Minnesota Profile: Dwarf mistletoe (Arceuthobium pusillum)

Range and Host

Minnesota's only mistletoe species is a parasitic plant that grows on black spruce trees and occasionally on white spruce and tamarack. It is found from the East Coast to Minnesota and central Saskatchewan. This parasite derives nutrition, especially amino acids and sugars, from the host, reducing its growth and eventually killing it. Infections also upset the tree's hormone balance, causing proliferation of twigs in dense clumps called witches'-brooms.

Identification

Dwarf mistletoe is readily identified by witches'-brooms in black spruce crowns. Within the broom, the mistletoe stems can be seen as yellow-red aerial shoots, which bear flowers and fruits. Dwarf mistletoe is not used for Christmas decorations because the plants are small and lose their seeds in the fall. The "kissing kind," called leafy mistletoes, are large plants with persistent berries. Several leafy mistletoe species provide U.S. markets with this holiday accessory.

Novel Seeds

Instead of a hard seed coat, mistletoe seeds have a sticky viscin, which glues them to host surfaces. Unlike most germinating seeds, mistletoe seeds are not sensitive to the pull of gravity. Their dominant growth impulse is to grow away from the light. Thus, a seed attached to the bottom of a twig will grow up into the twig; one atop a twig grows down. In either case they grow into the twig to infect the host tree.

Life History and Biology

Dwarf mistletoe plants are dioecious: male and female flowers bloom on separate plants. Wind and insects carry the pollen in April and May. In the fall, mature fruits fill with fluid, building up pressure until they burst and cast seeds around 20 feet at about 50 miles per hour. Seeds that land on black spruce twigs will germinate in spring. Some seeds stick to feathers of birds such as dark-eyed juncos and gray jays. The birds preen off the seeds, occasionally leaving them on a spruce far from the original source. About four years after a plant takes hold, its aerial shoots produce seeds, completing the life cycle.

Ecological Importance

Witches'-brooms provide nesting sites for some birds. Buds of infected spruce often open very early in spring, yielding sugars for ants and other insects, which, in turn, serve as food for birds.

Management

Dwarf mistletoe is the most serious pest of black spruce. DNR foresters recommend that when a stand is harvested all infested trees should be cut or burned to prevent infection of the next generation.

Don Knutson, past president, Minnesota Native Plant Society