Consider the two Pseudotsuga menziesii (Douglas-fir) infected by Arceuthobium douglasii in the image above. The tree at left is all brooms, stunted, and deformed. The tree at right is dying. The large brooms in live trees may provide preferred habitat for some animals, but neither tree will make a full-size snag that is most valuable to wildlife (see dwarf mistletoe ecology). For timber, the tree at left is largely worthless. The one at right could be merchantable, but only if it is harvested before or soon after it dies.
In western North America, dwarf mistletoes are generally considered the most damaging type of forest disease. There are areas where root diseases are more important, but mistletoes are important in many forest types, on most major tree species, over vast areas. Important forms of damage include:
- Growth loss
- Predisposition to other mortality agents (e.g., bark beetles) and decay fungi (after old stem infections become cankerous)
- Dead tops
- Defect (stem swellings, cankers, other deformities)
- Reduced production and viability of seed
An animation of the modeled impacts of Douglas-fir dwarf mistletoe shows the results of disease progression (Fig. 31 in ). With moderate amounts of dwarf mistletoe at age 80, after 100 years the stand actually has lower basal area than it started with! This is a combined effect of growth and mortality effects. Young trees are particularly vulnerable to permanent stunting, deformity and mortality. But even mature trees have reduced lifespan and growth potential when severely infected.
Significant growth impact is seldom seen until at least the lower 1/3 to 1/2 of the crown is heavily infected. Light infection throughout the crown or heavy infection in just the lower crown usually cause little or no damage, except perhaps when large brooms are produced or stem infections cause problems. Brooms seem to have a special effect on growth. A large broom or several smaller ones can devastate the rest of the crown, leading to severe crown thinning.
Dwarf mistletoes increase host mortality, the effect becoming larger as infection severity increases (following table; data from [10, 11]). Certain dwarf mistletoes are considered to be especially lethal . For example, 4 of the 5 dwarf mistletoes in the southern Rocky Mountains have that distinction.
Increase in 10-year mortality rate in stands infested with dwarf mistletoe
|A. abietinum||Abies concolor||1||3||6||10||15||21|
|A. americanum||Pinus contorta||2||4||5||6||7||8|
|A. vaginatum||P. ponderosa||1||4||8||15||23||34|
|A. douglasii||Pseudotsuga menziesii||1||2||4||9||15||23|
Mortality can be strongly influenced by water relations. It is greater on dry than on wet sites  and also during or following a drought . Large trees survive longer than small trees (next table; data from ), but even large, mature Pinus ponderosa can be killed by dwarf mistletoe .
Effect of dwarf mistletoe on host half-life
|Initial DMR||Half-life for|
4-9 in. DBH
> 9 in. DBH
ND: No decrease in longevity detected; half-life too long to estimate.
In P. contorta stands infested for 80 years, 15% of the standing basal area was killed by dwarf mistletoe (determined by subtracting standing mortality in similar but uninfested stands . However, most of the snags in that study were quite small.
Dwarf mistletoes frequently increase susceptibility to attack by bark beetles [10, 22]. In Colorado, dwarf mistletoe increases susceptibility of P. ponderosa to mountain pine beetle (MPB) and western pine beetle [3, 4, 12, 15, 26]. Infected P. ponderosa may also be selectively attacked by Ips spp. [13, 14]. A similar effect is seen in P. edulis (piñon) [17, 24].
In the case of Pseudotsuga menziesii, dwarf mistletoe may have little effect on bark beetle susceptibility . However, such predisposition has been suggested for northern Idaho .
Lodgepole pine dwarf mistletoe generally decreases susceptibility of Pinus contorta to MPB [16, 20]. However, some authors suggest little or no significant effect [9, 19]. Decreased susceptibility, where it occurs, is probably due to smaller diameter and thinner phloem caused by dwarf mistletoe (Roe & Amman 1970). However, stem infections, which often have thicker bark than the rest of the tree, may be selectively attacked by MPB (McGregor 1978).
It has been estimated that, in western USA, dwarf mistletoes cause direct, annual wood volume loss of 11.8 million m3 . This is roughly equivalent to 3.3 billion board feet. This estimate includes only commercial forest land on National Forests, and it doesn’t include defect and loss of wood quality. There is a great deal more dwarf mistletoe in non-timber lands and in other ownerships. That said, most losses are not realized, i.e., based on current management, much of it likely wouldn’t have been harvested even if healthy. In western Canada, a comparable loss estimate is 3.8 million m3 yr-1 .
