Because of the extensive timber loss caused by dwarf mistletoes in forests of western North America, considerable efforts have been made to control them. Some early forest pathologists in the West recognized that silvicultural controls were feasible and outlined suggestions for their implementation, notably Korstian and Long (1922) in the Southwest, Weir (1916c) in the Northwest, and Meinecke (1914) in California. Various alternative controls are applicable in different types of stands. Most emphasis, however, is given to silvicultural controls, as these are the only methods economically practical in most situations. Potentials for biological and chemical controls are also discussed, although their use is presently limited. Current efforts to develop dwarf mistletoe-resistant trees is also described.
In spite of the many organisms that are pathogens or predators of dwarf mistletoes (chapter 8), none have been sufficiently studied to consider developing them as biological controls (Hawksworth 1972). Insects (particularly various lepidopteran larvae) and some fungi are often highly destructive to dwarf mistletoes in some areas in certain years. However, these are indigenous organisms that have co-evolved with their hosts; their overall effects on dwarf mistletoe populations are probably negligible. Initial research has identified three destructive insect predators that are apparently endemic to Pakistan (Baloch and Ghani 1980), but no steps have been taken to test their applicability for introduction into North America. Other Asian dwarf mistletoes may also harbor candidates for biological control of New World dwarf mistletoes. The likelihood of developing insects or fungi as practical control agents in the near future, however, seems remote.
The development of a selective herbicide to control dwarf mistletoes has been a primary, but elusive, goal for decades. Gill (1955) listed 260 tests that had been made by 7 investigators in the western United States and Canada. Nearly 60 different chemicals were tested (most were various formulations of 2,4-D or 2,4,5-T), but none were effective without also injuring the host tree. Furthermore, none of the chemicals tested affected the endophytic system of the dwarf mistletoe, and resprouting typically occurred. Gill’s report was followed by additional studies (Quick 1962, Scharpf 1972). The results, however, were generally similar to those obtained in 1955, although Quick (1962) suggested that some formulations of 2,4,5-T showed promise as a practical control in high-value trees. Quick (1964) conducted extensive herbicide tests for dwarf mistletoes involving over 2,500 trees in California and concluded that the isooctyl ester of 2,4,5-T was the most effective in killing mistletoe shoots with the least damage to the host. In spite of its early promise, 2,4,5-T found little acceptance as an operational management tool for dwarf mistletoe control and is now banned because of concerns over environmental effects.
From 1970 to the early 1990’s, Arthur Moinat (personal communication) tested Arceuthobium vaginatum subsp. cryptopodum in Colorado against many herbicides and growth regulators including 2,4-D (Dacamine), MCPA, 2,4-DB (Butyrac), oxyfluofen (Goal), MCPM (Thistrol), silvex (Weedone), (Emulsamine), GA-41065 (Prime), and ethephon (Florel). He obtained high mortality of dwarf mistletoe shoots and minimal host damage with most of these herbicides. None, however, affected the endophytic system and resprouting commonly occurred. His experiments with systemic chemicals that might also affect the endophytic system have been inconclusive to date.
The ethylene-releasing growth regulator Florel—active ingredient, ethephon (2-chloroethyl phosphoric acid)—is the most promising chemical for inducing dwarf mistletoe shoot abscission (Hawksworth and Johnson 1989b), and it is the only chemical approved by the Environmental Protection Agency for use on dwarf mistletoes in the United States. The success with ethephon for controlling Arceuthobium pusillum on Picea mariana in Minnesota renewed interest in testing this growth regulator on various western dwarf mistletoes (Livingston and Brenner 1983a, 1983b; Livingston and others 1985). Tests with A. americanum on Pinus contorta in Colorado showed that ground spraying of this chemical was effective in causing dwarf mistletoe shoot abscission (Nicholls and others 1987a, 1987b). However, applications by helicopter of ethephon to P. contorta in Colorado (Robbins and others 1989) and to P. banksiana in Manitoba, Canada (Baker and others 1989) were not effective. Presumably, adequate deposition on dwarf mistletoes shoots could not be achieved by aerial application. Ground spraying of ethephon for controlling A. vaginatum subsp. cryptopodum on P. ponderosa in Colorado was also effective (Nicholls and others 1987a, Johnson and others 1989). To date ethephon has been tested on 9 host–parasite combinations (Hawksworth and Johnson 1989b, Frankel and Adams 1989):
Preliminary results from these tests are promising, yielding shoot abscission rates of 90 to 100% when coverage is thorough. However, rapid resprouting from the endophytic system in some species, e.g., Arceuthobium campylopodum (Parks and Hoffman 1991), may limit its effectiveness. Tests are currently underway to determine the extent of resprouting and the interval required to re-initiate flowering and fruit production. There has been limited, premature browning of older host needles in some tests, but in general there appear to be few serious side effects on the host (Nicholls and others 1987a). If resprouting is extensive following shoot abscission, then ethephon will not provide long-term dwarf mistletoe control. Seed production by the dwarf mistletoe, however, can be delayed by approximately 2 to 4 years, depending on the host–parasite combination and local environmental conditions.
