2.1. TAXONOMY
A detailed discussion of the taxonomy and taxonomic history of the genus Arceuthobium is provided by Hawksworth & Wiens (1996). The genus Arceuthobium is a member of the plant family Viscaceae and is a clearly defined group of small (generally less than 20 cm high), variously colored flowering plants that are aerial parasites on conifers of the families Pinaceae and Cupressaceae. They are considered to be the most evolutionary specialized genus of the family Viscaceae. Some features which make Arceuthobium unique among mistletoes include:
Arceuthobium has been previously included in the subfamily Viscoideae of the family Loranthaceae, however, the subfamilies Loranthoideae and Viscoideae are now generally considered to have family status (Loranthaceae and Viscaceae) although there does not appear to be one single characteristic used to separate them. Members of the family Loranthaceae are larger plants with large, conspicuous flowers and a rim-like growth (calyculus) on the ovary whereas the Viscoideae are smaller plants with inconspicuous unisexual flowers that lack a calyculus (Kuijt 1969). The Viscoidae contain the mistletoe genera Arceuthobium, Dendrophthora, Ginalloa, Notothixos, Phoradendron, Korthalsella and Viscum (Hawksworth & Wiens 1996).
It is speculated that the genus Arceuthobium evolved in eastern Asia. At one time Korthalsella was considered to be the closest living genus because species in this genus have mildy explosive fruits. This genus presently has a wide distribution from eastern Africa and Madagascar through southern Asia, Australasia, the South Pacific and Hawaii. Arceuthobium, on the other hand, may have undergone two major radiations. One radiation, on Juniperus spp, was westward through the Near East and Mediterranean region. The second radiation was northeastward through the Bering area into the New World. This latter group parasitizes only members of the family Pinaceae (Hawksworth 1987). More recent investigations suggest that the fruits of Korthasella may not be truly explosive and molecular analyses do not indicate which genus is the closest relative of Arceuthobium. Both Notothixos and Viscum also have centers of diversity and possible centers of origin in Asia (Hawksworth & Wiens 1996).
Arceuthobium oxycedri, the subject of this paper, was the first dwarf mistletoe species described and was originally included in the genus Viscum. This species was segregated into a new genus Razoumofska in 1808. Arceuthobium was first proposed as a genus ca 1819 and was generally used until the early 1900's. In 1905, the Vienna Botanical Congress conserved Arceuthobium over Razoumofska. However, because the American code for botanical nomenclature gave priority to the first designated name, Razoumofska remained in use in the New World until the 1930's.
As of 1996, 42 species of Arceuthobium have been described. Most species (34) occur in the New World and are found primarily in western Canada, the western United States and Mexico. The eight known Old World species are found in the Mediterranean Region of Europe and northern Africa, the Near East, the Indian subcontinent and China with outlying populations in the Azores, Ethiopia and Kenya. The genus is subdivided into two subgenera; Arceuthobium and Vaginata. The characteristic used to separate the two subgenera is branching patterns; verticillate (whorled) in Arceuthobium and presence of flabellate (fan-like) branching in Vaginata. All of the Old World species are in the subgenus Arceuthobium (Hawksworth & Wiens 1996).
2.2. BIOLOGY
The biology and life history of all dwarf mistletoe species is somewhat similar (Fig 1). The following summary of the generalized biology of dwarf mistletoes is taken from various sources (Hawksworth 1978, Hawksworth & Wiens 1996, Hawksworth & Johnson 1993)
2.2.1. FLOWERING AND SEED PRODUCTION - Dwarf mistletoes reproduce only from seeds. All dwarf mistletoes are dioecious and have both male (staminate) and female (pistillate) plants (Figs 2-3). Flowering and pollenation may occur either in spring or summer to early fall and is usually confined to a 2-4 week period. Exceptions are certain tropical species, such as A. aureum subsp. aureum, which flower continuously or species such as A. juniperi-procera which may have several distinct periods of flowering. Dwarf mistletoe flowers are pollinated by either insects or wind.
The seeds of most species mature in late summer or fall (July-November). Female plants produce clusters of berry-like fruits, each containing a seed with a single embryo and a viscous coating which is dispersed explosively.
