by Joe Elkinton and Jeff Boettner
This article appeared in the No. 3, 2016 issue of Massachusetts Wildlife Magazine.
For the first time in 35 years, widespread and severe tree defoliation by gypsy moths surged across Massachusetts, Rhode Island and Connecticut. This came as a surprise to researchers whose subsequent investigations revealed the outbreak is linked to the recent and severe statewide drought. The authors describe the introduction, spread, and population ecology of gypsy moths, and the past and current efforts to battle this exotic invasive.
The gypsy moth (Lymantria dispar) is a native of Europe and its history in North America compared to many exotic species is unusual in that we know exactly when and where it was introduced. The culprit was Etienne Leopold Trouvelot of Medford, Massachusetts, an amateur entomologist who later became famous for his astronomical illustrations of celestial objects and phenomena. He was trying to hybridize gypsy moth with native silk moths for the silk industry. He imported gypsy moths from France and they escaped from his home in 1868 or 1869. The first tree defoliation by these moths started in Medford and spread to surrounding towns in the 1880s. In 1890, the Massachusetts legislature earmarked funds for gypsy moth control and eradication. These efforts did not succeed and gypsy moths continued to spread, but very slowly, because even though female gypsy moths have wings, they do not fly. Only males fly. Instead, the females devote all of their energy to egg production, averaging 600 eggs per moth. Recently hatched larvae (caterpillars) spread from tree to tree by dangling from silken threads blown by the wind. Much of the actual spread of gypsy moth in North America is caused by human beings, who transport the overwintering egg masses on firewood, lawn furniture and other objects in their yards to other locations. It took over 50 years for gypsy moth to spread across Massachusetts. In 1922, a barrier zone was created along the Hudson Valley in an attempt to prevent the spread further south and west, but that effort failed. The gypsy moth spread is continuing today, with the leading edge of the infestation ranging from Minnesota in the Midwest to North Carolina in the South. It has taken 148 years for gypsy moth to attain its current distribution in North America.
As mentioned earlier, in the 1890s, the legislature funded a substantial eradication program which focused on mechanically destroying gypsy moth egg masses, found on the trunks of trees from August through April each year. In addition, there was a considerable pesticide spraying effort targeting the larvae. These pesticides consisted mainly of lead and copper arsenate. In those days, there was little to no appreciation of the environmental danger posed by these toxins. The pesticides were found to be largely ineffective, and failed to stem the spread of the population.
Beginning in 1905, efforts shifted to what was then a relatively new approach, biological control, which consisted of introducing the gypsy moth’s natural enemies from Europe. This tactic had just recently succeeded famously in California against the cottony cushion scale. For the gypsy moth effort, several dozen parasitic European wasps and flies, collectively referred to as parasitoids, were reared at a lab in Melrose Highlands and then released in Massachusetts over the next decade. This soon became the biggest biological control effort in history and continued until the 1970s. In the first decade, 10 species of parasitoids were established. All of these species caused some mortality, but their combined impact did not prevent gypsy moth outbreaks.
Depending on the species, parasitoids attack different life stages of the gypsy moth host, either developing on or in the host and killing the host in the process. Beginning with the gypsy moth egg masses, a small parasitic wasp (Ooencyrtus kuvanae), which is a good flier, actively seeks out gypsy moth egg masses on tree trunks and branches. O. kuvanae will lay an average of 200 eggs on the moth egg masses. Studies in several states show that 20–30 percent of the gypsy moth egg populations are destroyed annually by this egg parasitoid. A tiny round emergence hole in a mass of gypsy moth eggs indicates the parasitic wasp adult has emerged.
Two parasitoids, Cotesia melanoscela (a parasitic wasp) and Compsilura concinnata (a parasitic fly), attack the young gypsy moth larval (caterpillar) stage and are most effective when the gypsy moth caterpillars are still quite small. The process is somewhat of a science fiction horror event. The parasitic wasp C. melanoscela adults emerge from their cocoons at the same time as the gypsy moth caterpillars start hatching from their overwintered eggs. Attacking young caterpillars, an adult female wasp lays up to 500 solitary eggs. Along with each single egg, the wasp injects a polydnavirus which prevents the caterpillar from molting. The result is a zombie caterpillar, which lives longer but is unable to mature, allowing the wasp time to develop before tearing a hole in the caterpillar to exit and kill the host.
Brachymeria intermedia is a parasitic wasp that lays its eggs inside gypsy moth pupae. Not much is known about the life history of this wasp, although it appears to also parasitize other species of moths.
Another introduction from Europe to combat the gypsy moth scourge was Calosoma sychophanta, a large, bright green metallic ground beetle predator. Beetles in the genus Calosoma are called caterpillar hunters and they are aptly named, as both the adults and larvae are active predators. The larvae feed during the day and night, consuming gypsy moth caterpillars during a two-week developmental period. Adult beetles will eat caterpillars during their life span of two to four years.
