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Photograph: Good management practices can result in production of
high-quality Christmas trees like this Fraser Fir grown in
Michigan.
INTRODUCTION
What would Christmas be without a Christmas tree? Most people
don't think about Christmas trees until November or December, when
they go out to select that perfect tree for the family. However,
Christmas tree production is a multi-million dollar industry that
requires year-round management efforts. Christmas trees occupy a
unique niche, incorporating aspects of both agricultural production
and forestry. Insect and disease pests can have major effects on tree
growth, appearance and value. Managing these pests, along with weeds
and even rodents, is a critical component of tree production.
Our goals are 1) to provide an overview of Christmas tree
production and 2) to briefly discuss insect and disease management in
Christmas tree production. Many effective and economically viable
pest management practices can be readily integrated with other
aspects of tree production. Information presented here will largely
be drawn from our experience in Michigan and other Lake States, one
of the largest Christmas tree producing regions in North America.
THE CHRISTMAS TREE BUSINESS
Approximately 38 million natural Christmas trees are purchased
each year by American consumers. These trees represent the annual
harvest of an estimated 20,000 individual growers who annually plant,
trim, protect and otherwise manage trees on roughly 500,000 acres of
plantations. More than 15 different conifer species are grown for
Christmas trees.
Christmas tree production is a significant industry in the Lake
States, the Pacific Northwest, North Carolina and areas in the
Northeast. Traditionally, growers in these regions supply about 90%
of all trees offered for sale at retail outlets. Approximately
10,000,000 trees are produced each year in the Lake States of
Michigan, Wisconsin and Minnesota. Michigan is the largest producer,
annually harvesting 5,000,000 trees, valued at up to $100,000,000 in
some years.
Christmas trees are produced by two principal types of operations;
"wholesale" or "choose-and-cut" plantations. Most trees offered for
sale at retail outlets such as garden centers, nursery stores, high
volume discount stores and lots operated by service clubs, were
produced by wholesale growers. These are individuals or companies
that operate and manage large plantations of trees. They may sell
trees either directly to retail outlets or to "brokers" who in turn
market trees to retailers. Some large operations may sell 100,000
trees or more each year.
Choose-and-cut operations are usually smaller plantations that
manage and produce trees for direct sale to consumers. These
operations invite customers out to their fields and will either cut
the trees for the purchaser or allow the customer to cut his/her own
trees. Many choose-and-cut operations enhance the experience of
getting a Christmas tree by providing a variety of recreational
activities for customers. Choose-and-cut farms are found in every
state and are often concentrated near large population centers. These
operations vary considerably with regard to the species and quality
of trees produced, and the services offered.
Christmas tree production is a long-term process. The average 7 to
8 foot tree generally requires 8 years of growth after planting. Some
species require even longer production times. Likewise, larger-sized
trees which are becoming increasingly popular, require more years to
produce.
A major challenge for Christmas tree producers is to determine
which tree species will be popular with consumers several years in
the future. Although several species are utilized as Christmas trees,
the majority of trees sold at retail markets today are Douglas-fir,
Fraser fir, noble fir and Scotch pine. Species such as white spruce,
Austrian pine and red pine are no longer as popular as they were in
the past. Producers who are not sensitive to shifts in consumer
preference will have difficulty marketing even high-quality trees of
less preferred species.
CHRISTMAS TREE PRODUCTION PRACTICES
Each component of Christmas tree production must be addressed from
the standpoint of maximizing both productivity and quality. Failure
to appreciate or deal appropriately with any aspect can result in
management difficulties, pest problems, or production of low-quality
trees which are difficult to sell in competitive markets.
Site Selection
Not all species used for commercial Christmas tree production grow
equally well on all sites. Factors such as soil texture and
fertility, water availability and air drainage will affect growth
rates and tree quality. In general, true firs and Douglas-fir require
better quality sites than pines and spruces. Species that are planted
"off-site" will frequently experience stressful conditions. These
trees are more likely to be attacked by insect or disease pests, and
will be less likely to tolerate or recover from pest damage, than
healthier trees.
Site Preparation
In the past, Christmas tree plantations were sometimes established
with little or no site preparation other than removal of competing
woody vegetation. Survival and success of these plantation were
mixed. Where site preparation was minimal, pines were easier to
produce than spruces or firs. Modern Christmas tree operations often
spend much effort in site preparation activities including tillage,
use of cover crops and soil fertility enhancement. These efforts pay
off in higher seedling survival, fewer pest problems on young trees,
more rapid initial growth and an overall increase in tree
quality.