2.2.2. DISPERSAL - When the seeds mature, a hydrostatic pressure builds up in the seed coat which forcibly releases the seed from the fruit. When released, seeds are capable of traveling at a speed of about 27 m/second and distances of up to 16 m from the point of release. Dispersal distances of about 10 m or less are more typical, however. Most seeds are intercepted within 2 to 4 m by the foliage of host plants and approximately 40% of the seeds
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are intercepted. In dense forests, seeds are frequently intercepted by the foliage of trees containing the dwarf mistletoe plant resulting in secondary infections. Interception of seeds by foliage of host plants is the most common means of dispersal of dwarf mistletoes. This results in a typically patchy pattern of infection and a relatively slow rate of spread. Dwarf mistletoes spread at a rate of about 0.3-0.6 m per year through single storied stands. The rate of spread is more rapid in open stands than in dense stands. However, as stands become more open, the rate of spread drops and in very open stands, explosive dispersal is no longer effective. Spread is also faster in multistoried stands than in single storied stands. The most rapid spread is from an open overstory to a vigorous understory (Hawksworth & Dooling 1984) (Fig 4).
Less is known about long distance spread. Studies in North America implicate various species of birds as mechanisms of long distance spread, however, unlike the seeds of other mistletoes, seeds of dwarf mistletoes are destroyed if ingested. Therefore birds can only disperse seeds which accidentally become attached to their feathers (Hawksworth & Wiens 1996). At least in North America, long distance dispersal is considered to be quite limited and, from the standpoint of managing dwarf mistletoe infection, can probably be ignored (Hawksworth 1978). A single possible exception is A. verticilliflorum, a species indigenous to northern Mexico which has exceptionally large fruits which are not discharged explosively and may be dependent on birds for dispersal (Hawksworth & Wiens 1996).
2.2.3. INFECTION - A portion of the seeds which have been intercepted by foliage are washed to the twigs by rains. Probability of infection is high if the seed is washed to a shoot segment less than 5 years old (safe site). Only about 10% of seeds reach safe sites and less than 5% of these actually establish infections. Seeds which are successfully moved in this manner germinate the following spring and begin to penetrate the host tissue. Once host penetration occurs, the dwarf mistletoe develops an extensive haustorial absorptive system which consists of the production of "sinkers" which penetrate the host xylem. The sinkers connect with the trachieds of the host xylem and the parasite receives most of its carbohydrates, water and minerals from its host.
2.2.4. GROWTH AND DEVELOPMENT - Following initial infection, growth and development of dwarf mistletoe plants is slow. Depending on species, an incubation period of from 2-5 years can elapse before young aerial shoots of dwarf mistletoe begin to appear on the host plant. In the case of A. americanum, an important pest of Pinus contorta in North America, a small swelling begins to form on the branch of the host plant at the point of penetration during the second year of infection. The first aerial shoots typically appear during the third year following infection. Sexual dimorphism begins to become apparent during the fourth growing season following infection but flower production and pollenation does not occur until the fifth season. Seed development and dispersal do not occur until the sixth season following initial infection.
Dwarf mistletoes are obligate parasites and must have a living host in order to survive.
2.3. DISTRIBUTION AND HOSTS
Arceuthobium spp. are widely distributed in conifer forests of the northern hemisphere. In the New World, the 39 known taxa (34 species and 5 subspecies) are concentrated primarily in Mexico and United States. The 8 species present in the Old World range from Spain and Morocco east to the Himalayas of southwestern China with outlying species in the Azores and East Africa. Of the 21 taxa present in the United States, all but one are found in the western states. A single species, A. pusillum, occurs on Picea spp in the north-central and northeastern United States. Six of the taxa found in the United States also occur in Canada. There are 22 known taxa in Mexico, of which 8 are also found in the United States. Guatemala has 4 known taxa; Honduras, Belize and Hispaniola each have a single endemic species. Only one species, A. juniperi-procera, is found south of the equator (Hawksworth & Wiens 1996).
Hosts of dwarf mistletoes are conifers of the families Pinaceae and Cupressaceae. In the New World, Arceuthobium spp. are found only on the Pinaceae. Members of the genus Pinus are the most common hosts of various dwarf mistletoe species but Abies, Larix, Picea, Pseudotsuga and Tsuga also serve as hosts. In the Old World, members the conifer genera Abies, Keteleeria, Picea, Pinus (Pinaceae) and Juniperus (Cupressaceae) are hosts.
