Habitat Selection Model to Predict the Distribution and
Abundance of the Sagebrush Defoliator Moth (Aroga websteri
Periodic outbreaks of the sagebrush defoliator moth (Aroga
websteri Clarke [Lepidoptera: Gelechiidae]) can cause
widespread damage to rangelands in the western United States.
Sagebrush (Artemisia spp.) is the exclusive larval host
of A. websteri and, in high numbers, larvae can kill hostplants
and reduce the production of foliage and ﬂowering by surviving
plants for years. The overall goal of this project is to use
habitat data from sagebrush communities in southeastern Idaho to
determine which variables (e.g., presence, relative cover, or
height of sagebrush species; presence of other plant species;
presence of other moth and insect species; land use attributes;
or weather conditions) most strongly predict the presence or
absence and abundance of A. websteri. Development of a
predictive model would be a ﬁrst step toward identifying the
locations of potential A. websteri outbreaks. A better
understanding of the location, timing, and pattern of defoliator
Speciﬁc project objectives for 2007 were to:
Determine the presence and relative density
of A. websteri in 45 locations along 3 permanent INL
Site Breeding Bird Survey (BBS) transects;
Use counts of A. websteri on
sagebrush branches to develop an estimate of A. websteri
density and to calibrate trap efﬁciency;
Characterize vegetation and other habitat
attributes at each sampling location;
Compile and analyze ﬁeld data.
Project accomplishments for 2007 include the
Insect traps were used to sample A.
websteri in 40 locations along INL Site roads
spanning portions of 2 permanent BBS transects;
An inventory of A. websteri on
branches collected from sagebrush in each of the 40
trapping locations was completed;
Plant composition (relative abundance
rank), distance from each trap to the nearest sagebrush,
height, canopy width (in two compass directions) and
distance to nearest neighbor were obtained at each
location and non-target insect species captured in the
traps were preserved;
Temperature, precipitation, and
wind-speed for all sampling dates were obtained from
climatological monitoring stations at the Central
Facilities Area (CFA) and the Materials and Fuels
A. websteri was captured in 11
(about 28 percent) of the sampled locations (Figure
9-9). A maximum of three individuals was captured in any
Evidence of A. websteri was also
found in sagebrush branches from only eight of the 40
sites (20 percent), which could account for the low
numbers captured in traps;
Although collection of 45 samples along
three transects was planned, time constraints and
habitat destruction associated with a wildﬁre that
started on July 18, 2007, resulted in the elimination of
sampling along the Twin Buttes BBS transect. An
unintended consequence was that all sites with more than
one sagebrush host species were eliminated.
A. websteri and other specimens
were preserved and are in the process of being sorted
and mounted for identiﬁcation.
Plans for Continuation
To support an analysis of host-plant
composition, sampling in 2008 will be conducted in a
sagebrush community with two or more host-species
(location to be determined).
INL Site sites sampled in 2007 will be
re-sampled (or sub-sampled) in 2008 to verify low A.
websteri densities and to further quantify potential
correlations with densities of A. websteri and
other insects in grazed and un-grazed habitats.
Combined data from 2007 and 2008 will be
used to develop alternative models to determine which
habitat variables (e.g., relative cover or height of
sagebrush species, presence of other plant species,
presence of other moth and insect species, land use
attributes, or weather conditions) most strongly predict
the presence or absence and abundance of A. websteri.
Publication, Reports, Theses, etc.
A manuscript documenting project results
will be submitted for publication in a peer-reviewed journal
in December 2008. Results also will be included as a chapter
of my Ph.D. dissertation and will contribute to other
integrated presentations and publications on the biology and
outbreak dynamics of A. websteri and other insect
pests of western rangelands.
Investigators and Affiliations
Nancy Hampton, Ph.D.
Candidate, Department of Biological Sciences, Idaho State
University, Pocatello, Idaho
Dr. Nancy Huntly,
Professor, Department of Biological Sciences, Idaho State
University, Pocatello, Idaho
University Graduate Student Research and Scholarship Committee