In the early 1900s, when these natural enemies were introduced, no one cared or thought very much about the potential impact of these parasitoids on native moths. Compsilura concinnata, for example, was known to be a generalist, meaning it would prey on a wide variety of moths, not just gypsy moth larvae. Our research showed in 2000 that C. concinnata had become the dominant mortality factor for native giant silk moths such as Cecropia and Polyphemus moths. Nowadays, we know to avoid such generalist natural enemies and focus any introductions on specialists which have little or no impact on non-target hosts.
The Era of Pesticides
During World War II, the pesticide DDT was invented. It was cheaper and more effective than any previous pesticide. Following the war, widespread aerial application of DDT was made against gypsy moth. Entomologists in those days were convinced that DDT was a new tool that would solve most insect problems. By the 1960s, however, the environmental costs of DDT and related compounds were evident and were popularized by the famous book Silent Spring by Rachel Carson. DDT and its breakdown products persist indefinitely in the environment and accumulate in the fatty tissue of many animals. It was particularly damaging to birds, especially those at the top of long food chains, such as eagles and ospreys. The use of DDT and other chlorinated hydrocarbon insecticides was banned in the late 60s and 1970s. The Environmental Protection Agency was established in the early 1970s and laws were passed to require safety testing of all pesticides. Nevertheless, populations of birds such as eagles and ospreys took many decades to recover, a process that continues to this day.
Meanwhile, new pesticides were developed and used against gypsy moth. When we started our work on the gypsy moth in the 1980s, aerial applications of carbaryl were very popular. Carbaryl gave way to diflubenzuron, an insect growth regulator. By the end of the decade the bacterial insecticide Bacillus thuringiensis (B.t.) became popular. Its advantage was that it affected only foliage-eating insects, and not the adult stages of their insect natural enemies. Other bacterial insecticides such as spinosad were added to the mix in subsequent decades. Thus, in the modern era, we now have much safer pesticides that affect a more narrow spectrum of target and nontarget insects. However, we believe the days of aerial application of any pesticides against gypsy moth in New England are gone forever. We now know that the gypsy moth outbreaks will subside on their own and the forests will recover, even if there is significant tree mortality. Even the modern pesticides with a narrow spectrum will kill many non-target insects and aerial applications are too expensive to justify.
Applications to individual shade trees, however, are another matter. Homeowners place high value on these trees which provide beauty and shade to their yards. If a shade tree dies, it is expensive to remove. Homeowners are willing to spend significant funds to protect their trees, and many tree care professionals are available to help them to do that.
The Rise and Fall of Gypsy Moth Populations
Widespread and severe tree defoliations caused by gypsy moth occurred approximately every 10 years throughout most of the 20th Century in the generally infested region. Although 10 species of parasitoids were established in the early 20th Century, none of them were sufficient to maintain low-density populations of gypsy moth. This phenomenon contrasts with native species of caterpillars. There are hundreds of native moths that have the capacity to increase in density like gypsy moth, but all of them are maintained at low density by their natural enemies. It is important to understand that the combined mortality from all sources must exceed 99 percent, if a population of an insect like gypsy moth is to stabilize at a particular density. Since each female lays approximately 600 eggs, half of which are female, only one of the 300 females needs to survive to maturity for the population to remain stable. If two of them survive, the density will double in one year. If 10 survive, the density will increase tenfold. This same fact is true of all insects, indeed all organisms. The details depend on the fecundity of females.
In the 1960s and 70s, Robert Campbell of the U.S. Forest Service, led the first comprehensive research aimed at understanding the population ecology of gypsy moth in North America. His research suggested that predation by small mammals, in particular the white-footed mouse, feeding on the late larval and pupal stages, was the key to maintaining low-density populations in the years between outbreaks. Predation by birds, in contrast, was a much less important limiting factor. Many types of birds feed to some extent on gypsy moth caterpillars, but many are also deterred by the dense spiky hairs on the mature caterpillars.
We began similar ecological research based in forests on Cape Cod and near the Quabbin Reservoir in the 1980s. Our evidence supported the ideas promoted by Robert Campbell. We found that gypsy moth populations would rise when populations of white-footed mice declined. Mouse populations fluctuate with the acorn crops, their major overwintering food source and, as is true with many trees, acorn crops vary enormously from year to year. A variety of weather conditions, such as a late spring frost, or mid-summer drought, can nearly eliminate the acorn crop. We showed that when acorn crops failed, mouse populations then declined dramatically by the following summer and gypsy moth populations therefore increased. All of this was occurring at low gypsy moth density, when they were in a non-outbreak phase. At these densities gypsy moth populations might increase from less than 10 egg masses per acre to more than 100. There was no defoliation. Since each gypsy moth female lays up to 600 eggs and most of those eggs survive to the larval stage, 100 egg masses per acre might result in more than 50,000 larvae. Somewhere above 100 egg masses per acre, a density threshold is reached beyond which the proportion of gypsy moth larvae or pupae eaten by mice or other small mammals, such as shrews, decline with increasing gypsy moth density.
