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August 1934Volume 9, Number 5

DOES MISTLETOE KILL the TREES at GRAND CANYON By Donald Edward McHenry, Junior Naturalist.

WHEN THE EARLY DRUIDS venerated the mistletoe as a sacred or mystic plant, it was because this strange form of vegetation was found growing midway between heaven and earth, but, as they said, really belonged to neither. Little did they suspect that this interesting plant, far from being of the gods, was actually a brigand, exhausting the vitality of the very support which made possible its exalted position and gave it its semi-divine reputation.

Not in the same mystic sense, but none the less with considerable interest do the modern visitors to Grand Canyon National Park view the yellowish green clumps of mistletoe which dot some of our evergreen trees. Mistletoe is most conspicuous on the Utah Juniper (Juniperus utahensis). It has been identified as Phoradendron juniperinum. Much less conspicuous is the mistletoe, Razoumofskya crypotoda on the Western Yellow Pine (Pinus ponderosa). On the Pinyon or Nut Pine, (Pinus edulis), only a few plants of mistletoe, Razoumofskya divaricata, are to be found. As one discovers these parasites and then finds a dead tree near by, he frequently wonders whether or not the tree has been killed by mistletoe. Such a conclusion seems especially probable if the dead tree has the characteristic scrubby growth known as "Witch's broom" on its branches - the adventitious growth induced by the hemiparasitic mistletoe.

Spanish Moss Pinyon Pine on South Rim  McHenry Photo

The writer recently had occasion to carry on investigation in an attempt to answer the question of how much damage this mistletoe actually does. This investigation was undertaken along with the inspection of infected trees in the perk made by Mr. John Miller, Entomologist of the Forest Service, and Dr. E. P. Meinecke, Plant Pathologist of the Bureau of Plant Industry. A number of specimens were examined in various stages of infestation. In every case the early stages were first indicated by mild etiolation or yellowing of the leaves at the tips of the branches, usually uniformly over the entire tree. This became progressively more marked as the infection made greater in-roads. The final stage was characterized by a uniform straw color of the leaves on the Western Yellow Pine while on the Junipers the branches were almost if not completely stripped bare of foliage.

Although at first one might easily conclude that a tree in the condition just described had been killed by the mistletoe, further investigation seems to indicate that this plant served only to begin a series of events resulting finally in the death of the tree. The mistletoe apparently serves to sap the vitality of the trees. One result of this seems to be the reduction of the resin flow. As suggested by Mr. Miller, a tree in full vigor is probably able to produce enough resin to ward off insect attack. When the flow of resin is great, the attacking insect must either retreat from it or become engulfed. Mr. Miller does not recall having ever seen any evergreens such as those represented in this park, successfully attacked by insects unless the vitality of the trees has been first reduced by some other agency. A sapping action, however, can be directly attributed to the mistletoe on the evergreens. Following primary infestation, the attacking insects are able to bleed the plant of its resin until the flow has been sufficiently reduced to allow the entrance of the insect into the bark. Such insects then proceed to the cambium layer and, feeding upon it, actually bring about the death of the tree. It is thus probable that it is not the mistletoe but the secondary insect infestation which kills the tree. The insects involved include such forms as Long-horned beetles (Cerambyoidae), Click Beetles (Elateridare), Metallic Woodborers (Buprestidae), and other related forms. Even such destructive primary forms of insect infestation as represented by the Black Hills pine beetle (Dendrootonus ponderosae or D. barrberi) or Ips (Ips lecontei) are not able to make successful attacks until the vitality of the tree has been somewhat reduced, at least at the ends of some of the branches. In these latter cases, however, there is no evidence of mistletoe infection.

Another type of infection which causes yellowing of the ends of some individual branches of the Western Yellow Pines in the park gives no evidence of mistletoe attack. Upon close inspection Dr. Meinecke found that the etiolating needles first showed definite alternating transverse dark and light bandings which might at first suggest fungus attack. Careful examination with a hand lens, however, disclosed small red mites (Eriophus) in the axil of the infested leaves and these Dr. Meinecke thought might possibly account for the diseased appearance of the plant. This feature needs further investigation.

Infrequently Pinyon Pines will be found which are covered in varying degrees with Spanish Moss (Usnea florida). A few of these trees apparently are dying from the extremely heavy infestation of this peculiar lichen. Whether or not this plant will actually kill the tree or whether it acts in the same role as the mistletoe is a question which needs further study.

There are numerous trees which have suffered from primary agencies other than those suggested above. Some of them, especially the Western Yellow Pines, have been struck by lightning which has naturally reduced their vitality. As in the case of the devitalized condition induced by mistletoe, the vitality of a tree struck by lightning is reduced to a point at which secondary insect infestation easily sets in and kills the tree.

Occasionally one is struck with the singular appearance of an evergreen in which the entire upper portion is dead while the part beneath is apparently in good health. Examination of such specimens often reveals evidence of attack by porcupine in which the tree has been "ringed". When a porcupine gnaws completely around the stem, cutting off the passage of the sap, it kills all portions above the wound.

There are doubtless other injuries to our trees which could also be discussed with interest, but the writer is only endeavoring to suggest here some causes of the more manifest types. This is in no way intended to be an exhaustive discussion of the subject; nor should the reader conclude that the problem is in any way serious in the park area. Fortunately the most destructive forms of infestation have been all but eradicated by a consistent program of effective control measures. It is believed that very few living Black Hills beetles are to be found in this forest at present. The writer holds the opinion that in the majority of cases where trees infected with mistletoe have been killed, their death is due not to this parasite alone but to the presence of secondary insect infestation, therefore, no effective measure for the control of the spread of mistletoe is being followed. As stated before, the major mistletoe attack is to be found on the juniper trees of the area. Although it has been suggested that in a few cases some of these trees have actually been killed by the mistletoe alone the writer is not acquainted with a well authenticated case of this nature. Further study may reveal that the mistletoe is more deadly per se than we realize.

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