Dwarf mistletoes are generally host specific. Hosts of a single species of Arceuthobium can be ranked as "principal," "secondary," occasional" and "rare." An example of parasite/host relationships for dwarf mistletoe species occurring in Colorado, USA, the author’s home state, is shown in Table 1. Potential hosts that are not parasitized are designated as "immune."
| Arceuthobium | PRINCIPLE HOST |
SECONDARY HOST |
OCCASIONAL HOST | RARE HOST |
IMMUNE TAXA |
|
A. americanum |
Pinus contorta1 |
Pinus ponderosa2 |
Pinus flexilis |
Picea engelmannii |
Abies lasiocarpa |
|
A. cyanocarpum |
Pinus flexilis |
|
|
Picea engelmannii? |
Abies concolor |
|
A. divaricatum |
Pinus edulis |
|
|
|
Pinus ponderosa |
|
A. douglasii |
Pseudotsuga |
|
|
Abies lasiocarpa |
Pinus ponderosa |
|
A. vaginatum subsp. cryptopodum |
Pinus ponderosa |
|
Pinus contorta |
Pinus flexilis |
Abies concolor |
Source: Hawksworth & Wiens, 1996
* Includes only tree species indigenous to Colorado, USA.
1 var. latifolia
2 var. scopularum
3 var. glauca
The principle hosts of both New World and Old World dwarf mistletoes tend to be restricted to a single genus or for Pinus to a single subgenus (Hapoxylon or Dipoxylon) (Hawksworth & Wiens 1996).
2.4. SYMPTOMS AND SIGNS OF INFECTION
Symptoms of dwarf mistletoe infection include presence of branch swellings at the point of infection, witches brooms, branch dieback and tree mortality. The intensity of symptoms varies with dwarf mistletoe species and response of the host plant.
The earliest symptoms of infection is the development of a swelling at the point of penetration by the dwarf mistletoe (Fig 5). Usually this swelling is fusiform shaped, however in the case of A. azoricum, which infects Juniperus brevifolia in the Azores, rounded gall-like formations are formed (Hawksworth & Wiens 1976).
Infections often lead to the production of profusely branched, dense masses of distorted branches called witches brooms (Fig 6-8). Two types of witches brooms are formed:
Generally speaking, the type of witches broom formation is characteristic of the dwarf mistletoe rather than the host plant and broom type is an important taxonomic characteristic. Dwarf mistletoes such as the North American species A. americanum, A. douglasii and A.pusillum consistently form systemic witches brooms while non-systemic brooms are more common among other North American species. Several taxa (e.g. A. aureum subsp. aureum, A. globosum subsp. globosum) do not induce witches broom formation on host plants (Hawksworth & Wiens 1996).
Advanced infections can cause crown dieback and utimately, tree mortality. Trees killed by dwarf mistletoes often contain remnants of witches brooms (Fig 8).
Aerial shoots of dwarf mistletoe plants are a sign of infection. Shoot size and color varies by species and age of infection. Several species, including A.oxycedri, have abundant, conspicuous aerial shoot growth which is easy to detect. Other species such as the North American A. pusillum or the Asian A. minutissimum have small aerial shoots which are difficult to detect. Color of aerial shoots can be green, yellow-green, orange, dark brown or black, depending on species (Figs 9-12).
2.5. RATING INTENSITY OF INFECTIONS
A system for rating the intensity of dwarf mistletoe infection on trees and in stands is helpful to:
 Figure 5 - Branch swelling caused by A. americanum on Pinus contorta, Roosevelt National Forest, Colorado, USA.