Unlike gypsy moth parasitoids, changes in the density of vertebrate predators such as mice or birds are fairly constrained. Birds defend territories and so do mice. Thus the population densities of mice rarely increase beyond about 50 per acre. Gypsy moths in contrast can increase from 1 to 10 to 100 to 1000 to 10,000 egg masses per acre, which is characteristic of outbreak populations. At these higher densities, mice or birds can feed all day on gypsy moth and never make a dent in the population, whereas at lower gypsy moth densities the mice may consume most of the gypsy moth pupae in the forest. Thus, these vertebrate predators play almost no role in regulating the outbreak populations.
With many caterpillar species, parasitoids can regulate density and prevent outbreaks because their numbers can increase along with their hosts. Unfortunately, introduced and native parasitoids that attack gypsy moth in North America do not do this effectively. Their numbers are constrained for reasons that are poorly understood, and they never cause very high levels of parasitism. So once gypsy moth densities reach a threshold in the vicinity of 100 egg masses per acre, the gypsy moth population will grow inexorably over the next one or two years into an outbreak phase that results in widespread defoliation. Outbreak populations become limited only by the availability of green foliage. Few gypsy moth caterpillars actually starve in outbreak populations, but many fail to get sufficient food resources. As a consequence, the adults that arise from outbreak populations are smaller and the females might lay 100 eggs per mass, instead of 600.
Left: Predation on late larval and pupal stage gypsy moth by small predators, in particular the white-footed mouse, was the key to maintaining low-density populations of gypsy moth in the years that existed between outbreaks. Illustration © Debra Silva. Center: Since each female gypsy moth lays approximately 600 eggs, half of which are female, only one of the 300 females needs to survive to maturity for the population to remain stable. Over 40,000 eggs appear in this image! Photo © Marion Larson. Right: The armies of white-footed mice that prey on late larval and pupal stage gypsy moth depend heavily on acorns as an overwintering food source. Consequently, when acorn crops fail, mouse populations decline dramatically by the following summer and gypsy moth populations therefore increase. A viral pathogen has always served to terminate gypsy moth outbreaks. The introduction of a new fungal pathogen in 1989, prevented outbreaks since that time. The fungus, however, depends on rainfall in May and June, so summertime drought in the past two years caused the 2016 outbreak. Illustration © Debra Silva
Another important limiting factor is a virus called Nuclear Polyhedrosis Virus (NPV). It takes off within these outbreak populations and may kill 99 percent of larvae before they reach the pupal stage. Such viruses are common in outbreak populations of many insect species. Virus diseases reach epidemic proportions in outbreak populations because transmission from one caterpillar to another is much more likely at high population densities. In yet another science fiction-like horror scene, when the caterpillar dies from NPV, the virus causes the caterpillar cadaver to liquefy and virus particles spread over the leaf surface. Transmission occurs when a healthy caterpillar consumes virus particles released by these liquefied cadavers. Mortality from NPV starts in the early larval stages, but grows exponentially in the late larval stage and peaks just before the caterpillars form pupae. It is this epidemic that brings an end to gypsy moth outbreaks and causes the populations to retreat back to low density. Outbreaks will typically last for 1 to 3 years before this population collapse happens. In the years following collapse of the outbreak, predation by small mammals resumes as the dominant force of mortality and maintains gypsy moth at low density for subsequent years. We saw evidence of the virus at many Massachusetts sites in 2016, however, not in numbers high enough to cause population collapse.
A Fungal Assist
Outbreaks of gypsy moth in Massachusetts and New England occurred about every 10 years up until 1989. In that year, an epidemic of Entomophaga maimaiga, a new fungal pathogen of gypsy moth from Japan took hold. We remember it well. We walked out one day to our research plots at the Quabbin Reservoir, where densities of gypsy moth were fairly low, and we saw dead caterpillars hanging head down on the trunks of trees everywhere. We were accustomed to seeing lots of dead caterpillars killed by the virus in outbreak populations, but never in such low densities. This occurred all over southern New England that year. Densities of gypsy moth during that period were starting to increase and another outbreak was imminent, but this fungal pathogen prevented that from happening. It turned out that 1989 was an especially rainy year and subsequent research showed the fungus depends on rainy conditions in May and June for successful transmission to healthy larvae.