Ethephon can reduce the rate of spread of dwarf mistletoe and protect understory trees beneath infected trees, but its use cannot cure infected trees as the endophytic system remains active. Spraying infested stands without an understory is not recommended, and application should be restricted to trees in high-value areas, such as recreational, residential, and commercial sites.
Occasional evidence of host resistance to dwarf mistletoes has been noted in several host-parasite combinations:
Information on the resistance of western conifers to dwarf mistletoes is reviewed by Roth (1978) and Scharpf (1984, 1987). Genetic resistance of Pinus ponderosa to infection by Arceuthobium vaginatum subsp. cryptopodum in Colorado was discussed by Roeser (1926) and Bates (1927). They observed a slow-growing form of P. ponderosa that exhibited less infection than most other trees in the area and attributed the resistance to "lack of succulence" in the bark. However, nearly 50 years after outplanting seedlings grown from seeds of "resistant" or "susceptible" trees, there were essentially no differences between the two groups with respect to either the incidence or severity of infection by the dwarf mistletoe (Hawksworth and Edminster 1981).
The most extensive studies of resistance are by Roth (1953, 1966, 1971, 1974a, 1974b) and Scharpf and Roth (1992) for Arceuthobium campylopodum on Pinus ponderosa. Roth (1966) noted that some P. ponderosa from western Oregon had "drooping" needles. He suspected that some resistance was imparted by this characteristic because dwarf mistletoe seeds would tend to slide off needles onto the ground, rather than onto safe-sites where infection could occur. However, grafts from these trees did not exhibit the drooping needle habit, and there was no evidence of resistance (Roth 1974a). Furthermore, some Mexican pines that are consistently characterized by drooping needles (notably P. lumholtzii and P. patula) are severely infected by dwarf mistletoes (Hawksworth 1991a).
Roth (1971, 1974a) showed that seedlings of Pinus ponderosa from less susceptible parents also were subject to less infection and exhibited faster growth rates than comparable seedlings from more susceptible parents. Roth (1974b) also observed that susceptibility to infection decreases with increasing tree age to 50 years. He further demonstrated that marked differences existed among trees with respect to both infection susceptibility and subsequent pathological effects.
In a test on the Pringle Butte Experimental Forest of central Oregon, small trees produced by grafting scions from dwarf mistletoe-resistant selections of Pinus ponderosa were planted in a heavily infested stand of P. ponderosa. After 20 years, high levels of resistance were found in grafts of several selections from the Ochoco and Deschutes National Forests (Scharpf and Roth 1992). Grafts and seedlings from susceptible selections showed no resistance.
An example of resistance to infection by Arceuthobium campylopodum was observed in low-elevation Pinus jeffreyi from Placer County, California (Scharpf 1987). Seedlings from the Placer County population that were planted near infected trees showed much lower levels of infection 13 years after planting than seedlings of P. jeffreyi from other sources:
| Seed source |
Mean elevation (m) |
Percent of trees infected |
Average 6-class rating (DMR) |
|---|---|---|---|
| Alpine | 2,510 | 100 | 6.0 |
| El Dorado | 1,920 | 86 | 2.8 |
| Ormsby | 1,925 | 50 | 1.6 |
| Placer | 1,125 | 17 | 0.3 |
In a subsequent test, artificial inoculations were made on 7-year-old trees from the same Pinus jeffreyi seed sources. Again, the Placer County seed source showed higher levels of resistance (Scharpf and others 1992). Resistance in the Placer population is therefore heritable (Scharpf and others 1992), and outplantings from this seed source have been made in several state and national forest campgrounds in central and southern California.
