Douglas-fir Tussock Moth Outbreaks
Outbreaks of the Douglas-fir tussock moth have been observed throughout the region over the past few years, particularly in the Sandia Mountains in central New Mexico and the Tonto National Forest in Arizona.
Visitors to the Sandia Mountains east of Albuquerque in the Cibola National Forest have probably seen the areas of the forest that have been stripped of foliage during the past few years. This outbreak started along the bottom of Pino Canyon on the west side of the Sandia Mountains in 2004. The outbreak expanded considerably in 2005 into the upper portions of the Pino Canyon and portions of Domingo Baca Canyon to the north. Evaluations of the outbreak areas througout the summer and fall of 2005 found that many caterpillars were being affected by natural predators (both a virus and several parasites). Because of these predators, the outbreak on the west side of the Sandias collapsed in 2006. New activity, however, was observed on the east side of the mountains (see map below) in late 2005 and into 2006. This defoliation has been quite visible to visitors along the Crest Highway. The tussock moth activity observed during our 2007 surveys was north of the Crest Highway in Capulin, Las Huertas, and Media canyons. Smaller areas of defoliation were also observed in the Cañon del Agua area on the northwest side of the mountains. While significant areas of the mountain have been impacted since 2004, no new defoliation was observed during our 2008 surveys of the Sandia Mountains.
In 2007, defoliation was observed on the Tonto National Forest in central Arizona. Activity was first observed in the Workman Creek area. Defoliation was mapped over 1,500 acres in two locations: the Sierra Ancha Mountains, along the western boundary of the Sierra Ancha Wilderness and a smaller outbreak on the Pinal Mountains. Both of these areas have historically experienced outbreaks of the Douglas-fir tussock moth. Activity was last observed in the Pinal Mountains in 1999 and minor activity in the Sierra Ancha Mountains during the early 1990's. No additional defoliation was observed during our 2008 survey flights.
The Douglas-fir tussock moth is a native insect of the mixed-conifer forests of western North America. It usually goes unnoticed and causes little damage. During an outbreak, however, arge number of caterpillars can consume nearly all the foliage on the affected trees.
The caterpillars feed on the needles of Douglas-fir and true fir trees. Hikers in affected areas may notice the numerous young caterpillars hanging from silk threads as they dispersed in early summer. As the caterpillars mature, they grow to more than an inch in length and can consume large amounts of foliage. The mature caterpillars are quite distinctive due to their hairs and are visible during the summer and early fall.
The caterpillars pupate in a hairy cocoon and then emerge as small gray/brown moths. The male moths are attracted to the females by a pheromone (a chemical attractant) that the females produce. The flightless females lay their eggs near or on top of the cocoon from which they emerged. The hairy egg masses are a common sight on branches in the outbreak area.
Frequently Asked Questions
Will the caterpillars kill the trees?
Whether or not a tree survives depends upon how much of the tree’s foliage the caterpillars consume. During the 1970s outbreak in Bear Canyon, there was substantial mortality of Douglas-fir and white fir trees caused by the heavy feeding of the caterpillars. Many of the heavily defoliated white fir and Douglas-fir trees in Pino Canyon have died.
Why is it called a "tussock moth"?
The caterpillars develop dense tufts, or tussocks, of hairs as they mature. Some people are sensitive to these hairs and can develop rashes if the caterpillars are handled. There are several other species in the tussock moth family that defoliate other tree species and some shrubs.
How long with this outbreak last?
Outbreaks of the Douglas-fir tussock moth typically last around 3 years, though in NM they have been observed to last as long as 7 years. The damage in Pino Canyon became readily apparently in 2004 and increased substantially in early 2005. The insect population in Pino Canyon collapsed in late 2005 due to a virus, several parasites, and a lack of food. In 2006, the outbreak had progressed to the east side of the mountain. Most of the activity in 2007 was observed on the northern portion of the mountains. Outbreaks in both AZ and NM seem to have collapsed. No new defoliation was observed at either affected area.
Can we do anything to stop the outbreak?
Although insecticides can be applied, their use may only be partially effective. Natural enemies, including a virus, often stop outbreaks without human intervention as happend in Pino Canyon. Where the virus is not naturally present, it can be applied. Phermone treatments, which make it difficult for male moths to locate females, are being tested in the Northwest where outbreaks resulting in severe defoliation occur more frequently.
To learn more about the biology of this insect, visit the Douglas-fir tussock moth section of our field guide or download the Department of Agriculture's Forest Insect & Disease Leaflet No. 86 on the Douglas-fir Tussock Moth (PDF, 949 Kb).