective; no reliable relationships between numbers of trapped males and gypsy moth population density have yet been found. Currently, for detection surveys, APHIS recommends placing pheromone traps at a density of one trap per 1 to 4 square miles and at frequencies of every two or four years, depending on the potential for accidental introductions to occur in a particular area (Anonymous 1990). When moths are captured during a detection survey, a delimiting survey may be conducted in the vicinity of the trap catches. In delimiting surveys, traps are deployed at densities of 16-36 traps per square mile over areas of from 1 to 4 square miles. The pattern of trap catches can be used to estimate the approximate area of infestation.
Larval trapping: The collection of gypsy moth larvae under burlap bands, while not useful in quantifying population density, can serve as another early indicator of low (e.g., recently established) but building populations. The most convenient method involves tying a 12"-wide burlap band around the trunk of each tree to be monitored so that the top 6" of the band can be pulled down over the bottom, making a shaded flap in which larvae will hide during daylight hours. Bands should be monitored two times each week and any trapped larvae should be destroyed. The presence of gypsy moth larvae in such traps indicates that a population may be developing in the vicinity of the trap site and that other survey methods should be used to determine whether treatment is required. Tar-paper wrappings and plastic tree flaps can be used instead of burlap.
Egg mass counting: Several methods have been developed for determining the number of gypsy moth egg masses in an infested area. Egg mass counts can be done from the time of oviposition (usually June-August) until egg hatch the following April or May. Counts are easier and probably more accurate, after the leaves have dropped from deciduous trees. The walks generally follow an "M"-shaped pattern through the area to be sampled, which helps to eliminate an edge effect. In forest situations, edge trees have found to have 2.4 times more egg masses than interior trees (Bellinger et al. 1989). Methods currently in use include:
Threshold walk: An observer walks through the area to be monitored, counting all new (current season) egg masses. The walk ends when the count reaches a predetermined number (see Threshold/Action population levels, below). This method gives no approximation of the actual gypsy moth density in an area, but it is easily done, and in areas of high gypsy moth density it may be useful in making a treat/no-treat decision using accepted threshold values.
Five-minute walk: Two observers walk through the area to be monitored for a five-minute period; each counts every detectable new egg mass. The average of the two counts is calculated and converted to an approximate number of egg masses per acre by the following equation:
estimated number of egg masses per acre = (average number of egg masses observed x 20) + 15
(Schneeberger 1987). The estimated number of egg masses/acre can be compared to established threshold levels to determine whether treatment is necessary. A recent analysis of this method
(Liebhold et al. 1991) recommends against its use because density estimates vary too much among observers and because it is generally too imprecise.
Intensive search. This method is used for very small populations (i.e., no evident defoliation, but with multiple adult male catches in pheromone traps) and simply involves examination for egg masses on all surfaces in the vicinity of traps with trapped males, including under bark flaps, on rocks, and in tree holes. It can be quantified somewhat by reporting the number of egg masses found per person-hour of searching. Intensive searching is recommended to support pheromone trapping for the discovery of new infestations.
The following two methods are currently the only methods available for quantifying gypsy moth egg mass density. Both methods consist of a complete census of all egg masses occurring within a predetermined number of randomly located sample plots. The number of egg masses per acre is estimated from the samples.
Fixed-and variable-radius plot counts. This method is described in detail by Wilson and aine (1978). At each sample plot, all of the egg masses are counted on trees selected from the plot center using a prism (a tool commonly used by foresters for estimating tree basal area). Egg masses occurring on the ground are counted within a fixed-radius plot located around the same plot center.
Fixed-radius plot. The observer counts every new egg mass on trees and on the ground within a circle with a radius of 18.6 feet (1/40th acre or 0.01 ha) around a chosen point. This count multiplied by 40 gives an estimate of the number of egg masses per acre. This method is more cost effective than the fixed- and variable-radius plot method (Kolodny-Hirsch 1986; Thorpe and Ridgway 1992) and is currently the most widely used method.
In addition to providing an estimate of the number of egg masses in an infested area, these methods can provide the opportunity for the observer to judge the health of the gypsy moth population. Egg masses that are thick and of large size (about that of a 50-cent piece), showing little or no parasitoid damage (such as small holes) and containing large quantities of undamaged fertile eggs indicate a healthy population. In many cases, a numerically large population of small egg masses or those showing predator/parasitoid injury may indicate a declining gypsy moth population which may not require treatment. Unfortunately, assessment of gypsy moth population quality must be done subjectively, as analytical guidelines do not exist.