Pinus contorta trees that are apparently resistant to mistletoe have been found in a small stand of fast-growing trees on the Colorado State Forest (G. Fechner personal communication). Preliminary inoculation tests on seedlings from this site and on seedlings from susceptible sources suggest that initial infection after 2-3 years is lower on the "resistant" source, but results are not yet conclusive (unpublished data).
Smith and others (1993) found significant differences in resistance between artificially inoculated trees propagated by grafting from selected "resistant" and "susceptible" Tsuga heterophylla. Resistance was considered to be operating "within rather than outside the host branch."
Guidelines for silvicultural control of dwarf mistletoes were discussed by Baranyay and Smith (1972), Graham (1967), Hawksworth and Johnson (1989a), Hawksworth and Shaw (1984), Johnson and Hawksworth (1985), Kimmey (1957), Kimmey and Graham (1960), Scharpf and Parmeter (1978), Schwandt (1977), Weir (1977), and Wicker and Hawksworth (1988).
Various options exist for managing forests infested with dwarf mistletoes. Actual treatments of stands, however, must be decided on an individual basis. Dwarf mistletoes are most easily and economically controlled by sound silvicultural practices and not through separate management activities. Detailed guides have been developed for some of the more economically important dwarf mistletoes (Hawksworth and Scharpf 1978):
Management of stands infested by dwarf mistletoe involves detection, evaluation, prevention, and suppression of the pathogen. These activities must follow in a carefully planned sequence for successful reduction of disease. Several features of dwarf mistletoes make them ideal candidates for cultural management (Johnson and Hawksworth 1985):
Several successful silvicultural strategies have been developed specifically for dwarf mistletoe control. However, these practices need to be integrated into management plans that also reduce susceptibility of stands to other pests. For example, outbreaks of Dendroctonus ponderosae (mountain pine beetle) may increase the proportion of trees infected by dwarf mistletoes by killing more uninfected trees, thus increasing spread and intensification of dwarf mistletoe in the remaining stand and ultimately increasing mortality from dwarf mistletoe. In this case, the management objective should be to reduce the incidence and severity of both dwarf mistletoe and insect, thereby creating a healthier, pest-resistant forest.
Infestations of dwarf mistletoes affect not only timber value but also recreation, aesthetics, fire hazard, wildlife habitat, and watershed. Depending upon stand age and management objectives, appropriate strategies should be designed to either suppress the dwarf mistletoes or prevent them from entering the stand. Priorities in control programs usually should be placed on prevention as it is much more effective than removing dwarf mistletoes after they have entered stands or than replanting severely damaged stands. The following actions should be considered:
These strategies all reduce the likelihood of dwarf mistletoe spreading into subsequent stands. For stands that are already lightly infested, infected overstory and infected understory trees should be removed by sanitation thinning. Crop trees should be disease free; however, if necessary, lightly infected trees may be retained to meet minimum stocking guides. Replacing severely infested stands with healthy stands by clearcutting, roller chopping, or prescribed burning and regenerating may be required.
Effects of dwarf mistletoes on a stand depend on a combination of factors: intensity of infection, stand density, stand structure, and composition. For a given intensity of infection, effects are most pronounced in dense stands. For example, in 60-year-old stands of Pinus contorta with moderately heavy infestations (stand DMR of 2) estimated volume reduction is about 10, 20, or 30% for stands of growing stock level 12, 24, or 36 m2/ha, respectively.
The techniques employed depend on individual situations, including stand age, structure, density, species composition, number of years to harvest, incidence and distribution of dwarf mistletoe, and length of time the stand has been infested. Tree, stand growth, and dwarf mistletoe infection models are available to assist the resource manager simulate yields of stands with infected trees (Demars and Barrett 1987, Edminster 1978, Edminster and others 1991, Hawksworth and others 1992c). Yields for a stand can be predicted under various management regimes and compared to no-treatment alternatives. By comparing outputs and economic analyses of control costs, foresters can determine the preferred management alternative for each infested stand.
The opportunity to control dwarf mistletoe is greatest at the time of final harvest and, secondly, in recently regenerated stands, 5-15 years old. Sanitation is the primary emphasis of management in these young stands. The greatest dwarf mistletoe threat to regeneration exists where harvest of the previously infested stand was incomplete, and infected residual trees were left on site. Such residual trees are often left because they have no commercial value. Timber contracts should stipulate felling of diseased, nonmerchantable trees to prevent infection of the regeneration. Unfortunately, dwarf mistletoe control was not adequately addressed in many past timber sales, and remedial silvicultural management work is needed in such stands.
