Umatilla National Forest

 
DOUGLAS-FIR TUSSOCK MOTH

2001 INFESTATION ON THE HEPPNER RANGER DISTRICT

Map of Area and Photos

Douglas-fir tussock moth (DFTM) is a native insect of the Blue Mountains; it was not introduced from somewhere else like larch casebearer or white pine blister rust.  Tussock moth defoliates true firs (grand fir primarily) and Douglas-fir from the top down, killing trees outright or setting them up for future attack by bark beetles such as Douglas-fir beetle or fir engraver.  Unlike western spruce budworm, another major defoliator that caused widespread tree damage in the Blues between 1980 and 1992, the tussock moth can consume a tree’s entire foliage in a hurry – infested trees begin to turn reddish-brown in June and may be entirely defoliated (all of the needles are gone) by mid-July.

Tussock moth populations are cyclic, rising on average about every 9 years in the western United States.  Not every population peak results in an outbreak – in northeastern Oregon, it appears that an outbreak happens to coincide with every second or third population peak.  Outbreaks were recorded for the Blue Mountains in 1928-1929, 1937-39, 1946-48, 1963-65, 1972-1974, and 1992-93.  However, dendrochronology studies have shown that tussock moth has been active in the Blues for as long as suitable habitat (mixed-conifer forest) has been present (dendrochronology is the interpretation of tree cores to infer climatic cycles, fire cycles, insect outbreaks, etc.).  For example, dendrochronology analysis indicates that tussock moth may have defoliated mixed-conifer stands in the Drumhill Ridge area (Walla Walla Ranger District) from 1843 to 1845, 1852 to 1854, and in 1875.

The Heppner Ranger District is currently experiencing an infestation of tussock moth in the following areas:

  • Indian Creek
  • Lovlett Creek
  • Big Willow Springs Creek
  • Happy Jack Spring
  • The Knob

The last severe outbreak on the Umatilla National Forest occurred in the early 1970s, when tussock moth affected areas in both northeastern Oregon and northeastern Washington.  The first damage was noticed as 2,400 acres of defoliation in the Okanogan Valley of north-central Washington in 1971.  In 1972, over 197,000 acres were defoliated in Oregon and Washington.  Perhaps some of the worst damage occurred on the north end of the Umatilla National Forest.  By 1974, 44% of the defoliated acreage in the DFTM outbreak area (including state, private, and other federal ownerships) was on the Umatilla – a total of 353,850 acres!

      

Drawing of fully developed tussock moth larvae.

Adult Male Tussock Moth

Adult Female Tussock Moths along with cocoons.  Females are completely flightless.

How do we know when an outbreak will occur?  As a result of the 1970s outbreak, a early warning system was developed to monitor tussock moth population levels.  As part of the early warning system, pheromone traps are strategically placed throughout eastern Oregon and Washington.  These traps attract the adult male moth.  By trapping the moths, resource specialists can determine if populations are stable or increasing.  If the average trap count exceeds 40 moths/trap ground sampling is initiated to determine actual population levels.  In recent years, the Umatilla has seen an overall increase in trap counts with some counts exceeding the 40 moth/trap threshold and ground sampling has indicated sub-outbreak to outbreak levels.

What has the Forest done to address the increase in population levels?  In 1999, a team of resource specialist began to address an anticipated outbreak (as of result of the early warning system) of tussock moth on nine National Forest in Oregon and Washingtion.  This included the Colville, Okanogan, Wenatchee, Umatilla, Wallowa-Whitman, Malheur, Ochoco, Winema, and Fremont  In preparation, the team began the process of formulating an Environmental Impact Statement (EIS) to identify key issues and to formulate a proposed action.  In April 2000, a final EIS was published and released.  In May the Record of Decision (ROD) was signed by the Regional Forester to allow the use of the insecticide TM-BioControl to treat areas (of concern) identified in the EIS.  The TM-BioControl is made of a natural virus of the tussock moth.  This virus is specific to the tussock moth and other species of western tussock moths.  The virus is the primary cause of tussock moth populations to collapse. In June of 2000, 6,125 acres were treated with the TM-BioControl on the Pomeroy and Walla Walla districts.  Although, not all areas on the Forest are covered under the EIS and cannot be treatment with the TM-BioControl, other actions may be needed if an outbreak was to occur in these areas.

If no direct action is taken to suppress tussock-moth populations and mixed-conifer stands suffer partial or complete tree mortality, it is possible that salvage sales would be completed to remove some of the dead trees.  Once again, salvage would only be considered for situations where tree removal was compatible with other values.  Since tussock-moth can kill entire stands of susceptible host type, and do it quickly, the future risk of forest fires would also need to be considered.  Damaged areas in the urban-wildland interface zone might pose a particularly high fire risk, so any salvage operations would probably be considered there first.

map of infested area

   Click on map at left and photos below for enlarged view  
image003.gif (310626 bytes)  

Photograph taken of the Indian Creek drainage showing areas of defoliation

Legal descriptions of the defoliated areas include:
 T.7 S., R.27 E., sections 7, 8, 17, 18, 20, 31, 32, and 33
T.8 S., R.27 E., sections 4, 5, 6, 7, 8, 9, 17, and 18
T.8 S., R.26 E., sections 10, 11, 27, and 28
Most of the defoliation seems to be occurring within the Indian Creek area.  It is estimated that the current outbreak has affected a total of 3,500 acres with moderate to extreme defoliation. The following photographs were recently taken within the Indian Creek area.
image005.png (530599 bytes) Tussock moth larvae feeding on a Douglas-fir. 
larvae Tussock moth larvae working their way up a ponderosa pine tree in search of food. This particular tree measured 45 inches in diameter and was approximately 110 to 120 feet tall.  Unknowing to these larvae, the tree had already been defoliated. 
image009.jpg (173991 bytes) Tussock moth cocoon along with several larvae feeding on a young ponderosa pine.
image017.png (1958199 bytes) Active defoliation on a young ponderosa tree (sapling).  Notice the needle cast (die off) caused by feeding larvae. poster6.JPG (413573 bytes)
tree3.JPG (115989 bytes) Large ponderosa pine trees (left and below) completely defoliated.  During severe outbreak conditions, non-host species can fall victim to tussock moth.  This usually occurs when their primary food source becomes depleted and they move onto other sources for food.  They have even been known to feed on grasses and shrubs during extreme outbreak conditions. 
defoliated trees

 defoliated trees

tree1.JPG (223013 bytes)

tree tops

tree2.jpg (198046 bytes)

Return to top

HOME / Forest Products / Location Map / Annual Report / Recreation Opportunities / Forest Plan /NEPA/ 
Maps / Natural Resources / Fire Information / Monitoring / Tower EIS / Tussock Moth EIS / Phone Numbers & Addresses

Updated 08/09/01
Edited by: L. Dillavou
Contact: Don Justice