Dwarf Mistletoe Lesson Packet
- August 17, 2020 -

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Learn about host/parasite interactions and dwarf mistletoe (Arceuthobium) through this lesson. 

Get this lesson: You can download the full packet here or read a condensed version of this unit below.

Worksheet: Download just the worksheet or there’s a copy included in the packet.


Host/Parasite interactions: In evolutionary ecology, a parasite is an organism, plant, or fungus that lives in or on another organism, called the host. Without a host, a parasite cannot live, grow, or multiply, so it does not usually kill its host. Both parasite and host evolve together, in a relationship where the parasite benefits from the host, harming it in some way. Some hosts develop ways of getting rid of or protecting themselves from parasites, while others live with the relationship for some time.

Dwarf Mistletoe is a small, leafless parasitic plant which parasitizes native ponderosa and lodgepole pines by slowly robbing them of food and water. They will generally focus their life cycle on one species of pine tree. Dwarf mistletoe has very little chlorophyll so they must put roots into the host tree to extract nutrients. Often at the anchor site this parasitic species will secrete hormones to produce a structure called a “witches broom” which is an overgrowth and will disrupt the branching structure. Dwarf mistletoe can stunt tree growth, reduce seed production and wood quality, and occasionally kill the host tree in times of drought or forest stress.


Dwarf mistletoe also has a well studied connection to fire events. As early as the 1970s it was evident that fire suppression was a primary driver of increased dwarf mistletoe abundance in North American forests. A denser forest will aide the reproduction success of this parasite. The resulting witches brooms can act as fire ladders.

Some benefits of this parasite are that squirrels and blue grouse like to eat the mistletoe and infected branches, and witches’ brooms can serve as ideal nesting platforms for birds and small mammals.  

Video resources:

Dwarf mistletoe overview – Visuals and an explanation of what dwarf mistletoe is, plus a comparison between that and the “holiday” version of this plant. 

Dwarf mistletoe music video –  A fun, approachable music video made by students about the seed “explosion” process.

Mistletoe/tree interactions – A concise description with footage of actual infected lodgepole pines discussing the impacts of mistletoe growth. 

Sample Research Project:

Description: students analyze forests near their home for signs of dwarf mistletoe interaction.

Methods:  Students look for signs of parasitism in their local forests within a study area measured with a transect. Students track findings in multiple areas to get an average. If known beetle damage is present, students can also observe localized areas within a single tree using a quadrat.

Sample Research Questions: 

  • Is there more mistletoe on trees at a lower or higher elevation?
  • Is mistletoe the same size or different sizes on each tree? How might this correlate?
  • Do trees that are closer together have more mistletoe?
  • Do larger or smaller trees have more mistletoe?
  • Which part of the tree has the most infection?

NGSS Standards:


MS-LS2-2; MS-LS2-3; MS-LS2-4 Ecosystems: Interactions, Energy, and Dynamics

HS-LS2-3; HS-LS2-6; HS-LS2-8 Ecosystems: Interactions, Energy, and Dynamics


  • Analyzing and interpreting data
  • Constructing Explanations and designing solutions
  • Scientific knowledge based on empirical evidence
  • Planning and carrying out investigations


  • Stability and change
  • Patterns
  • Cause and effect




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