Tracing Moth Migrations
Male (left) and female gypsy moths, Lymantria dispar.
Knowing the travel patterns of these mobile pests can help growers plot
Former Agricultural Research Service entomologist Douglas C. Ferguson (now
retired) spent a half century tracking moth species across the continent.
Knowledge of a moth's frequent-flier mileage can influence the timing of
pest control measuresespecially components of integrated pest management
programs. And timing's becoming increasingly critical as control strategies
need to cross county and state lines to deal with these mobile pests.
Many of the moth species that make up the insect order Lepidoptera are
serious pests affecting crops and forests, says Ferguson. These
include corn earworms, Helicoverpa zea, as well as armyworm,
Pseudoletia unipuncta, tobacco budworm, Heliothis virescens, and
fall armyworm, Spodoptera frugiperdia. Though these are among the
most costly U.S. insect pests to control, he says, many other moths
are useful in helping to control weeds.
A world expert on Lepidoptera, Ferguson still works on gypsy moths and
inchworm moths at ARS Systematic Entomology Laboratory as a taxonomist,
searching for ways to tell moth species apart.
During his career, he has collected over 200,000 moth museum specimens in
the United States and Canada. They are now part of the national collection of
insects at the U.S. National Museum of Natural History in Washington, D.C.
Largely as a byproduct of his research, Ferguson has been accumulating and
analyzing data on the migratory moths of eastern North America since the
To collect specimens, Ferguson ties a white bedsheet between two trees and
shines a light on itusually an ultraviolet fluorescent or mercury vapor
lamp. The illuminated sheet attracts the moths without damaging them, leaving
them in good condition for examination. Ferguson has spent thousands of
nocturnal hours doing just this. Sometimes he also sets out light traps or
applies fermented fruit and sugar bait to tree trunks.
"The bait technique has been used since the 1830's and is still one of
the best," he says.
As a graduate student in the 1960s, Ferguson got the idea that moths
migrate unbelievably long distances and wrote a term paper on the subject. He
says this early training alerted him about what to look for in doing this
"I needed to know enough about moths in generalnot only the
peststo do moth migration research," he says.
In the United States, there are more than 11,000 moth species. Ferguson
categorizes them all, including the economically important ones, into two
groups: migratory and nonmigratory. Unlike butterflies, most moths fly at
night, so their migrations have long remained a secret.
"Our understanding of this aspect of their behavior has developed very
slowly," he says.
Assisted by the right wind and weather conditions, some migratory species
can cover at least 600 miles in one nonstop flight. Evidence is ample that a
few have flown much farther than that.
Nonmigratory moths do not expose themselves to transport by major weather
systems and are thought to be more sedentaryperhaps wandering only a few
miles per year.
Ferguson devised ways to shed more light on both styles of dispersal.
Corn earworm moth, Helicoverpa zea.
For example, from his years of field detective work and using the
museums extensive moth collections, he determined which are the
long-range migratory moths of eastern North America. To verify his assumptions,
he then investigated the moth fauna of Bermuda, using the islands 600 miles
from Cape Hatteras, North Carolina, as a natural laboratory.
He thought that "if most of the migrants of the East Coast are also
present in Bermuda, then the most likely explanation is that they can fly
there," he says. This hypothesis was further substantiated by data from
other islands worldwide. In 1991, Ferguson published a detailed account of
"Of the 40 migratory pest species that occur in the eastern United
States," he says, "38 are now reported from Bermuda."
Previously, it had been suggested that some of these were unintentionally
taken to Bermuda by people. But Ferguson questioned why such a disproportionate
number of migratory species would be introduced there accidentally.
As to why moths fly so far, Ferguson says that migrants not only
exploit new food sources, they also evade predators, parasites, and diseases
that have no way of catching up with them. Their one long flight is probably
related to favorable winds that tend to carry them northward in spring and
summer and southward in fallthat, coupled with their basic urge to ensure
Ferguson believes that moths fly quite high, taking advantage of low level
jet streams. These currents can carry them up to 68 miles per hour, at a height
of 1,000 to 3,000 feet, for many hours.
But sometimes the weather betrays the moths and carries them far off course
over the Atlantic or too far north to breed successfully.
Monitoring the Slow Movers
More recently, Ferguson turned his attention to finding out the travel
distances of more sedentary mothsthe nonmigrants. However, this proved
The only way seemed to be to measure the rates of spread of introduced
species. So Ferguson investigated the case histories of 10 foreign moth species
for which sufficient information was available. These included the potato stem
borer, Hydraecia micacea Esper, and the large yellow underwing
(cutworm), Noctua pronuba L.
He established for the first time how far these nonmigratory insects had
spread and the average rate of movement per year for a known timespan. Until
then, only the gypsy moth, Lymantria dispar, had been examined in this
Studying how introduced species spread gives you a starting point, and
knowing how far they travel gives you a hint as to what to expect if a new pest
should invade the country, says Ferguson.
Two cutworm moths were the fastest spreading of the accidentally
introduced moth species, he says. One champion traveler appears to
have spread 121 miles in a year and the other, 99 miles in one continuous
flightover water. These pests attack a wide variety of garden,
crop, and weed plants.
Ferguson found that other introduced species spread from 8 to 28 miles a
year. Of the 10 foreign moth species he studied, the European gypsy moth spread
the slowestan overall average of about 8 miles a year, since being
introduced around 1868. This was probably because of its flightless females.
The browntail moth, Euproctis chrysorrhoea, and satin moth, Leucoma
salicis, which are in the same family but with flying females, spread much
fasterabout 26 and 28 miles per year.
Ferguson estimates that the time required for various nonmigrant, introduced
species to occupy all suitable habitat in the United States and Canada varies
from 30 to 100 years. Yearly mileage totals may vary, he says,
for moths rarely fly in a straight line. Most of their flights are in a
And, curiously, he adds, 6 of the 10 Eurasian immigrants
studied entered the United States independently, from both East and West
coasts. Seaports, rather than airports, have been the major points of
Nearly all accidentally introduced Lepidopteraan
estimated 125 speciesare of European origin.
Like most of the ARS Systematic Entomology Laboratory entomologists,
Ferguson is located at the U.S. National Museum of Natural History. The
laboratory maintains headquarters and part of its staff at the Beltsville
(Maryland) Agricultural Research Center. By Hank Becker, ARS.