Predicting the Distribution  of the Sagebrush Defoliator Moth

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 was to use habitat data from sagebrush communities in southeastern Idaho to determine which variables (e.g., abundance or height of sagebrush; presence or abundance of other plant species; presence of other moth and insect species; or land use attributes) most strongly predict the presence or absence 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 outbreaks would allow land managers to better maintain and manage critical sagebrush habitats.

 

Objectives

  • Compile and analyze 2007 and 2008 field data and document results

  • Prepare specimens for taxonomic identification.

 

Accomplishments through 2009

Simple correlation and linear regression were used to test the strength of relationships between 9 independent variables (IVs) used to characterize the habitat at each trapping location (table below). The height of rabbitbrush was eliminated to minimize effects of colinearity, and logistic regression was used to fit the presence and absence of A. websteri to the 8 remaining IVs, individually and in combination.  Models for grazed and un-grazed habitats were analyzed separately for 2007 and 2008. All combinations and interactions between IVs were tested using forward and backward stepwise methods.
 

Independent variables analyzed for their usefulness in predicting the presence or absence of Aroga websteri. 

Independent Variable

Definition

Soil type

1 = Pancheri-Polatis, 2 = Beriniceton-Aecet, 3 = Terreton-Zweifel, 4 = Malm-Bondfarm-Matheson (Olson and Jeppeson 1995)

Relative abundance of sagebrush and rabbitbrush in 2007 and 2008

Ranking of 1 = rare, 2 = present, 3 = common but not dominant, 4 = dominant for sagebrush (Artemisia tridentata) and green rabbitbrush (Chrysothamnus viscidiflorus) in each sample location

Number of sagebrush 

The number of sagebrush (Artemisia tridentata) recorded along three 20m transects in each sample location

Number of rabbitbrush 

The number of rabbitbrush (Chrysothamnus viscidiflorus) recorded along three 20 m transects each sample location

Number of other shrubs

The number of  shrubs other than sagebrush or rabbitbrush recorded along three 20 m transects in each sample location

Dead sagebrush

Number of dead sagebrush recorded along three 20 m transects in each sample location

Height of sagebrush

Average height of sagebrush measured along 3 20 m transects in each sample location

Height of rabbitbrush

Average height of rabbitbrush measured along 3 20 m transects in each sample location

Other moths captured

Total number of moths other than A. websteri captured in each location in 2007 and 2008

 

Results

Alone or in combination, the simplistic metrics used to characterize habitat for each trapping location were not significant in predicting the presence or absence of A. websteri.  Although vegetation and soil characteristics differed between grazed and un-grazed locations, the difference in the number of A. websteri captured in each habitat type (table below) was not significant and grazing by domestic livestock was also shown to be an insignificant predictor of the presence or absence of A. websteri.

 

 

Summary of the presence and absence of A. websteri in sagebrush habitat at the Idaho National Laboratory.

 

Habitat

Total Sample Locations

Locations with

A. websteri

(no. captured)

Locations without

A. websteri

2007

2008

2007

2008

2007

2008

All

40

42

11 (15)

11 (21)

29

31

Grazed

21

25

5 (6)

7 (13)

16

18

Un-grazed

19

17

6 (9)

4 (8)

13

13

 

Over 100 macrolepidopteran specimens representing nearly 30 species captured in 2007 and 2008 were pinned, labeled, and are being submitted for identification as qualified taxonomists are located.  Efforts to sort several hundred microlepidopteran specimens to morphospecies are ongoing.

 

Plans for Continuation

With the exception of continued efforts to curate and identify specimens, this study will not continue once results have been documented.