Rotation Length
Most conifers planted for Christmas tree production require 7 to
10 years to reach maturity, depending on the species, the size of
trees produced, and the intensity of management. Pines generally can
be produced faster than spruce or fir trees, although size of
planting stock, soil fertility and water availability can
significantly affect rotation length. Damage from insect or disease
pests that affect tree appearance, form or growth rate, can increase
rotation length and production costs. Even after a pest population is
controlled, trees may require 1 to 3 years to outgrow or recover from
the damage.
Planting
Most Christmas tree plantations are established using planting
machines. Seedlings may be planted by hand on adverse sites or to
fill in plantations where mortality occurred in previous years.
Nearly all growers plant 2 to 4-year-old seedlings or 3 to 5-year-old
transplants in early to mid-spring. A few growers may produce their
own planting stock, but the majority of producers purchase stock from
private seedling-transplant nurseries. Pests, particularly diseases,
may be transported on infested nursery stock. Purchasing stock from
reputable dealers or buying inspected and certified stock can help
prevent establishment of new pest problems.
Shearing
Christmas trees are typically sheared or shaped each year,
beginning 2 to 3 years after planting, and continuing on through
harvest. Shearing accomplishes two goals. First, shearing develops
the characteristic tapered shape associated with high-quality
Christmas trees. Second, shearing controls the amount of annual
growth and in some species, increases bud set. This results in
greater density and uniformity of the foliage. Timing of shearing
depends on the species of conifer. Pines are usually sheared during
June and July; shearing of other species usually begins in August and
continues until finished in the fall.
Shearing can affect pest infestation or damage. For example, tight
shearing can result in very dense crowns with little air circulation.
This situation can lead to problems with needlecasts or other foliage
diseases. Effectiveness of insecticide or fungicide applications may
be poor if sprays do not penetrate the dense outer canopy. On the
other hand, shearing can remove much of the damage caused by
shoot-boring insects, Pales weevil (Hylobius pales) and other
pests. A good understanding of potential pest problems as well as
consumer preferences, should help growers develop suitable shearing
practices.
Harvesting
In late summer, trees with that will be harvested that year are
identified and marked with tags or flagging. Many growers spray trees
with a water-soluble green latex pigment in August or September. The
green paint is applied to mask the characteristic yellowing of
foliage which commonly occurs in some varieties of Scotch pine,
eastern white pine and Douglas-fir. Needles begin to turn yellow in
fall, in response to shorter photoperiod and cooler temperatures.
Actual harvesting begins in late October and will continue on
through mid-December. After cutting, trees are shaken to remove dead
foliage and debris, then baled with string or net. Trees are then
transported from the plantation to a storage or loading yard where
they will be stored until shipment. Shipment to retail centers and
stores is usually well underway by November 18 to 20.
MAJOR CHRISTMAS TREE SPECIES
Although several conifer species have been used for Christmas
trees, most commercial production involves four genera. These are:
pines (Scotch, Austrian, red and white), spruces (Colorado blue,
white, and the Blackhills variety), firs (balsam, Fraser, Concolor
and Noble), and Douglas-fir. A few other individual species may be
produced in specific regions or local areas, but these species rarely
account for significant numbers in the national market place.
Douglas fir
Major Pests: Rhabdocline needlecast (Rhabdocline
pseudotsugae), Swiss needlecast (Phaeocryptopus gaumanni),
Cooley's spruce gall adelgid (Adelges cooleyi)
This species is native to western regions of the United States and
Canada and is widely planted in the Lake States and northeast.
Because Douglas-fir breaks bud early in the spring, it must be
planted on sites with good air drainage to prevent injury from late
spring frosts. Douglas fir does best on well-drained, loam to sandy
loam soils and will grow rapidly once it is established. It will not
tolerate heavy soils which are poorly drained. Needle retention is
generally good, although trees that have not experienced freezing
temperatures before harvest may loose some needles during the display
period.
True Firs
Major Pests: Balsam twig aphid (Mindarus abietinus), Spruce
spider mites (Oligonychus ununguis), Balsam gall midge
(Paradiplosis tumifex), Lirula needlecast (Lirula
sp.)