In the US study  the highest losses on an area basis were about 1.5 m3 ha-1 yr-1 for Pseudotsuga menziesii and Larix occidentalis in certain regions.
To put this in perspective, 11.8 million m3 is:
- enough wood to build 1.1 million small, 3-bedroom houses each year (3000 bd ft each)!!!
- over 13% of the 2016 harvest volume from the same western states (87.9 million m3) . This is an underestimate because the harvest data include non-USFS land.
- From an ecological perspective, it is enough wood to make trees a lot larger and older if they were neither infected nor harvested.
- 1.Baranyay JA, Safranyik L. 1970. Effect of dwarf mistletoe on growth and mortality of lodgepole pine stands in Alberta. Publication 1285. Canadian Forestry Service, Department of Fisheries and Forestry, Ottawa, Ontario, Canada.
- 2.Drummond DB. 1982. Timber loss estimates for the coniferous forest of the United States due to dwarf mistletoes. Report 83–2. USDA Forest Service, Forest Pest Management, Methods Application Group, Fort Collins, Colorado, USA <https://books.google.com/books?id=bZjlc-6icacC>.
- 3.Frye RH, Landis TD. 1975. Mountain pine beetle and dwarf mistletoe, Lake Creek area, San Carlos Ranger District, Pike-San Isabel National Forest. Biological Evaluation R2-75-4. Denver, Colorado, USA: USDA Forest Service, State and Private Forestry, Region 2.
- 4.Fuller LR. 1983. Incidence of root disease and dwarf mistletoe in mountain pine beetle killed ponderosa pine in the Colorado Front Range. Biological Evaluation R2-83-2. Denver, Colorado, USA: US Dept. of Agriculture Forest Service, Rocky Mountain Region, Forest Pest Management. 8 pp.
- 5.Furniss MM, Livingston RL, McGregor MD, Hedden RL, Barras SJ, Coster JE. 1981. Development of a stand susceptibility classification for Douglas-fir beetle. In: Hazard-rating systems in forest insect pest management. General Technical Report WO-27, pp. 115–128. Washington, DC: USDA Forest Service.
- 6.Geils BW, Cibrián Tovar J, Moody B, eds. 2002. Damage, effects, and importance of dwarf mistletoes. In: Mistletoes of North American Conifers. General Technical Report RMRS-GTR-98, pp. 57–65. Ogden, Utah, USA: USDA Forest Service, Rocky Mountain Research Station.
- 7.Hawksworth FG, Geils BW. 1990. How long do mistletoe-infected ponderosa pines live? Western Journal of Applied Forestry 5(2):47–48.
- 8.Hawksworth FG, Hinds TE. 1964. Effects of dwarf mistletoe on immature lodgepole pine stands in Colorado. Journal of Forestry 62:27–32 <10.1093/jof/62.1.27>.
- 9.Hawksworth FG, Johnson DW. 1989. Biology and management of dwarf mistletoe in lodgepole pine in the Rocky Mountains. General Technical Report RM-169. Fort Collins, Colorado, USA: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station <https://www.google.com/books/edition/Biology_and_Management_of_Dwarf_Mistleto/zu4QPyYdACQC?hl=en&gbpv=0>.
- 10.Hawksworth FG, Wiens D, Geils BW, Nisley RG. 1996. Dwarf Mistletoes: Biology, Pathology and Systematics. Agricultural Handbook 709. Washington, DC: USDA Forest Service. 410 pp.
- 11.Hawksworth FG, Williams-Cipriani JC, Eav BB, Geils BW, Johnson RR, Marsden MA, Beatty JS, Shubert GD. 1992. Interim dwarf mistletoe impact modeling system: users guide and reference manual. , p. 90.
- 12.Johnson DW, Yarger LC, Minnemeyer CD, Pace VE. 1976. Dwarf mistletoe as a predisposing factor for mountain pine beetle attack of ponderosa pine in the Colorado Front Range. Technical Report R2-4. Denver, Colorado, USA: USDA Forest Service, Rocky Mountain Region, Forest Insect and Disease Management. 7 pp.