|  Figure 6 - Large witches brooms on Pseudotsuga menziesii caused by A. douglasii, Winema National Forest, Oregon, USA
|
 Figure 7 - Dieback and witches brooms on Pinus contorta caused by A. americanum, Roosevelt National Forest, Colorado, USA
|  Figure 8 - Pinus ponderosa killed by A. vaginatum subsp. cryptopodum, Roosevelt National Forest, Colorado, USA
|
 Figure 9 - Arceuthobium cyanocarpum on Pinus flexilis, Roosevelt National Forest, Colorado,USA
|  Figure 10 - Arceuthobium tsugense on Tsuga heterophylla Tongass National Forest, Alaska, USA
|
 Figure 11 - Dark colored form of A. vaginatum subsp. cryptopodum on Pinus ponderosa, Rist Canyon, Colorado, USA
|  Figure 12 - Light colored form of A. vaginatum subsp. cryptopodum on Pinus ponderosa, Rist Canyon, Colorado, USA
|
A number of systems have been devised for rating intensity of dwarf mistletoe infection in North America. These are summarized by Hawksworth (1977). A 6-class rating system (DMR), (in actuality a 7 class system including a 0 rating) was devised in the 1950's (Hawksworth & Lusher 1956) which has become more or less a standard and can be applied to virtually any species of dwarf mistletoe or host tree. The system works best for host trees with an open crown structure (e.g. Pinus ponderosa) and less well for trees with globose or dense crowns. On trees with the latter crown characteristics, it is important to view the tree from several perspectives before assigning ratings.
To use the 6-class system, the live crown of a tree is divided into thirds. Each third is rated as follows (Fig 13):
0. No dwarf mistletoe.
1. Light dwarf mistletoe - < ½ of the branches infected.
2. Heavy dwarf mistletoe - > ½ of the branches infected.
A tree with a bole infection, but no branch infections is rated as 1, otherwise bole infections are not considered in the rating system.
The ratings for each crown level are added to obtain a tree rating (DMR). A stand rating (DMR) is the average of the individual tree ratings. The system is simple to use and different observers tend to rate an infected tree similarly (Hawksworth 1977).
DMR can be applied to trees of any size class, but in practice, it is difficult to use for very small trees and is not generally recommended for trees less than about 3 m in height. The system is also difficult to use for small, suppressed trees with sparse crowns or for dense stands with short crowns. Another limitation of this system is that it is difficult to use on very tall trees. The 6-class system is useful for evaluating the effects of the parasite on growth of infected trees but it does not necessarily provide a measure of the infection potential of a tree. For example, a Class 6 tree may be of such poor vigor that it is producing fewer dwarf mistletoe seeds than a more vigorous Class 2 tree.
Successful applications of the 6-class dwarf mistletoe rating system include quantification of height and diameter growth loss in infected trees, quantification of infection in stands and prediction of stand mortality and loss of height growth (Hawksworth 1977).
2.6. IMPACTS OF DWARF MISTLETOE INFECTION
Dwarf mistletoes are considered serious pathogens of conifers throughout their ranges, especially in North America. They retard growth of infected trees and cause extensive loss of timber through direct mortality of host trees by the dwarf mistletoe itself or through indirect mortality by secondary pests, particularly bark beetles (Coleoptera: Scolytidae) which often attack and kill infected trees. Dwarf mistletoes can also reduce seed production and wood quality, especially in Abies and Tsuga. Infections by dwarf mistletoe can also provide entrance points for decay fungi.
Reductions in both diameter and height growth have been documented for various dwarf mistletoe parasite/host combinations on important timber species in the United States. These reductions are related to the severity of infection, expressed as DMR, and to a lesser degree, the parasite/host combination (Tables 2-3). A similar relationship exists for periodic (10 year) mortality rates (Table 4).
In many parts of western North America, dwarf mistletoes are the most damaging pathogens of conifers. The extent of annual economic loss caused by these parasites has been estimated at about 11.3 million m3 in the western USA (Drummond 1982) and 3.8 million m3 in western Canada (Sterner & Davidson 1982). No quantitative information on economic losses is available outside of the USA and Canada but they are known to have serious effects on forest production (Hawksworth & Wiens 1996).
