How would you like to be called dung-on-a-twig? There certainly are worse things in life but certainly much better things as well. Dung-on-a-twig is one of the root meanings for mistletoe, which grows on trees. This common name comes from two parts of Anglo-Saxon speech. “Mistel” a common word for dung, and “tang” the word for twig, combine to form the word mistletoe or “dung on a twig.” This name became prevalent as it was noticed that mistletoe would appear where many birds had landed on branches and deposited their excrement, nice. This puts a whole new context on the holiday tradition for kissing under the mistletoe, eh?
Whether in holiday greenery or in deciduous trees, mistletoe is a visible symbol during the winter months. While it might be good for kissing your loved-one under, it’s certainly not good for trees. Mistletoe negatively impacts both tree health and structure. It is a parasitic plant (actually it’s hemi-parasitic but I’ll explain that later) which instigates branch decline and wood decay as well as creates entry points for insect pests and disease.
Because mistletoe occurs world-wide, there are hundreds of species. Two kinds of mistletoes are native to the United States: the dwarf mistletoe and the one commonly associated with our kissing customs. The dwarf mistletoe is found from central Canada to western United States and Mexico. The kissing kind is found in trees from Long Island south to Florida and west through Ohio to Texas. It’s commonly known as leafy mistletoe, Eastern mistletoe or American mistletoe. Over time there have been numerous attempts to name it. In fact, the scientific name has been changed 20 times over the past 225 years as different taxonomists labored to categorize the genus Phoradendron. The most recent re-classification was in 2003 naming it Phoradendron serotinum. Or, you could just stick with the Anglo-Saxons and call it dung on a twig. Or, you could go with the Greek meaning of Phoradendron, which more aptly names mistletoe ‘thief of the tree’.
No matter what we call it, its presence and perpetuation on a tree is not desired by arborists. Mistletoe commandeers growth resources such as water, essential elements and food from trees. Mistletoe is an evergreen, perennial, parasitic flowering plant. More precisely, it is classified as hemi- or partially parasitic because even though it does manufacture most of its own food, it also steals all other essential resources from its host. This ancient plant has an aerial portion consisting of leaves, stems and reproductive parts like other flowering plants. It also has a resource-gathering organ, which establishes and grows inside a tree twig or branch. This structure, called a haustorium, is akin to something you’d see in an alien invasion movie; the host is infected with the embryo of an alien being and it begins to grow and take over the body. Ok, well not quite, but it’s close. Let’s go back to the dung part of the story. Once a digested seed is deposited on a twig, a series of steps enfold which develop this haustorium. They are root-like structures that penetrate through the twig bark, infiltrate wood tissue and trick the tree into thinking that it’s another twig. In turn, it readily extracts vital water and nutrients from the tree.
Year after year, the haustorium expands in length and girth, within and on top of the twig. Over time, the mistletoe decreases branch health, girdles the twig, dries out the wood and creates entry crevasses for insect pests and decay fungi. All in all, the twig or branch is weakened both in health and structure, making it more susceptible to break. Once mistletoe gets a foothold in a tree, it seems to perpetuate more readily. We attribute this to the simple fact that birds hang out, eat and poop within the canopy of the same tree.
American mistletoe is seen primarily on willow, water and laurel oaks, Bradford pear and river birch trees but make no mistake; it has a wide range of host trees it will invade, such as maples, pecans and elms. Even though I’ve seen trees with so much mistletoe in the canopy it looks like summer in the winter, I’ve never seen it actually kill a full grown tree. However, the greatest threat for a tree is loss of water from mistletoe infection. This certainly can affect tree health, especially during prolonged periods of drought and may eventually affect tree longevity.
Mistletoe not only has a negative impact on trees. According to Web MD “American mistletoe is UNSAFE. All plant parts are considered hazardous if ingested and can cause nausea, vomiting, diarrhea, decreased heart rate, hallucinations and heart problems.” However, a study in the Journal of Toxicology 34(4):405-408 cited that out of 92 Americans who reported mistletoe ingestion, only 11 of them presented any symptoms, such as stomach and intestinal upset, mild drowsiness, eye irritation, seizure and uncontrolled muscle movements. No deaths were reported.
On the flip side, mistletoe is, according to the US Geological Survey, important in other ways. It can provide essential food, cover and nesting sites for a variety of critters in the United States. Extensive studies in western forests reveal the importance of dwarf mistletoe (American mistletoes’ western cousin) for particular species of birds, butterflies and insects. Berry-eating birds such as grouse, mourning doves, evening grosbeaks, American robins and cedar waxwings are among those who benefit from mistletoe.
According to butterfly expert and Colorado State University professor Paul Opler, three kinds of butterflies in the United States are dependent on mistletoe for their survival. The great purple hairstreak, says Opler, is the only butterfly in the United States that feeds on American mistletoe. This beautiful butterfly lays its eggs on the mistletoe, where the resulting caterpillars thrive on a mistletoe diet. In the northwest, the great purple hairstreak and the Johnson’s hairstreak caterpillars feed on dwarf mistletoes.
Regarding insects, the hickory horned-devil moth can consume a large volume of American mistletoe foliage in a short time. Mistletoe may also be a source of pollen and nectar for honeybees and other native bees, says Erik Erikson, a (former) bee researcher at the USDA Bee Research Lab. And then there are the mammals. The berries and leaves of mistletoe provide high-protein fodder for many mammals, especially in winter. Researchers have documented that animals such as elk, cattle and deer eat mistletoe during winter when fresh foliage is rare. In Texas, some ranchers even consider mistletoe on mesquite trees as a forage crop. Other mammals that eat mistletoe include squirrels, chipmunks and even porcupines.
So what’s an arborist to do? The easiest thing is nothing and allow Mother Nature and the cycle of life to take its course. On the other hand, if mistletoe is overwhelming a tree or just beginning to infect a young tree, it can be pruned. A plant cluster can be removed either by breaking off the main stem of the mistletoe from the twig or to cut the branch at least 14 inches away from the point of mistletoe attachment. The first method is quick and easy but the mistletoe will likely grow back in a few years. The second method helps ensure that the wood infected by the haustorium is removed and cannot re-grow from within the twig. A third method is chemical application to the canopy or injected into the trunk. The cost is generally high and the efficacy not always successful.
American mistletoe is a distinct plant which has a unique ecological role. Its benefits for wildlife are compelling and must be considered and balanced with its long-term control, particularly in urban landscapes where socially significant or valuable trees, which cannot be easily replaced, are present.
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