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Dwarf mistletoes are parasites of native conifer forests that can cause severe damage.

Dwarf Mistletoe: Parasitic Plants

Forty-two species of dwarf mistletoe are known worldwide; five species are found in Colorado’s forests. Most dwarf mistletoes are native to western North America, from Alaska south through the western United States, Mexico and Central America.

Dwarf mistletoe, a common problem in Colorado forests, predominantly affects ponderosa and lodgepole pines, although they can attack Douglas-fir, piñon, limber and bristlecone pines.

Dwarf mistletoes are parasites of native conifer forests that can cause severe damage. These plants tend to confine their attacks to one species of tree, with only occasional attacks on other species.

Dwarf mistletoe infections can retard growth and reduce seed production and wood quality; heavy, long-term infections can kill trees.

Some dwarf mistletoe species induce abnormal tree growth at the point of infection, and produce a structure known as a witches’ broom, which disrupts the typical branching structure.

Dwarf mistletoes are small, leafless parasitic flowering plants that kill by slowly robbing the tree of food and water.

Symptoms of Infestation

The first symptom of dwarf mistletoe infection is a slight swelling of the bark at the infection site. The parasite is identifiable when the yellow to green or brownish-green segmented shoots protrude from the infected part of the tree, which form about 2-3 years after the infection. Because dwarf mistletoes are slow killers, long-term management options are feasible.

 Dwarf Mistletoe Species of Colorado’s Forests

Dwarf Mistletoe Species Lodgepole Pine Dwarf Mistletoe
Arceuthobium americanum
Douglas-Fir Dwarf Mistletoe
Arceuthobium douglasii
Southwestern Dwarf Mistletoe
Arceuthobium vaginatum cryptopodum
Limber Pine Dwarf Mistletoe
Arceuthobium cyanocarpum
Piñon Dwarf Mistletoe
Arceuthobium divaricatum
Range in Colorado Widespread; roughly 50% of lodgepole pine in Colorado have some degree of infection Southwestern Colorado and the Western Slope Spotty distribution throughout host range in Colorado Rocky Mountain range throughout Colorado, primarily along the Front Range Throughout host range in Four Corners region and western Colorado
Primary Host Species Lodgepole pine Douglas-fir Ponderosa pine Limber pine; Rocky Mountain bristlecone pine Piñon pine
Plant Characteristics Olive-green to yellow, 2-4 inches long Olive-green, less than 1 inch long Orange, reddish-brown to black, 4-6 inches long Yellow-green, 1-3 inches long Olive-green to brown, 3-5 inches long
Host Response Conspicuous witches’ brooms, heavily infested crowns appear thin Fan-shaped and spherical witches’ brooms Small but conspicuous brooms on branches Small and compact witches’ brooms Small and compact witches’ brooms
Impact Infestations can significantly reduce growth by 80-90% and predispose trees to other insects and diseases Witches’ brooms may weigh in excess of several hundred pounds and cause severely swollen branches Reduced growth, foliage discoloration and dieback reduce vigor of host tree Considered second most important disease of high-elevation five-needle pines, causing severe mortality Can cause increased mortality of piñon pines and significant deformation of host

Source: 2010 Report on the Health of Colorado’s Forests

Management Options

  • Pruning and removing infected trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations.
  • Plant resistant trees under infected ones to replenish the forest after infected ones are removed.
  • If planting the same species as the infected tree, use approved sprays to reduce the risk of infestation. (Always carefully read and follow all label instructions before applying insecticides.)

Learn More About Dwarf Mistletoe

Annual Insect Activity

CSFS Forest Entomologist Dr. Dan West produces an Esri Story Map for the Annual Forest Health Report, which offers users an interactive way to find out about forest conditions and insect activity in their area.

Type your address or the city or town where you live into the search field on this map.

Map powered by the Colorado Forest Atlas from the Colorado State Forest Service