Living with the gypsy moth

By Wayne and Mary Ellen Koser
Following are excerpts of an article that first appeared in the Summer 1974 edition of The Explorer, a scientific magazine published by The Cleveland Museum of Natural History. Thirty years later it is still relevant as gypsy moths continue to plunder our forests.

Man has become increasingly aware of and concerned with the worldwide spread of disease, pest outbreaks, and disturbance of basic food chains through the introduction of foreign organisms into a relatively stable environment. Separation by continents and other physical boundaries previously helped to isolate species, but man’s mobility now provides a means for spreading non-endemic organisms to new areas. Some of these introduced species can adapt to their new environments, causing little notice or damage, but many examples can be listed where non-endemic species have caused intense dislocation of plant and animal communities. No one really knows how many different species have been spread beyond their natural boundaries, though some ecologists have estimated the number to be in the many thousands. The control of introduced species is often necessary in order to preserve living resources, to prevent diseases, and to ward off economic dislocation, all of which ultimately affect human life.
They gypsy moth is an example of a pest that has spread through Europe and Asia regardless of efforts to control it. Like previously introduced epidemics of the Dutch elm disease, the American chestnut blight, and the Japanese beetle, the gypsy moth (Porthetria dispar) is considered by many entomologists to be potentially one of the most devastating influences on North American hardwood forests. In 1973 alone over 1.5 million acres of forest were defoliated. It is feared that if the moth spreads in epidemic numbers to the Allegheny, Appalachian, and Ozark Mountain regions, where much of the nation’s hardwood timber is located (over 112 million acres), the damage and control expense would be astronomical. Data obtained by the New Jersey Department of Agriculture showed that oaks are most vulnerable to mortality after repeated defoliation by the gypsy moth. Most oak trees, as well as other hardwoods, can withstand one season of partial to complete defoliation, but tree mortality after the second year of severe defoliation will increase considerably, with oak losses ranging from 25 to 64 percent. Conifers, however, will suffer approximately the same mortality after only the first season of defoliation. Some of the immediately surviving trees are weakened; so that some long-range deaths can also be attributed to the defoliation that left the trees more vulnerable to other insect pests, disease infection, and adverse weather conditions.
Life cycle and Ecological Effects
As do all insects having a complete metamorphosis, the gypsy moth develops in four stages: egg, larva (caterpillar or feeding stage), pupa (resting stage), and adult (breeding stage). In early August the adult female lays a one to one-and-a-half inch, buff-colored egg mass containing from 400 to 1,000 eggs. The buff color comes from the female’s body hairs becoming attached to the overwintering mass as she deposits the eggs on any hard, sheltered surface. With the advent of spring (late April), the caterpillars hatch and begin satisfying their voracious appetites. From late April until early July, or approximately ten weeks, each caterpillar can consume as much as one square foot of leafage per day, and they have been known to denude a tree in a matter of minutes. After having devoured all the leaves on a tree, they move to new foliage. The gypsy moth is a serious threat to our forests because its list of preferred food species is extensive. The larvae (caterpillars) will greedily devour the leaves of oak, apple, aspen (poplar), maple, basswood, alder, willow, hawthorn, linden, shrubs, and even shade and ornamental trees. Although not as readily, the insect will also attack cherry, elm, hemlock, hickory, pine, spruce, sassafras and birch. It has even been observed to try green Astroturf and plastic flowers! Trees not favored by the gypsy moth include red cedar, holly, locust, sycamore, catalpa, balsam fir, dogwood and black locust.
The gypsy moth larva, which may remain a maximum size of two or three inches, is dark gray with many long hairs along the sides of its body. On top of each segment (except the first) is a par of warts called tubercles. The first pairs are blue: the last six red. The body hairs have been known to cause skin irritation on susceptible people. In once case a child’s eyes were affected by contact with the body hairs. The long hairs covering the young larva are hollow, giving it buoyancy to balloon into the air. Thus the larvae have the ability to spread into new areas by wind currents or by their own mobility. It is for this reason that the peripatetic insect was given the name “gypsy” moth. The normal direction of spread is toward the southeast, related to prevailing northwestern winds during summer months.
