Developing a Habitat Selection Model to Predict the Distribution and Abundance of the Sagebrush Defoliator Moth (Aroga websteri Clarke)


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.

Accomplishments Through 2007

Project accomplishments for 2007 include the following:

  • 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 Complex (MFC).


  • 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 location.

  • 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

Funding Sources

Idaho State University Graduate Student Research and Scholarship Committee

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