The fungus apparently was introduced from Japan accidentally. Earlier researchers, notably Dr. Ann Hajek at Cornell University, had tried to introduce the fungus apparently without success. Over the next few years, the fungus spread rapidly across the Northeastern United States. Unlike the virus, the fungus spreads by way of airborne spores released from the caterpillar cadavers. This is the manner in which low-density populations can acquire high levels of mortality. Beginning in 1992, and working with our colleague Dr. Hajek, we spread the fungus to Virginia and Michigan by transporting fungal-infected soil from our Quabbin research plots and we successfully established new infestations of the fungus at these locations. The fungus, however, soon spread on its own across the gypsy moth infested regions of the Northeastern United States by means of airborne spores, so that by 1996 most of the infested regions had the fungus firmly established. In New England, the fungus caused a major change in status of gypsy moth, dropping it from a serious pest to one more like the hundreds of other native species of caterpillars that are present in our forests. Gypsy moth populations retreated to low density where they have remained for the last 35 years. They are maintained at low density by natural enemies, and little or no tree damage occurs. Gypsy moth populations in areas further south, such as Pennsylvania, have continued to experience periodic outbreaks despite the presence of the fungus. Laboratory tests showed the fungus does best in cooler conditions. Temperatures in May and June in the mid-Atlantic states are much warmer than in New England.
So what happened in 2016? The answer is very simple: drought. Transmission of the fungus begins in the early larval stage of gypsy moths in early May. It takes about a week for the fungus to kill a caterpillar. Spores from the fungus-killed cadaver spread by air to other caterpillars, but rainfall primarily from May 1 through June 20 is required for the fungus to germinate and penetrate the integument (skin) of the new host. In a normal year with sufficient rainfall the fungal epidemic steadily increases as larvae mature over several rounds of transmission and mortality. In 2016, drought conditions prevented this from happening in southern New England. Our drought began in May 2014 and June 2016 was even drier. We observed some gypsy moth larvae killed by the fungus in 2016, but not in the epidemic proportions required for a population reduction.
Impact on Trees and Forests
Like people, gypsy moth caterpillars favor certain foods. They prefer to feed on trees such as various oaks, aspen, apple, and willows. Less favored species, such as maples and conifers, will be defoliated if they are growing in stands where outbreaks are occurring. There are only a few tree species, notably ash trees, that are almost never defoliated by gypsy moth. Gypsy moth outbreaks tend to occur in forest stands which consist mostly of the favored trees. Oak trees, for example, dominate the forests in eastern and central Massachusetts, whereas maple trees are more numerous in the Berkshires’ higher elevation forests. Thus, gypsy moth outbreaks are generally more common in the eastern and central parts of the state, as they were in 2016.
Most trees will survive one or more complete defoliations, provided they are in a healthy condition. Many trees, however, are currently confronting a variety of stresses. A notable one this year is the effect of severe drought. Following defoliation by gypsy moth, deciduous trees will put out a new set of leaves in July. In 2016, because of the drought, many trees were still struggling to do this in August. In eastern Massachusetts, some trees had already been defoliated in May by the winter moth, Operophtera brumata, another invasive defoliator from Europe. They were then defoliated a second time by gypsy moth after they had already started putting out new leaves. On Cape Cod, there is yet another oak-feeding insect in outbreak conditions, the black oak gall wasp, Zapatella davisae. We are worried that stresses from all the sources may cause a lot of oak mortality this year in Massachusetts.
Homeowners who want to protect their shade trees should look for tawny brown egg masses on the trunks of trees in their yards this fall and winter. Large numbers of egg masses mean that high populations can be expected next year. If opting to apply pesticides, homeowners should contact local tree care professionals who have the training and equipment to choose appropriate pesticides and to apply them safely. This should be done in May of 2017. Other measures, such as destroying egg masses, have not worked very well, because in the spring after the larvae hatch, they blow in from neighboring trees. As for the general forest health prognosis, we can only hope for abundant rainfall next May and June. If that occurs, gypsy moth populations should decline and retreat to the non-outbreak status that we have enjoyed over the last 35 years. However, this past August, we observed large numbers of egg masses in many places, so it is likely there will be significant defoliation again next year even if the fungus epidemic occurs.
About the Authors
Joe Elkinton (left), Ph.D., is a Professor of Environmental Conservation at the University of Massachusetts, Amherst. He conducts research on population dynamics and biological control of invasive forest insects. His early work focused on gypsy moth and the impact of small mammal predators and viral and fungal pathogens on that system. More recently his projects focus on the population dynamics of browntail moth, hemlock woolly adelgid, and winter moth. Jeff Boettner (right) is the Lab Manager at Elkinton Lab at the University of Massachusetts, Amherst. He began working with Joe in 1986 on a gypsy moth project in the Quabbin Reservoir area. His recent work involves traveling to British Columbia to collect and rear Cyzenis albicans, a species-specific tachinid fly, which he and Joe are importing to use to control winter moth outbreaks in New England.