Arceuthobium pusillum (Dwarf Mistletoe) with ripening fruit growing on the branch of a Black Spruce (Picea mariana) in an intermediate fen. Smaller shoots (about 1 cm tall but maximum height can be 3 cm) are growing below it and to the right. Dwarf Mistletoe commonly parasitizes Black Spruce and occasionally White Spruce (Picea glauca), Tamarack (Larix laricina), and Jack Pine (Pinus banksiana). When the fruit is ripe the seeds of Dwarf Mistletoe are ejected at speeds of about 5 miles per hour (~8 km per hour) for a distance as far as 50 feet (~15 meters). The sticky coating on the seeds helps them adhere to the bark of their host plant where they will germinate and send roots (haustoria) into the tree's tissue to extract nutrients and water.
The Dwarf Mistletoe's growth deforms the branches on which it grows creating clusters of shoots called "witches' brooms" and also flattening and contorting the branches. Some birds like the Gray Jay (Perisoreus canadensis) use these as nesting sites.
''Arceuthobium pusillum'' is a perennial, obligate parasitic plant in the sandalwood family. Its common names include Dwarf mistletoe or Eastern dwarf mistletoe. It is one of the most widespread dwarf mistletoes within its range which covers the eastern United States and Canada, from Saskatchewan to Nova Scotia and New Jersey. The species name "pusillum" derives from Latin "pusillus", meaning very small.