Another stage of metamorphosis begins as the larva pupates in early July. After 10 to 14 days it emerges as an adult moth. The moth does not feed; therefore it is not a direct threat to the forest. Reproduction is the only function of the adult. The brownish one-and-one-half-inch male emerges first and is mobile during the day. It is not unusual to see hundreds of thousands of males flying aimlessly through an infested area. Males have black irregular bands on the first pair of wings, and have feathery antennae. In contrast, the larger, two-inch, white-bodied female is generally immobile due to her size and weight. She has faint black markings along the margin of her wings. In order to attract a male, the immobile female secretes gyptol, a chemical sex attractant. In August the cycle is repeated when the female again lays the overwintering eggs for succeeding generations.
Infestation and Control Methods in the United States
The gypsy moth was introduced from Europe in 1869 at Medford, Massachusetts, by a French naturalist and astronomer, Henri Trouvelot. Trouvelot, employed by the Harvard Observatory, tried to crossbreed several silk-producing moths with the aim of developing a strain resistant to a disease threatening the French silk industry. Inadvertently a storm damaged his outdoor laboratory, thus scattering some of the eggs and caterpillars. Little notice was taken until approximately 20 years later when the area was inundated by epidemic numbers of the gypsy moth caterpillars. “The numbers were so enormous that the trees were completely stripped of their leaves, the crawling caterpillars covered the sidewalks, the trunks of shade trees, the fences, and the sides of houses, entering the houses, getting into the food and into the beds … The numbers were so great that in the still, summer nights the sounds of their feeding could plainly be heard, while the pattering of their excremental pellets on the ground sounded like rain.” (Howard, 1930). The result of this invasion was the destruction of shade and fruit trees, recreational land and wildlife habitats, and in reduced real estate values. The moths’ activities changed forest composition, caused soil erosion, and increased fire hazards.
After such an extreme outbreak, the Massachusetts government became concerned and began experimenting with direct control methods. By the summers of 1898-1899, more effective treatments, such as spraying with lead arsenate and the physical removal of the insect, had been developed. The new treatments, coupled with three years of adverse weather conditions, aided in reducing gypsy moth populations.
Another factor which may have eased Massachusetts’ plight was the observable three-year gypsy moth population cycle. During the first year of infestation there is a gradual build-up in numbers. The second year notes a high gypsy moth density, resulting in severe defoliation of trees, as was experienced in Massachusetts in 1899. The third year shows a sudden collapse in population, apparently due to a viral infection similar to the one that causes cold sores in humans. The insect carries the virus at all times but manifests it only in over-population conditions, which result in high competition stress. Succeeding generations are weakened and subject to viruses and parasites because their favorite foods have been stripped by their ancestors, thus leaving hard-hit areas relatively free of heavy infestations for seven to 10 years.
Observations by the New Jersey Department of Agriculture have shown that forest stand structure and the composition of vegetation change because of gypsy moth feeding patterns. Tree stands which were susceptible to the gypsy moth have been killed and replaced by species more resistant to, or not favored by, the insect pest. In February of 1900 the Massachusetts legislature, viewing no further damage, withdrew support for controls. This action was taken in spite of experts’ warnings that the pest had not been totally eradicated. During the next five years the insect population increased enormously. The situation became so critical that in 1905 Massachusetts resumed its control program. At this time Massachusetts tried releasing parasites as a means of biological control.
In 1906 an appropriation was made to Congress, and the Secretary of Agriculture was authorized to cooperate with concerned states in developing control programs. The major objective of the federal government was to prevent the spread of the pest to new areas. Local inspection and quarantine of transported produce and products led in 1912 to the enactment by Congress of the Plant Quarantine Law.