Infected residuals over 3 m tall should be felled. Shorter infected trees pose little threat, because infections will be located in the lower half of the crown and dwarf mistletoe seed dispersal will be minimal. Also, trunk infections on these trees will usually kill very small trees. All visibly infected trees, however, should be removed during subsequent precommercial stand entries.
Infected trees along the edges of openings should be felled back 20 m before the regeneration is 1 m tall or 10 years old. This will markedly reduce infection of the regeneration. If infected residuals have been present for more than 10 years, then regeneration is probably already infected and will require subsequent sanitation to prevent future losses. A survey of infection intensity in regeneration will indicate if sufficient stocking of noninfected and acceptable trees is available for future crop trees. If an infected residual stand has been present for 20 or more years and regeneration is heavily infested, then it may be necessary to clear and replant the site. Guidelines are available for stands of Pinus contorta based on stand age and average stand DMR (Hawksworth and Johnson 1989a).
The appropriate control measure for precommercial stands should be based on survey data that include: (1) size and location of dwarf mistletoe-infested areas, (2) approximate number and location of infected residual trees, and (3) number of potential crop trees. In most precommercial stands, an intensive, systematic survey provides the best method of collecting these data. Data points (fixed or variable radius plots) should be arranged in a systematic grid over the entire area to insure adequate coverage. Spacing between plots should not exceed 200 m, and intervals of 100 m or less are recommended. The extent of dwarf mistletoe infestation should be noted when traversing from plot to plot.
Sanitation should be an integral part of all stand entries for thinning, especially in lightly infested precommercial stands from which all overstory or residual infected stems have been eliminated. Severity of infection, not strictly stand age, is the best criterion to decide whether sanitation is practical. Stands of Pinus contorta with more than 40% of the trees infected (or stand DMR ≥3) are too severely infested to attempt strict sanitation cutting (removal of all infected trees). The removal of this many trees would reduce stocking below minimal acceptable levels (Hawksworth 1978, Hawksworth and others 1977).
The highest priority for precommercial sanitation thinning should be for stands 10 to 20 years old with less than 40% of the trees infected. Potential crop trees should have no visible mistletoe infections. Stands should be thinned and sanitized only if evaluations indicate that a minimum acceptable stocking can be achieved with noninfected trees. Severely infested stands that lack acceptable stocking of potential crop trees should be harvested or destroyed (if no products can be salvaged), and the site regenerated. Simulation programs such as described by Edminster and others (1991) and Hawksworth and others (1992c) can be used to project growth of the stand to determine whether or not replacement is the best alternative. In stands where potential crop trees average ≥5 cm diameter at breast height (dbh), the order of priority for crop tree selection is:
If acceptable stocking cannot be obtained within these guidelines, then perhaps the stand should not be thinned. Thinning crews must be able to recognize infected trees for sanitation treatments to be effective. Stands sanitized prior to harvesting should not need to be controlled for dwarf mistletoe again until they are entered for a commercial timber sale.
Survey information that includes the location and intensity of dwarf mistletoe infection is essential to determine the need for control. Yield projections for these stands are invaluable to determine whether infection levels are high enough to influence growth and yield. Sanitation thinning is recommended only where stand DMR is 3 or less and where removal of infected trees does not reduce stocking levels below accepted minima. Severely infested stands that lack acceptable stocking of potential crop trees should be harvested early, and the site regenerated. If nonsusceptible species are present, then those species should be favored during stand regeneration.
Mature stands that are infested and scheduled for harvest and regeneration offer the greatest opportunity for disease control by replacement with noninfected regeneration. Severely infested stands with sufficient cones may be clearcut, and the cone-bearing tops scattered to obtain natural regeneration. Alternatively, slash may be burned, and the site replanted. Clearcuts within infested stands should have as large an area-to-perimeter ratio as allowable to minimize re-invasion from infected trees along bordering stands. Narrow strips should be avoided, and cutting units should be no less than 8 ha to minimize edge effect (Dooling and Brown 1976). However, smaller cutting units may be needed where natural regeneration is desired for trees with nonserotinous cones and border trees provide the only seed (Alexander 1986a). Wherever possible, cutting boundaries should be located in noninfected stands, nonsusceptible timber types, and natural or artificial openings to prevent reinfection of the regenerated stand from adjacent infested stands.