Sequential sampling plans. A sequential sampling plan for gypsy moth has been used successfully in Shenandoah National Park as well as in several urban areas in northern Virginia for five years (Ravlin 1991; Ravlin 1994). Sequential sampling is a process in which a given number of samples are taken and, based on how far above or below the threshold you are at the end of the sample, a decision is made to either stop sampling and apply a treatment, stop sampling without a treatment, or continue sampling to gather more information before a decision about gypsy moth management is made.
Of course, any egg masses found in areas outside the established North American infestation area may represent the spread of the gypsy moth and may require treatment, since isolated infestations can usually be eradicated. Within the infested region, management of the moth population to limit defoliation and population growth is the most sound approach, since eradication is impossible.
In addition to directly sampling the gypsy moth population in a particular area, site managers may wish to indirectly track zones of defoliation to determine where to treat otherwise unidentified populations, where to set up traps next spring, and the spread of existing infestations. Defoliation should also be monitored to assess the efficacy of any treatments that were applied. Defoliation is generally monitored during the period of peak larval development in one or more of the following ways.
Ground estimation: An observer may make estimates of percentage defoliation of particular trees by walking through the infested area and examining tree crowns through binoculars. A slightly more comprehensive method involves using the fixed- radius or fixed- and variable-radius plot designs noted above (under Egg Mass Counting), and again estimating the percentage defoliation noted on each tree observed. Comparing photographs of a sample area taken at regular intervals will allow the observation of changes in canopy density due to defoliation. These methods are very time-consuming and are subject to errors of interpretation. They are discussed by Talerico (1981).
Sketch mapping: An observer may fly over the area to be monitored in an aircraft, sketching zones of light, medium, or heavy infestation on a U.S. Geological Survey map of the area. Talerico (1981) details the procedure and interpretation of such maps. As in ground observation methods, interpretation of the results is largely a matter of experience.
Threshold/Action Population Levels
To date, efforts to construct reliable predictive models for gypsy moth defoliation based on population density have been only partly successful at best (Ganser et al. 1985, Montgomery 1990). Current defoliation thresholds are rough estimates (Figure 1). The following population values are currently used by the U.S. Forest Service and APHIS in their gypsy moth management programs. The National Park Service follows U.S. Forest Service guidelines. It should be noted that the goal of the U.S. Forest Service Forest Pest Management program differs depending on whether or not the gypsy moth population is within the area of the United States that is recognized as being generally infested. Contact your regional U.S. Forest Service office or regional Integrated Pest Management coordinator for this information.
For areas in established infestation zones: At 250-500 egg masses/acre, gypsy moth populations may produce noticeable defoliation. Treatment is recommended for high-use recreational areas (campgrounds, trailer parks, and other areas with transient traffic) and residential areas.
For areas outside established infestations: The capture of any male moths in a detection survey may indicate the need to conduct a more intensive delimiting survey. The decision to delimit the following season should be based on such factors as history, host vegetation, local resources, and movement of people in the vicinity (Anonymous 1990). If the delimiting survey indicates that an isolated population has developed, further delimiting surveys, intensive searches for egg masses, and eradication treatments may be indicated.
NON-CHEMICAL CONTROL OF GYPSY MOTHS
Individual Tree Treatments
Egg mass destruction: Scraping and removing egg masses is one of the oldest methods used against the gypsy moth in North America. In residential areas, where 50% of the egg masses may occur within reach of the ground (Thorpe and Ridgway 1992), this approach could destroy a significant portion of the population. However, because of the tendency of larvae to migrate in from adjacent areas, scraping should not be relied upon for effective control. Vegetable oils have been shown to be effective ovicides when applied to egg masses in the fall (Ralph Webb, in manuscript), and a soybean oil product is registered for use on gypsy moth egg masses.