For many years the Hudson River provided a natural barrier to the spread of the gypsy moth, but due to a series of hurricanes around 1938 the moth was dispersed along the river southwestward into adjacent states. Within five years new isolated outbreaks were discovered in New York, New Jersey, Pennsylvania, and Ohio. Most of these outbreaks were contained by local control programs.
In Michigan, there were many undocumented gypsy moth sightings in the late 1940s. However, the first officially recorded outbreak was near Lansing in May of 1954. From then until 1962 a total of 282,015 acres of infested woodland in Calhoun, Clinton, Eaton, Ingham, Ionia, and Shiawassee counties were treated with DDT as an immediate control measure. No new infestations were recorded in the state from 1962 until the insect was once again found in Calhoun County in 1966. By that time DDT had become generally outlawed.
In 1967 a total of 24,182 acres were treated with the newly developed insecticide sevin (carbaryl), a less ecologically destructive insecticide which breaks down more quickly than DDT and which leaves no residue in milk, pasture land, and crops. In general, it has a low toxicity to fish, birds, and warm-blooded animals. It is harmful to honey bees and kills certain aquatic insects and organisms which provide essential food for fish. This aerial application of sevin supposedly eradicated that invasion. Michigan was once again though to be gypsy moth free, until an infestation was discovered near Mt. Pleasant (Isabella County) in 1972. It is believed that the egg masses that started this outbreak might have been dropped from a house trailer used by vacationers on a trip from one of the eastern states. Another example of the unwitting spread of the gypsy moth was caused by trailer and recreational vehicle owners who stopped at infested campgrounds in Connecticut. From these vacationers new outbreaks were traced in 1970 to California, Florida, Minnesota, Texas, Virginia and Wisconsin. In addition, male gypsy moths were trapped in Iowa, North Carolina, Ohio, South Carolina, Tennessee, and West Virginia during a 1972 trapping program carried out by the federal government.
Michigan, in an effort to determine the spread and numbers of gypsy moths throughout the state, and not as a control measure, distributed approximately 4,000 traps, resembling a paper cup and baited with the synthetic sex attractant disparlure. The male moth reacts to disparlure as he would to gyptol, a substance secreted by the female moth. Once inside the trap, the male is entangled in a sticky substance. These traps were placed in infested or possibly infested areas.
Update on gypsy moths
According to Michigan State professor Deb McCullough, the Department of Agriculture ceased spraying sevin from airplanes during the 1970s after farmers filed numerous lawsuits. Sevin is a poison that interferes with the nervous system of insects as well as other animals, having adverse effects on all. Michigan gave gypsy moths up for dead in the 70s, then began treating them again in reaction to new outbreaks in the 1980s with BT (Bacillus Thuringiensis). BT is a bacterial disease that only affects caterpillars which have eaten the foliage that has been sprayed, and is subsequently much safer on the environment. Michigan no longer sprays all forests, but only highly-populated areas like campgrounds. A method used to eradicate gypsy moths in areas where they are not yet rampant is pheromone flakes, which have been applied in Michigan’s Upper Peninsula, as well as Wisconsin, Indiana and Ohio. Pheromone flakes confuse the males because it causes everything to smell like females, preventing them from fertilizing their eggs, thus preventing them from breeding. “We are in a much better position to deal with this than we were in the 1970s,” said McCullough.
Even so, the gypsy moth began defoliating major forest areas like never before in the 1980s, first defoliating the Clare-Midland area before moving up to the center of the Lower Peninsula. They moved east and west in the early 1990s and were responsible for the largest area of defoliation in Michigan history in 1992. Gypsy Moths defoliated 700,00 acres of Michigan forest in 1992 and just under 400,000 the following year. Those figures spiraled downward until 1996, and in both 1998 and 1999 they defoliated between 100,000 and 300,00 acres per year. But once again, the numbers have declined ever since.