- 13.Kenaley SC. 2004. Bark beetle and dwarf mistletoe interactions in northern Arizona. Master of Science in Forestry thesis. Northern Arizona University.
- 14.Kenaley SC, Mathiasen RL, Daugherty CM. 2006. Selection of dwarf mistletoe-infected ponderosa pines by Ips species (Coleoptera: Scolytidae) in northern Arizona. Western North American Naturalist 66(3):279–284.
- 15.McCambridge WF, Hawksworth FG, Edminster CB, Laut JG. 1982. Ponderosa pine mortality resulting from a mountain pine beetle outbreak. Research Paper RM-235. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado, USA <https://doi.org/10.2737/RM-RP-235>.
- 16.McGregor MD, Kibbee DL, Berryman AA, Amman GD, Stark RW. 1978. Management of mountain pine beetle in lodgepole pine stands in the Rocky Mountain area. In: Theory and Practice of Mountain Pine Beetle Management in Lodgepole Pine Forests, Symposium Proceedings, pp. 129–139. Moscow, Idaho, USA: Forest, Wildlife and Range Experiment Station, University of Idaho <http://www.usu.edu/beetle/documents2/1978Cole_Mngmt%20Strategies%20for%20Preventing%20MPB.pdf>.
- 17.Negron JE, Wilson JL. 2003. Attributes associated with probability of infestation by the piñon ips, Ips confusus (Coleoptera: Scolytidae), in piñon pine, Pinus edulis. Western North American Naturalist 63(4):440–451 <https://www.fs.usda.gov/treesearch/pubs/39854>.
- 18.Oswalt SN, Smith WB, Miles PD, Pugh SA. 2019. Forest Resources of the United States, 2017: A Technical Document Supporting the Forest Service 2020 RPA Assessment. General Technical Report WO-GTR-97. USDA Forest Service, Washington, DC <https://www.fs.usda.gov/treesearch/pubs/57903>.
- 19.Rasmussen LA. 1987. Mountain pine beetle selection of dwarf mistletoe and comandra blister rust infected lodgepole pine. Research Note INT-367. USDA Forest Service, Intermountain Research Station, Ogden, Utah, USA.
- 20.Roe AL, Amman GD. 1970. The Mountain Pine Beetle in Lodgepole Pine Forests. Research Paper INT-71. Ogden, Utah, USA: USDA Forest Service, Intermountain Forest and Range Experiment Station <https://www.google.com/books/edition/_/clgTOe3N05sC?hl=en&gbpv=1>.
- 21.Roth LF. 2001. Dwarf-mistletoe-induced mortality in northwest ponderosa pine growing stock. Western Journal of Applied Forestry 16(3):136–141.
- 22.Stevens RE, Hawksworth FG. 1984. Insect-dwarf mistletoe associations: an update. In: Biology of dwarf mistletoes: Proceedings of the symposium. General Technical Report RM-111, eds Hawksworth FG, Scharpf RF, pp. 94–101. Fort Collins, CO: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station.
- 23.Weir JR. 1916. Mistletoe injury to conifers in the Northwest. Bulletin of the U.S. Department of Agriculture 360. USDA, Washington DC, USA <https://www.google.com/books/edition/Mistletoe_Injury_to_Conifers_in_the_Nort/9I-t1PAuqCcC?hl=en&gbpv=0>.
- 24.Wilson JL, Tkacz BM. 1992. Pinyon ips outbreak in pinyon juniper woodlands in northern Arizona: a case study. In: Ecology and management of oak and associated woodlands: perspectives in the southwestern United States and northern Mexico. General Technical Report RM-218, eds Hernandez C. VM, Ortega-Rubio A, Hamre RH, Ffolliott PF, Gottfried GJ, Bennett DA, pp. 187–190. Fort Collins, Colorado: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station <https://doi.org/10.2737/RM-GTR-218>.
- 25.Worrall JJ, Geils BG. 2008. Dwarf mistletoes. The Plant Health Instructor, American Phytopathological Society <10.1094/PHI-I-2006-1117-01>.
- 26.Ziegler RS. 1978. The vegetation dynamics of Pinus contorta forest, Crater Lake National Park, Oregon. Master of Science thesis. Oregon State University.