Gypsy Moths Keep Fluttering

Nature gets a little out of control sometimes, and despite our best chemical efforts to level the playing field, we simply can't win. That's the case with the gypsy moth. This destructive insect pest has been plaguing the Northeast U.S. and parts of Canada for more than a century. A bevy of innovative chemical solutions has been devised over the years, but the moth continues to annually defoliate substantial tracts of forest and blemish suburban landscapes.

At last month's ACS national meeting in San Francisco, David R. Lance, an assistant director at the U.S. Department of Agriculture's Center for Plant Health Science & Technology, in Buzzards Bay, Mass., provided an overview of the chemical history of gypsy moth management in North America. Lance's presentation was part of the symposium "Invasive Species: Is Chemistry Up to the Task?" sponsored by the Division of Agrochemicals.

"The story of gypsy moth battles is an interesting one," Lance told C&EN. "It runs from spraying trees with heavy doses of lead arsenate a century ago to currently using relatively small amounts of a nontoxic sex pheromone to disrupt mating."

By studying the chronology of chemical treatments between those two extremes, he added, scientists have learned a lot about what works and what doesn't work, limitations put on chemistry because of environmental concerns, and how the need for better solutions leads to innovation. Sharing those lessons—the magic bullets and the duds—was the purpose for holding the invasive-species symposium.

The European gypsy moth, Lymantria dispar, is native to temperate forests in Europe, Western Asia, and North Africa, Lance explained. The species name "dispar" comes from the disparity in appearance between the grayish-brown mottled male and the larger, white bespeckled female.

The moth was accidently introduced into North America near Boston in 1869 by French artist Étienne Léopold Trouvelot, who as an amateur scientist studying silk-producing insects lost control of his charges, Lance noted. After about 10 years, the itinerant moths were abundant enough in the wild to start wreaking havoc on local fruit and shade trees—and to start spreading. Today, gypsy moth outbreaks occur primarily in forests along the East Coast from central Virginia to New Brunswick, and west to the Great Lakes states and other Canadian provinces, he said.

Gypsy moth females lay egg masses on tree trunks and branches in the fall, Lance continued. Protected by a leathery matrix of short hairs, the masses easily last through the winter. Larvae emerge on their host tree in spring as leaves emerge, and the larvae start climbing toward branches or disperse on silken threads to take up residence on nearby trees. The larvae munch on leaves, and as they grow they pass through several molting stages in which they shed a chitin-based exoskeleton.

To anyone who has witnessed the destruction of a major gypsy moth outbreak, the leaf munching and excrement falling down like a light rain are audible as the larvae rapidly defoliate trees. Despite the damage, gypsy moth activity in a single season typically doesn't permanently hurt trees, Lance said. But multiple years of defoliation can kill trees or weaken them substantially so that other pests, such as pathogenic fungi or wood-boring beetles, finish them off.

Ever since the gypsy moth started to proliferate in North America, it has been targeted for eradication, containment, or simple reduction of the negative effects of outbreaks, Lance said. Lead arsenate, in the form of either PbHAsO₄ or Pb₅OH(AsO₄)₃, was specifically developed to treat gypsy moths, he noted. A concoction of 30 lb of lead arsenate suspended in 150 gal of water was one recipe. Despite the known toxicity of arsenic and lead, lead arsenate was widely used on millions of acres as the principal gypsy moth treatment until the end of World War II.

"The amount of arsenicals put out was incredible," Lance observed. "Workers would even load spray rigs on barges and go up and down rivers spraying trees. It's amazing today to reflect back on those early efforts and to think how much things have changed."

Arsenicals were followed by chlorinated hydrocarbons such as dichlorodiphenyltrichloroethane, better known as DDT. Aerial DDT spraying for gypsy moths was common in the 1950s, with some 3 million acres treated in 1957.

"DDT was highly effective, and it started to become the answer to everyone's gypsy moth prayer," Lance said. "But it became clear rather quickly that public concern for DDT residues on food and feed crops, and the chemical's adverse effects on fish and wildlife, was going to squelch everything." DDT was phased out for gypsy moths starting in 1958, he said.

That chemical was replaced by other broad-spectrum synthetic pesticides, such as carbamates and organophosphates. In particular, carbaryl, a carbamate sold commercially as Sevin, became the primary gypsy moth control agent.

Carbamates were a breakthrough of sorts in pesticides, Lance pointed out, because they don't have the persistence of chlorinated hydrocarbons. Carbaryl works by inhibiting the enzyme acetylcholinesterase and disrupting its role in controlling the activity of the neurotransmitter acetylcholine in the nervous system. A drawback is that carbaryl and organophosphates are not very selective, so they are now seldom used for gypsy moths because of their harmful effects on nontarget creatures, he added.