2.7. ECOLOGICAL RELATIONSHIPS
Several factors are believed to influence the distribution and abundance of dwarf mistletoes, including climate, topography, site quality and fire history. These are discussed in depth by Hawksworth & Wiens (1996) and summarized in the following sections.
| PARASITE | HOST | DMR=1 | DMR=2 | DMR=3 | DMR=4 | DMR=5 | DMR=6 |
|
A. abietinum A. americanum A. camplyopodum A.douglasii A. laricis A. vaginatum |
Abies concolor Pinus contorta Pinus ponderosa Pseudotsuga menziesii Larix occidentalis Pinus ponderosa |
100 100 100 98 94 100 |
100 100 100 97 92 100 |
98 100 98 85 88 98 |
95 94 86 80 84 86 |
70 80 73 52 58 73 |
50 59 50 44 54 50 |
* Expressed as a percentage growth of uninfected trees.
Source: Hawksworth & Wiens 1996.
| PARASITE | HOST | DMR=1 | DMR=2 | DMR=3 | DMR=4 | DMR=5 | DMR=6 |
|
A. americanum A.douglasii A. laricis A. vaginatum |
Pinus contorta Pseudotsuga menziesii Larix occidentalis Pinus ponderosa1
|
100 94 99 100 |
100 94 99 100 |
98 84 87 96 |
93 84 87 92 |
81 75 83 89 |
72 75 83 85 |
* Expressed as a percentage growth of uninfected trees.
1 10-year average stand growth.
2 Total height of 55 year old trees.
Source: Hawksworth & Wiens 1996.
| PARASITE | HOST | DMR=1 | DMR=2 | DMR=3 | DMR=4 | DMR=5 | DMR=6 |
|
A. abietinum
A. americanum A.douglasii A. vaginatum |
Abies concolor
Pinus contorta Pseudotsuga menziesii Pinus ponderosa |
1
2 1 1 |
3
4 2 4 |
6
5 4 8 |
10
6 9 15 |
15
7 15 23 |
21
8 23 34 |
* Expressed as % additional 10 year-stand mortality when compared to uninfected stands.
Source: Hawksworth and Wiens 1996.
2.7.1. CLIMATE - Climate may be a major factor responsible for restricting the distribution of several dwarf mistletoe species which do not occur across the entire natural ranges of their principle hosts. A classic example is A. vaginatum subs. cryptopodum which does not occur further north than northern Colorado in the USA although the natural range of its principle host, Pinus ponderosa subsp. scopularum extends more than 700 km further north. This dwarf mistletoe does not appear to occur in locations where mean January temperatures are <6o C.
A. douglasii is found throughout most of the natural range of its principle host Pseudotsuga menziesii except:
Part, but not all of these distributional anomalies can be attributed to climatic factors. Other factors which have been suggested include fire occurrence, physiographic processes, forest succession and fire.
2.7.2. TOPOGRAPHY - The influence of topographic factors such as elevation, topographic position, steepness of slope and aspect have been studied for several species of dwarf mistletoes in North America.
Studies on the distribution of A. vaginatum subsp. cryptopodum in southern New Mexico indicate that the distribution of this species is strongly related to elevation. The highest proportion of infections occurred in the mid-elevation range of Pinus ponderosa subsp. scopulorum, its principle host (2350-2400 m). Similar studies in other parts of the range of this parasite confirm its affinity for the mid to upper elevational ranges of its principle host.
The upper elevational limit of A. americanum is at least 185 m below that of its principle host Pinus contorta and varies with latitude which ranges from 2800 m in norther Wyoming to 3350 m in central Colorado. In southeastern Alaska, A. tsugense has an upper distributional limit well below that of its host, Tsuga heterophylla. Although the host can grow at elevations in excess of 610 m, the parasite occurs to only 365 m and is rare above 150 m.
The relationship of dwarf mistletoes to topographic position apparently is quite variable. A. vaginatum subsp. cryptopodum on Pinus ponderosa tends to be most abundant on ridge tops, intermediate on slopes and least common on bottomland sites. Similar patterns of distribution have been established for A. americanum on Pinus contorta in the central Rocky Mountains and on Pinus banksiana in Alberta, Canada. Occurrence of A. douglasii on Pseudotsuga menziesii in New Mexico is weakly correlated with topographic position. Additional studies have shown dwarf mistletoes to be more abundant on middle slopes than upper slopes ( e.g. A. douglasii on Pseudotsuga menziesii and A. vaginatum subsp. cryptopodum on Pinus ponderosa in central Arizona) or a high incidence on upper slopes (e.g A. vaginatum subsp. cryptopodum on Pinus ponderosa in Colorado and Arizona).
