Where Has the Gypsy Moth Gone? 
Gypsy Moth caterpillars, ubiquitous in the 1980s, were 
seldom seen in 2003 
By Clive Jones, Ecologist, 
Institute of Ecosystem 
Studies 
Originally printed in the Pough- 
keepsie Journal 
In 1869, imported silk was 
popular but expensive. Etienne 
Trouvelot thought he could 
make money crossbreeding 
sensitive silk moths with hardy 
European gypsy moths that 
would survive our cold climate. 
His breeding experiments failed, 
he did not make his fortune, 
but our forests paid a high 
price for the few gypsy moths 
that escaped from his backyard in Medford, 
Massachusetts. 
The insect spread. Young caterpillars ballooned 
in the wind on silken threads, and egg masses 
hitched a ride on vehicles and garden items 
when people moved. Over a century later, the 
moth has wandered north into Canada, west 
to the Great Lakes, and south to Virginia. 
Ten year cycles 
Every ten years or so, moth populations 
increase rapidly, sometimes to millions of 
caterpillars per acre. Once they strip oaks bare 
of their leaves, the caterpillars feed on other 
trees, including conifers. Oaks can withstand 
a year of complete defoliation, but are often 
killed by successive years of heavy damage. A 
single defoliation kills conifers. 
Eventually, lack of food, a viral disease, and 
predatory insects kill most caterpillars, and the 
outbreak collapses. 
Our long-term studies at the Institute of 
Ecosystem Studies in Millbrook show that 
moths are too rare most years to cause the 
widespread damage to trees like they did in the 
past. Two unlikely organisms - white-footed 
mice and a fungus — have, to a large degree, 
kept moth populations in check. 
Gypsy moths arrived in Dutchess County in 
the 1940s. In New York State, small amounts 
of defoliation occurred in the mid-40s, 
increasing in the mid-50s, 60s and 70s. 
with much less defoliation. 
Since the end of the 1980s outbreak, the 
gypsy moth seems to have receded into the 
background. Is it becoming an innocuous forest 
occupant or will it periodically resurge? 
In all but three years since the 1980s outbreak, 
caterpillar densities have been well below levels 
that could cause serious damage to trees. In 
some years, moths have been so scarce that 
we found only one or two egg masses after 
searching acres of forest. 
We now understand why moths are usually rare. 
Intriguingly, the very oak trees gypsy moths eat 
indirectly prevent moth outbreaks by co-opting 
the help of the white-footed mouse. 
Mice depend on oak acorns for food; mouse 
populations increase dramatically the year after 
a bumper acorn crop. Mice are also voracious 
predators on moth pupae. In summers 
following a fall with acorns, mice consume 
most moth pupae before they get the chance to 
emerge as adults and lay eggs. This keeps moth 
populations in check, preventing outbreaks. 
When acorns are scarce, mouse populations 
decline, fewer moth pupae are preyed upon, 
and moth populations start to rise rapidly. 
When acorn crops remain low for more than 
a year or two, moth populations can rise to 
levels where mice can no longer control them, 
even when mice become abundant again due 
to acorns. 
Volume 20, Number 6 
November - December 2003 
Editor's Note 
Early morning frost clings to the hardy 
plants that remain in the Gifford Garden, 
and the Institute's grounds are covered 
in winter's first blanket of snow. The 
grounds staff has set up a bird feeding 
tree, with an array of delicacies. The 
combination of bare trees and hungry 
birds makes it an excellent time to view 
our avian visitors. 
There are many pathways to learning 
about ecology. Some people find a pas- 
sion for the natural world through an 
engaging teacher or mentor. Others will 
have their imaginations sparked by read- 
ing a newspaper article or purchasing an 
Earth-friendly product. By participating 
in the Poughkeepsie Journal's new Envi- 
ronment Page, Institute scientists hope to 
reach a new community of learners. The 
lES Ecology Shop encourages visitors to 
think about the origins of products and 
the impacts of consumption, another 
opportunity for education. 
The lES Newsletter is published by the Institute 
of Ecosystem Studies, located at the Mary Fla- 
gler Cary Arboretum in Millbrook, New York. 
Director: Gene E. Likens 
Administrator: Joseph S. Warner 
Head of Education: Alan R. Berkowitz 
Writer & Editor: Lori M. Quillen 
Production Assistance: Pamela Freeman 
Address newsletter correspondence to: 
Public Information Office 
Institute of Ecosystem Studies 
Education Program, Box R 
Millbrook, NY 12545-0178 
e-mail: QuillenL@ecostudies.org 
Printing: Spectrum Graphics & Print 
Poughkeepsie, NY 
Many will recall the massive outbreak of the 
early 80s when nearly 2.5 million acres of 
forest were defoliated statewide in both 1980 
and 1981. Outbreaks reoccurred in the early 
1990s and the turn of the new century, but 
Density, but not defoliation 
This happened at lES in the early 1990s. Moth 
density rapidly increased over a two-year period, 
resulting in patchy, light defoliation in 1994. In 
continued on page J 