Barrier bands: Sticky barrier bands placed on tree trunks can prevent larvae from crossing (Webb and Boyd 1983); there is some evidence that under outbreak conditions and on isolated oak trees, barrier bands can reduce defoliation can be reduced (Blumenthal 1983). However, under gypsy moth population densities capable of causing less than 60% defoliation, larval populations on banded trees are reduced only an average of about 25%, and defoliation reduction is highly variable (Thorpe et al. 1993). Sticky barrier bands are available commercially or can be made from duct tape and Tree Tanglefoot
Burlap bands can be used as a control tactic if they are checked frequently and all larvae resting beneath them are destroyed. The efficacy of this method has not been quantitatively evaluated.
Natural Enemies of Gypsy Moth
Naturally occurring predators and parasitoids of the gypsy moth, while numerous and abundant, are not capable of preventing outbreaks. Efforts to control gypsy moths by rearing and releasing large numbers of parasitoids have not been successful
(Blumenthal et al. 1979, Kolodny-Hirsch et al. 1988). The best way for a site manager to make use of available natural enemies of the gypsy moth is to use management alternatives (e.g., B.t. or no treatment) which will not adversely affect the natural enemies, leaving them to function as a part of a gypsy moth integrated pest management program. See Blumenthal et al. (1981) for a detailed discussion of predator/parasitoid research. Egg mass surveys and larval surveys can include observations of predator/parasitoid presence as a guide to maximizing their effectiveness.
Bacteria: The naturally-occurring bacteria Streptococcus faecalis and Pseudomonas spp. occasionally cause high levels of mortality (up to 60%) under outbreak conditions (Podgwaite 1981).
Nucleopolyhedrosis virus: A virus of the genus Baculovirus is closely associated with all North American gypsy moth populations. Its effects are most often seen under outbreak conditions, when a large proportion of the larval population may be killed. For more information on this disease, see the following section on area-wide suppression of the gypsy moth.
Entomophaga fungus: For the first time in 1989, the fungal disease Entomophaga maimaiga was reported causing widespread mortality to North American gypsy moth populations (Hajek and Soper 1992). This disease was known to cause extensive mortality in Japan. It is now known to occur in 13 states from Maine to Virginia (Elkinton et al. 1991). The appearance of larvae killed by Entomophaga is similar to that of virus-killed larvae, and definitive identification requires examination by an expert.
Since 1905, more than 40 species of parasitic flies and wasps have been introduced into North America to control the gypsy moth. Among the 10 which have become established are the egg parasitoids Ooencyrtus kuvanae and Anastatus disparis, the larval parasitoids Cotesia melanoscela, Blepharipa pratensis, and Parasetigena silvestris, and the pupal parasitoid Brachymeria intermedia. Another introduced larval parasitoid, Compsilura concinnata, which has a wide host range, attacks many species of larvae in addition to the gypsy moth. The egg parasite O. kuvanae is usually abundant and typically attacks from 10 to 40% of all gypsy moth eggs (Brown 1984). However, because it can reach only the outermost eggs in an egg mass, its effectiveness is limited. The larval parasitoid C. melanoscela typically is abundant, but high rates of overwintering mortality and poor synchronization with host development limit its impact. Simons et al. (1979) provides a guide to gypsy moth parasitoid identification.
Invertebrate predators: Ground beetles, ants, and spiders are known to feed on gypsy moth larvae and pupae. One predatory beetle, Calosoma sycophanta, was successfully introduced into North America from Europe. This ground beetle sometimes becomes abundant in outbreak gypsy moth populations, but usually lags one to three years behind (Weseloh 1985).
Birds: Many species of birds feed on gypsy moths, but they are not a major diet item for any of the common species (Elkinton and Liebhold 1990). Most birds are deterred by the long hairs on larvae. Nuthatches, chickadees, towhees, vireos, orioles, catbirds, robins, and blue jays are probably the most important species in innocuous-phase gypsy moth populations. Cuckoos and flocking species such as starlings, grackles, red-winged blackbirds and crows may be attracted to outbreak populations (Smith and Lautenschlager 1978).
Mammals: Shrews and white-footed mice eat larvae and pupae and may be a major factor in the maintenance of low gypsy moth populations (Elkinton and Liebhold 1990). There is some evidence that regional changes in small mammal density may account for the region-wide onset of gypsy moth outbreaks (Liebhold and Elkinton 1989).
