Science: Meet me under the … parasite?
By Shannon Lee
One of the most iconic visuals of winter in our area are the hulking leafless oak trees with limbs draped with Spanish moss and trunks dappled with lichens. The changing of the season also reveals dark green clumps of mistletoe, surprisingly vivacious in character and in a stark contrast to the general pattern of cold-weather shutdown throughout the landscape.
Mistletoe is the common name for a group of species from the order Santalales. All these plants are parasitic, feeding off their host plant through connective structures called haustoria. Mistletoe also nourish themselves through photosynthesis, and so more accurately these types of organisms are called hemiparasites, or “half-parasites.” Although many associate mistletoe with temperate and polar ecosystems, the greatest diversity of mistletoe is found in subtropical and tropical areas.
Records and mentions of mistletoe go all the way back to Roman mythology and the story of Aeneas. Greeks referred to these plants as “oak sperm,” and several Celtic and Norse stories and traditions describe mistletoe as useful in herbology, and also as a symbol of fertility, friendship, and healing. The more recent Christian/Christmas tradition of “kissing” under the mistletoe has various origin reconstructions stemming from ancient vitality associations, stealing an intimate moment from a maiden unable to refuse, and the warding off of, or protecting oneself from, evil spirits.
There are more than 1,000 different species of mistletoe, and while one species is native to North America — eastern mistletoe ( Phoradendron leucarpum) — that is not what we have here in Sonoma County. At the turn of the last century, European mistletoe ( Viscum album) was introduced to Sebastopol, and over the past 120 years its distribution has radiated out about 8 miles from the initial location. The geographic spread has occurred mostly through the dispersal of seeds through the guts of birds who feed on mistletoe berries.
In our county, this parasite has been found in more than 20 different tree species, including oak, maple, alder, locust, apple, and cottonwood. The host trees are fed on by the parasite, and that harm can prove fatal. It is possible to reduce an infection, if caught early on, through the process of removing sections of plant that may be impacted by the haustoria structures. It is possible for the parasites to regenerate from any unremoved haustoria, so it is always best to call in an expert botanist if you are worried about a favorite tree.
Mistletoe is toxic to humans and animals, and Viscum album is one of the most toxic of the species, although mostly not fatal. These plants, however, have been used medicinally and can be effective with proper care and dosing.
Although we might consider parasites to be bad, their importance in a biological community cannot be overlooked. Many organisms rely on the winter-thriving mistletoe as a food source. Trees infected with these parasites often put more energy into reproducing and have better reproductive success. There is growing recognition that the presence of mistletoe may act to enhance biodiversity of the natural community, thereby functioning like a keystone species in the ecosystem.