In areas where retention of infected trees is needed to protect regeneration from adverse climatic conditions or where wildlife or aesthetic values require protection, a shelterwood prescription may be required. However, once the site has regenerated and a new stand is established (5 to 10 years), all infected trees should be felled or harvested to reduce infection in the regeneration. In Colorado, Pinus ponderosa trees rated as DMR class 3 or higher should not be left as seed trees (Alexander 1986b). In the Southwest, Heidmann (1983) recommends that 5 to 9 m2/ha (20 to 40 ft2/acre) of basal area in uninfected seed trees should be retained; if infected trees must be retained, then the basal area in seed trees should be doubled to provide an adequate seed source.
Suburban residential developments are now common in forested areas. These sites often are located in, or contain, stands that are infested by dwarf mistletoes. The object of dwarf mistletoe control in these sites is to retain tree cover by reducing the effects of infection on tree vigor and longevity and by preventing spread of dwarf mistletoe into areas not yet infested. Effects of dwarf mistletoes on tree growth rates are of minimal concern, except as they might eventually affect host vigor and mortality. Silvicultural techniques discussed earlier for commercial forests (clearcutting, sanitation thinning) are less acceptable alternatives in developed areas. Emphasis should therefore be placed on the introduction of nonsusceptible species into the understory and on favoring existing nonsusceptible species. Pruning of infected branches and witches’ brooms and establishing buffers to prevent spread of dwarf mistletoe are additional options.
Pruning infected branches usually is not economical in large commercial stands because repeated treatments are often necessary to eliminate latent infections. Pruning may be practical, however, to save trees that are needed for stocking and to prolong the life of high-value trees in developed recreation, administrative, or home sites (Brown 1978, Hawksworth and Johnson 1993, Laut 1978, Lightle and Hawksworth 1973, Perry 1922, Weir 1923). Candidate trees for pruning should be infected only in the lower half of the crown, rate as DMR class 3 or less, and have no infections on or near a main stem less than 15 cm dbh. Because dwarf mistletoe infections on stems over 15 cm dbh have little impact on growth and produce few seeds, they are consequently of little management concern (Mark and Hawksworth 1974, Walters 1974).
To minimize the chances of pruning branches in which the dwarf mistletoe’s endophytic system may already have entered the trunk of the tree, the following species-specific pruning guides have been developed:
All live branches in the two whorls above the highest visibly infected branch, should be pruned if possible, but removal of more than one-half of the tree’s live crown is not recommended. Pruning usually does not eliminate all the dwarf mistletoe because many small infections are overlooked or latent. Pruned trees should be re-examined 3 to 5 years later to see if additional branch removal is needed.
Pruning of witches’ brooms may also be effective in prolonging the life of individual trees (Lightle and Hawksworth 1973), but all visibly infected branches need not be removed. Dramatic recovery has been demonstrated in crown vigor and tree longevity in broom-pruned Pinus ponderosa in Arizona (Lightle and Hawksworth 1973), P. jeffreyi in California (Scharpf and others 1987), and P. contorta in Colorado (Hawksworth and Johnson 1989a).
Some additional activities that should help mitigate effects of dwarf mistletoes in high-value stands include: (1) supplemental watering, (2) application of nitrogen fertilizer, (3) minimizing soil compaction by humans or machinery, and (4) protection of trees from bark wounding. In planning locations for new recreation sites, dwarf mistletoe-infested stands should be avoided when possible, or at least treated, before developmental activities are initiated.
With increased awareness of the complex interactions among dwarf mistletoes and other biotic associates (chapter 8) and heightened interest in biodiversity and forest health, mistletoe control assumes a new dimension. Dwarf mistletoes are no longer viewed as "pests" requiring eradication but as biotic agents of disturbance that can profoundly effect forest communities. From this perspective, emphasis is on remediation, and mistletoe control can be used to achieve non-traditional objectives with new management techniques. For example, Pinus ponderosa trees severely broomed by Arceuthobium vaginatum subsp. cryptopodum could be selectively killed by prescribed burning of individual trees to provide snags for cavity-nesting birds and roost trees for wild turkey while also reducing sources of infection to young trees (Conklin and others 1991). Because ecological effects of dwarf mistletoe infestation are very complex, attempts to use mistletoe as a natural disturbance tool in management will require continued monitoring of mistletoe distribution, abundance, and population trends.