Bacillus thuringiensis: This spore-forming bacterium produces a crystalline protein during sporulation that is toxic to the larvae of many species of butterflies and moths, including the gypsy moth. Predators and parasitoids of the gypsy moth are not harmed by the toxin, nor are humans, plants, or other animals. A complete review of the properties and action of B.t. toxin can be found in Dubois (1981). B.t. is an effective alternative to chemical pesticides when used against the gypsy moth and is currently available in a number of commercial formulations. Label directions should be followed at all times.
Under most conditions, B.t. is generally effective at protecting foliage, although it is less effective at reducing populations (Twardus and Machesky 1990). Two applications of B.t. separated by three to seven days may increase the effectiveness of the treatment (Webb et al. 1991). Because it is most effective against very young larvae, the first application of B.t. should be made when 50% of the larvae are second instars and oak leaves are at least 50% expanded.
More detailed discussions of B.t. dose, adjuvants, dilution, and nozzle type and configuration, as well as spray calibration, characterization, and evaluation, can be found in Reardon (1991).
Nucleopolyhedrosis virus (NPV): This virus is the cause of an endemic wilt disease of gypsy moth larvae in the United States and Europe and is a major cause of naturally-occurring gypsy moth population decline. Its effects are most obvious under outbreak conditions, where a high proportion of the larval population may be killed. It is often referred to as "wilt" disease, because of the limp appearance of infected larvae. Infected larvae eventually rupture, releasing a brown fluid containing virus particles. Transmission of the disease occurs within a generation from contact with infected individuals and contaminated surfaces, and to some extent by gypsy moth parasitiods and predators (Podgwaite 1981). Transmission from generation to generation occurs through exposure to contaminated surfaces (Woods et al. 1989).
A review of the natural occurrence, culture, and testing of NPV as an artificially- applied larvacide can be found in Lewis (1981). Gypchek
Gypsy Moth Pheromone
The chemical structure of the sex pheromone produced by female gypsy moths to attract males, known as disparlure (cis-7,8-epoxy-w-methyloctadecane) was identified in 1970 (Bierl et al. 1970) and can now be synthesized for use in management programs. While disparlure is widely used to monitor adult male population levels (see Population monitoring section), it has also been used to control small populations (e.g., isolated outbreaks along the leading edge of the infestation) by trapping males in pheromone-baited traps and by disrupting mating behavior (Plimmer et al. 1982). Currently, APHIS uses pheromone traps (at a density of three to ten traps per acre) in attempts to eradicate small outbreaks in selected areas of the United States (Anonymous 1990).
Mating disruption for gypsy moth management can be effective in certain situations. It cannot be used in areas which are quarentine regulated or experiencing outbreak population levels. Mating disruption has been used effectively to control new infestations in areas that currently have no gypsy moth problem or on leading edge zones of current infestations. (The 100-mile border of current infestations which border uninfested areas.) Mating disruption is used in areas where there are fewer than 10 egg masses per acre, which corresponds to an average of 20 male moths/trap/season, or a maximum of 40 male moths/trap/season.
There are two types of dispensing systems for the pheromone; a flake formulation, which is currently on the market and a bead formulation, which will be fully registered by the end of 1994. The flake is expensive to apply because specialized aircraft application pods are required. It is long-lasting (eight weeks) and has a steady release rate over that time, so it provides more flexibility in time of application. The bead is less costly to apply, since a regular aircraft spray boom can be used. It is less effective than the flake because it tends to release quickly, so two applications are usually needed. Available in several bead sizes, the smaller bead releases more quickly than the larger. Temperature governs release rate and will be faster in warmer weather and slower under cooler conditions. See Leonard et al. (1989) for more information on this technique.
The release of sterilized gypsy moths has been attempted as a means of control, but is still in the research and development stage. See Mastro et al. (1981) for a detailed discussion of the USDA sterile gypsy moth release research program.
Since the demise of the American chestnut as the dominant overstory tree in the eastern United States deciduous forests, oaks have become a dominant species. Unfortunately, oaks are also the favored hosts of the gypsy moth throughout its range. In the absence of external control measures, repeated defoliation of favored trees may result in a shift of dominance to nonhosts and less favored hosts, such as maples. This will ultimately reduce the magnitude of the gypsy moth problem in these areas. While selective removal of favored gypsy moth hosts is an impractical (at best) solution for most park sites, selection of planting material for areas under development (e.g., urban parks) to exclude favored hosts is definitely feasible and should be strongly encouraged.