Other studies have attempted to relate dwarf mistletoe occurrence to steepness of slope. These have also produced variable results depending on parasite/host combination and/or location. For example, in southern New Mexico A. vaginatum subsp. cryptopodium was somewhat more abundant on gentle slopes (57%) than on steep slopes (45%) while in central Colorado incidence of the same parasite was 87% on gentle slopes, 51% on moderate slopes and 34% on steep slopes and another study in Arizona showed no correlation between slope and incidence of infection by A. vaginatum subsp. cryptopodum.
Studies relating incidence of dwarf mistletoe infection to aspect are equally variable. A study conducted in southern New Mexico established that A. vaginatum subsp. cryptopodum on Pinus ponderosa was more abundant on west and south facing slopes and less abundant on north and northeast facing slopes. Another study, conducted in Arizona, found that A. vaginatum subsp. cryptopodum was more abundant on east and south facing slopes while A. douglasii was more abundant on south facing aspects.
2.7.3. SITE QUALITY - The relationship between site quality for host growth and abundance of dwarf mistletoe is complex. Some dwarf mistletoe species tend to be more abundant on poor sites while others show little or no relationship to site quality. While incidence of a dwarf mistletoe species may not be related to site quality, effects of the parasite on growth and mortality of its hosts can be strongly influenced by site quality.
2.7.4. DWARF MISTLETOE - FIRE INTERACTIONS - The inter-relationship between fire and dwarf mistletoe occurrence is particularly interesting. Wildfires are considered to be the most important single factor governing the distribution and abundance of dwarf mistletoes. They can limit dwarf mistletoe populations because trees usually re-occupy burned over areas much faster than do the dwarf mistletoes. In some situations, however, fire can increase dwarf mistletoe abundance and distribution because spotty fires can leave scattered, infected trees which regenerate and re-infect new stands. Fires can also increase dwarf mistletoe incidence by maintaining early seral forest types susceptible to dwarf mistletoes rather than permitting succession to late seral species which are often immune to infection. For example, in the Rocky Mountains, Canada and USA, fire tends to favor Pinus contorta, a species highly susceptible to A. americanum, and discourage establishment of Abies lasiocarpa and Picea engelmannii, two trees which are not principle hosts of dwarf mistletoes. In other situations however, such as the juniper forests of Balochistan, it is the late seral species that is infected Witches brooms caused by dwarf mistletoes tend to be highly flammable. In addition, infected branches tend to be more resinous, larger in diameter and persist on host trees longer than uninfected branches. This results in greater fuel loading on dwarf mistletoe infected sites making wildfires hotter and more damaging.
2.8. DWARF MISTLETOE MANAGEMENT
2.8.1. RATIONALE - The procedures for managing dwarf mistletoe infections given in the following sections apply to all species since their life histories are similar.
Hawksworth (1978) regards the following features of dwarf mistletoes as making them amenable to the implementation of pest management tactics.
Effective management of dwarf mistletoes does not require eradication of the parasite. Instead, the objective of dwarf mistletoe management should be to reduce the amount of infection to a low level (Hawksworth & Johnson 1993).
2.8.2. PREVENTION - In North America it is recommended that priority be given to preventing establishment of new infections because this is much more effective than removing established infections. The following actions have proven to be effective methods of preventing spread and establishment of infections under North American forest conditions (Hawksworth & Wiens 1996):
2.8.3. SILVICULTURAL CONTROL IN INFECTED STANDS - Silvicultural tactics for management of dwarf mistletoe infections include removal of infected trees, pruning of lightly infected trees and favoring of tree species which are less susceptible or immune to infection.
In commercial forest areas, mature forests which are infected and scheduled for commercial harvest and do not have a manageable understory offer the greatest opportunity for replacement of infected stands with uninfected stands by clearcutting. Thinnings and intermediate selective harvests provide opportunities to remove infected trees in immature forests. Priority should be given to stands 10-20 years old with less than 40% of the trees infected and potential crop trees should have no visible infections. Stands should be thinned and sanitized only if acceptable stocking can be achieved with uninfected trees (Hawksworth & Wiens 1996).