Starting in the 1970s, a new generation of products based on microbial proteins and insect growth regulators rose to the top, Lance said. For example, Dimilin, a urea-based compound, is a commercial insect growth regulator that is extremely effective against the gypsy moth's larval stages at low application rates. It kills larvae by interfering with development of a new exoskeleton during the molting stages. Dimilin is less toxic to fish than organophosphate pesticides, but it's toxic to aquatic invertebrates and can't be used near water.

The soil bacterium Bacillus thuringiensis (Bt) is also part of the gypsy moth arsenal, Lance continued. In particular, the subspecies Bacillus thuringiensis kurstaki produces the insecticidal protein δ-endotoxin that is currently one of the most widely used pesticides in gypsy moth control. When larvae ingest the endotoxin, the low pH in their gut activates the protein, which subsequently causes epithelium cell lysis and kills the larvae. Bt is generally considered environmentally friendly, Lance said, because it reduces the amount of traditional chemical pesticides needed and it's not harmful to most beneficial insects or to people.

Another development in the gypsy moth war is the nucleopolyhedrosis virus (NPV), which infects only gypsy moth larvae, Lance told C&EN. NPV causes a "wilt" disease that is spread by larvae living in close quarters. The disease can reach epidemic proportions, killing up to 90% of the larvae in gypsy moth populations.

In 1978, the Environmental Protection Agency approved an NPV-based product, called Gypchek, for use against gypsy moths. In a unique development, USDA produces Gypchek, Lance explained. Sterile larvae are infected with the virus, and then the diseased larvae are raised, freeze-dried, and ground into a powder. It takes about 750 larvae to produce enough Gypchek to treat one acre.

Drawbacks to Gypchek are its labor-intensive and expensive production process and its short half-life in the environment, Lance added. Thus, Gypchek is currently used only when necessary in the most sensitive habitats, such as near streams and lakes.

Putting a final nail in the coffin of the gypsy moth might never be possible, Lance said. But USDA is attempting to control the spread of the insects by establishing a 60-mile buffer zone along the North Carolina-Virginia border and then northwest through West Virginia and Kentucky and onward to Wisconsin and Minnesota. This "Slow the Spread" program relies heavily on a sex-attractant pheromone called disparlure, both to monitor gypsy moth colonies and to disrupt mating.

Female gypsy moths emit disparlure to attract males. Lance described how entomologists started taking advantage of the pheromone in the 1890s by using live females as bait to attract males to sticky traps. In the 1930s, scientists figured out how to harvest the pheromone from the tip of the female's abdomen, which allowed broader use for traps.

In the 1970s, chemists determined that there are two disparlure enantiomers, and that only one, (+)-disparlure—(7R,8S)-2-methyl-7,8-epoxyoctadecane—attracts males. After researchers identified the correct enantiomer and worked out a synthesis, use of the pheromone really took off, Lance said.

Today, some 100,000 (+)-disparlure-baited traps per year are placed along the gypsy moth battlefront to track forward progress, he noted. In addition, traps are used in western states to check for moths that hitchhike in from the east or arrive on ships at Pacific Ocean ports. When the moths are discovered penetrating new areas, USDA acts to wipe them out, typically by administering Bt.

The monitoring traps use small amounts of (+)-disparlure, which costs several hundred dollars per gram, Lance noted. But the U.S. Forest Service uses large amounts of the racemic mixture in a slow-release formulation, such as flakes of laminated plastic with (±)-disparlure in the core, that can be bought by the barrel and applied by aircraft to disrupt mating.

"If you permeate the air with pheromone, the males can't locate individual sources of the sex-attractant—that is, the females," Lance said. "Mating disruption is now the primary control method for the Slow the Spread program."

Lance concluded by noting that scientists and government agencies had pretty much given up on trying to eradicate gypsy moths by the 1960s. The federal government has now fallen back to a fail-safe position of keeping the moths from spreading as much as possible.

"Economic models suggest that by stopping moths from spreading, we are keeping down the control costs," he noted. "The cost-benefit analysis shows the program is more than paying for itself. The gypsy moth front is still moving forward at about a dozen miles per year, but at some point it is going to run out of trees and stop."

"It was a real benefit to take a trip back in time with Dave Lance," said Tracy Ellis, an entomologist for California's San Diego County and a co-organizer of the Division of Agrochemicals symposium. "As a society, we don't reflect enough on where we have been and where we want to go with the treatment and control of insects."

While growing up in central Massachusetts, Ellis witnessed gypsy moth destruction. "My childhood memories are filled with gypsy moths and Japanese beetles eating everything," she noted. "We have made enormous strides in the safety and effectiveness of the chemistries applied to insect pest management since then.

"We still lament that we need more chemistries," Ellis added. "That is why we had the symposium—to raise awareness for the need for better pheromones, pesticides, and pest-management tools. History shows us that the very act of discussing the problems, describing the needs, and visualizing the next level sets us on the path to developing it."