In managed forests, one option for gypsy moth management that is available to the resource manager is silvicultural control. This is the selective harvest of trees to reduce the susceptibility (likelihood of defoliation) and vulnerability (likelihood of mortality after defoliation) of the forest stand to gypsy moth outbreaks. This is done by maximizing tree growth and vigor, removing high-risk trees, manipulating the habitat of the gypsy moth and its natural enemies, and increasing forest diversity. Further discussion and guidelines for silvicultural management can be found in Gottschalk (1993).
APHIS has designated most of New England, the mid-Atlantic states, and portions of Michigan as "gypsy moth high risk areas" (Anonymous 1990). Other areas of the United States may be designated by APHIS as high-risk areas if isolated infestations develop there, until gypsy moths are successfully eradicated. Individuals moving household or recreational items from these areas into or through other areas of the United States must have such items inspected and certified ``gypsy-moth-free'' by a USDA-trained inspector. Since gypsy moths may be carried on surfaces of vehicles, camping equipment, and other outdoor items, inspection of the vehicles and equipment belonging to park visitors from high-risk areas may enable park personnel to discover and destroy egg masses and other gypsy moth life stages which could give rise to new infestations. Distribution of educational materials (e.g., Don't Move Gypsy Moth [Anonymous 1983]) to prospective visitors of all parks outside high-risk areas, along with the erection of prominent informational displays outside park boundaries, are recommended as methods to encourage visitors to voluntarily participate in such a program. Contact your regional National Park Service Integrated Pest Management coordinator or local APHIS office for help in setting up such a program.
The establishment of a pheromone-trapping program in areas of high vehicular traffic and other visitor use is recommended as an adjunct to any inspection program, to permit the discovery of isolated infestations caused by egg masses or other life stages slipping through the inspection program. Contact your local U. S. Forest Service office for details and assistance in conducting a trapping program.
Currently, APHIS has responsibility for the eradication of isolated infestations of 640 acres or less. Suppression efforts over larger areas and within the generally infested area are the responsibility of the U.S. Forest Service. Some parks receive U.S. Forest Service funds for gypsy moth management and contract for their own management programs. APHIS uses either insecticides, including multiple applications of B.t., or mass trapping, to eradicate isolated populations.
CHEMICAL CONTROL OF GYPSY MOTHS
Several chemical insecticides are currently registered for gypsy moth control. National Park Service policy states that these pesticides may only be used in historic or developed park areas in which B.t. or other biological methods (or pheromone trapping) are ineffective. Contact your regional National Park Service Integrated Pest Management coordinator for further information.
Systemic injection: Injections or implants of insecticides registered for this purpose and applied to oak trees at budswell provide significant protection from gypsy moth defoliation (Webb et al. 1988). Some wounding to the tree occurs with this procedure, with white oaks exhibiting a more severe wound response than red oaks (Reardon and Webb 1990). It appears that most of the wounds close and trees recover within three years.
Ground application of insecticides: Individual trees in areas accessible to vehicles can be sprayed with registered insecticides from the ground, using hydraulic sprayers or mist blowers to protect foliage. Although relatively expensive, this method can be quite effective. Since the entire infested area may not be treated, the potential exists for reinfestation of treated trees from the surrounding area. Sticky barrier bands on treated trees may be helpful in preventing reinfestation.
The U.S. Forest Service is responsible for conducting gypsy moth population monitoring programs on all Federal lands. Each park manager should contact his/her regional U.S. Forest Service office for assistance in setting up an appropriate gypsy moth monitoring program for high-use areas. For further information regarding U.S. Forest Service services, contact:
U.S. Forest Service U.S. Forest Service Forest Pest Management Forest Health Protection 1720 Peachtree Road 5 Radnor Corporate Center Atlanta, GA 30367 Suite 200 (404) 374-2989 P.O. Box 6775 Radnor, PA 19087 (215) 975-4125
In historic and developed parks (including campgrounds, visitor facilities where shade is an important attraction, and specimen trees), survey programs may trigger suppression or eradication activities. Under National Park Service policy, natural areas, areas containing endangered species, or areas with special natural features may receive no treatments; existing natural enemies must be allowed to exert their long- term effects in such areas.
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