In developed and recreational sites, trees with dead tops and branches and/or heavily infected trees should be removed. Lightly infected trees which are isolated from uninfected trees or surrounded by non susceptible species can be retained.
Small trees with light infections can be retained by pruning. All live branches up to the highest infected branch should be removed. Isolated branches which are left after pruning have a good likelihood of being infected even if they appear to be free of infections. If there are sufficient branches above the highest visible infected branch on a tree, removing branches for approximately 2/3 m above this point will help eliminate young infections (Hawksworth & Johnson 1993).
Dwarf mistletoe root systems extend for several cm beyond the shoots inside the branches of trees. Therefore, if a plant occurs on a branch close to the trunk, the roots of the parasite may have already entered the trunk and new shoots will appear within several years. An infected branch can be pruned effectively if the closest aerial shoots are at least 15 cm from the trunk. If an infection does occur within 15 cm of the mainstem of a tree, it does not necessarily mean that the tree must be removed. Dwarf mistletoe infections on the mainstems which are more than about 12 cm in diameter are generally not harmful to the tree and produce relatively few seeds. Consequently, particularly desireable trees with infections on or in close proximity to trunks can be retained by periodically removing the aerial shoots (Hawksworth & Johnson 1993).
Felled trees or cut branches do not have to be burned or destroyed because the aerial shoots die quickly and are not a source of new infection (Hawksworth & Johnson 1993).
In areas of extremely heavy infections, often the only acceptable management alternative is planting of alternative tree species with have either a low susceptibility to the dwarf mistletoe present in the area or are resistant. Ability to make effective use of this tactic is the availability of suitable alternative tree species which will meet management objectives.
2.8.4. CHEMICAL CONTROL - Ethephon, a growth-regulating chemical that has been used on commercial foods to hasten fruit ripening, has been found to be effective in reducing the rate of spread of several species of North American dwarf mistletoes. When sprayed on dwarf mistletoe shoots during the summer before seed dispersal, the shoots will become dry and fall to the ground. Thorough coverage of infected branches is needed to obtain good results. This growth regulator does not effect the root systems of the dwarf mistletoe plants, however and if the infected trees are not sprayed every 3-5 years, new shoots and fruits will develop. This technique is justified only in high value areas such as homesites or developed recreation areas. Cost is approximately $US 3-5/tree (Hawksworth & Johnson 1993, Hawksworth & Wiens 1996).
2.8.5. OTHER MANAGEMENT TACTICS - A number of insects, mites and fungi are associated with dwarf mistletoes and may have potential as biological control agents. Insects and mites associated with dwarf mistletoes have been studied in the USA and Pakistan. In the USA representatives of the insect orders Coleoptera, Hemiptera, Lepidoptera and Thysanoptera and four species of mites are known to feed exclusively on dwarf mistletoes (Stevens & Hawksworth 1970,1984). Insects associated with dwarf mistletoes in Pakistan are discussed in section 4.6 of this paper. Fungi associated with dwarf mistletoes and kill shoots, fruits and seeds. Unfortunately none have been studied sufficiently to consider developing them as biological controls (Hawksworth & Wiens 1996). Recently, there has been renewed interest in biological control of dwarf mistletoes in Canada where research at the Pacific Forestry Centre (PFC), Victoria, British Columbia is being focused on hyperparasitic fungi associated with shoots, seeds and swellings of dwarf mistletoes, their pathogenicity and their potential as biological control agents (Personal communication, Simon Shamoun, PFC, Canadian Forestry Service, Victoria, BC, Canada) .
Occasional evidence of resistance to dwarf mistletoe infection has been observed in several host-parasite combinations, however this phenomenon has not been studied sufficiently for development of practical pest management measures (Hawksworth & Wiens 1996).
2.8.6. FOLLOW UP EVALUATION AND TREATMENT - It is easy to overlook infections, especially from very young dwarf mistletoe plants during direct control projects. Consequently, it is important to revisit areas were treatment has been undertaken at 2-3 year intervals and re-examine them for the presence of new dwarf mistletoe plants. The need for additional follow-up treatment should be anticipated as a normal procedure. Localized infections may be treated effectively by pruning infected branches while heavier infections may require removal of additional trees (Hawksworth & Johnson 1993).