Fraser fir: Closely related to Balsam fir, Fraser fir is
native to high elevations in the southern Appalachians, but has been
widely planted in other production areas. Fraser fir requires ample
soil moisture and fertility, and a soil pH of less than 6.5. It will
not grow in very wet or dry locations. Fertilization and sometimes
irrigation are frequently used in well-managed plantations. Because
of its excellent needle retention, attractive aroma, straight stem
and dark greensilvery blue needles, the popularity of this
species among consumers has increased tremendously in recent years.
It is now widely recognized as a premier Christmas tree species and
is one of the most popular species among both growers and
consumers.
Balsam fir: Native to the northeastern United States and
adjacent Canada, balsam fir has been long used as a Christmas tree.
Like all firs, its soft, fragrant foliage is prized by consumers. For
many years wild balsams were harvested from natural stands and
marketed throughout the northeastern United States. Most trees are
now harvested from plantations located in New England and much of
Quebec and the Maritime provinces of Canada. Growth is best on loam
soils which are well-drained. This species responds well to
fertilization and intensively managed plantations regularly supply
trees with nitrogen fertilizer to promote growth and enhance foliage
quality. Balsam fir foliage is often used for wreaths, garland, and
other Christmas greenery.
Concolor fir: Native to states in the Rocky mountains and
intermountain west, Concolor or white fir is an important timber
species throughout much of its range. However, because of its
attractive bluish-green needle color and soft foliage it has been
planted as both an ornamental and a Christmas tree. Concolor fir is
adapted to a wide variety of sites and soils. It will tolerate fairly
dry sites, especially after it is well-established, and also grows
well under varying soil pH conditions.
Noble fir: Noble fir has become a popular Christmas tree
and is produced primarily in the Pacific Northwest. This species is
known for its excellent needle retention and symmetrical branching
habit. Within its native range, Noble fir grows at elevations ranging
from 2000 to 5000 feet above sea level. Efforts to grow this species
outside of its native range have met with limited success, especially
in the Lake States and northeastern United States. Some efforts are
underway to identify seed sources which might prove hardy outside the
region. Because of its popularity among consumers, many trees of
Pacific Northwest origin are sold throughout the central and eastern
United States.
Spruces
Major Pests: White pine weevil (Pissodes strobi),
Cooleys spruce gall adelgid, Pine needle scale (Chionaspis
pinifoliae), Rhizosphaera needlecast (Rhizosphaera
kalkhoffii), Cytospora canker (Cytospora kunzei)
Colorado blue spruce: This species is planted extensively
throughout much of the United States. Although native to the central
Rocky Mountains, it is adaptable to a wide variety of sites and
soils. Colorado blue spruce is more popular in choose-and-cut
operations than in large wholesale plantations, largely because of
lack of consumer demand at the retail level. While its foliage is
attractive, the stiff, sharp needles make for difficulty in handling
and display. Needle retention is generally better than other spruce
species.
White spruce: For many years white spruce was a favorite
species of many eastern growers. It is adaptable to diverse planting
sites throughout much of the Lake States and northeastern United
States. Its tendency to grow into a tree with a "natural" Christmas
tree shape contributed to its popularity because only a minimal
amount of shearing and shaping was required to produce a quality
tree. However, its popularity has declined in the last several years
due primarily to its reputation for poor needle retention. The small,
short needles have a tendency to dry rapidly when the tree is
displayed in a heated room with low humidity. A variety known as
Blackhills Spruce has better needle retention characteristics and
continues to be planted by a few growers.
Pines
Major Pests: Pine root collar weevil (Hylobius
radicis), Zimmerman pine moth (Dioryctria zimmermani),
pine needle scale, white pine weevil, Lophodermium needlecast
(Lophodermium seditiosum), Dothistroma needlecast
(Dothistroma pini), Cyclaneusma needlecast (Cyclaneusma
pinus), Sphaeropsis (Diplodia) shoot blight (Sphaeropsis
sapinea)
Scotch pine: For many years this species enjoyed the
position as the nation's most popular Christmas tree. Native to
Europe and Asia, Scotch pine grows well on a wide range of sites and
soils throughout much of the United States and Canada. Many varieties
were imported from Europe and Asia and exhibit considerable variation
in such traits as needle length and color, stem straightness, branch
angle, and hardiness. Scotch pine continues to be widely planted,
especially in the Lake States, where it has long been viewed as the
"bread and butter" species of the Christmas tree industry. Scotch
pine adapts readily to plantation culture and responds well to
management practices such as shearing. It has excellent needle
retention and a long shelf life following harvest, even in warm and
dry locations. However, more than 40 insects and diseases attack
Scotch pine, making pest management a major aspect of Scotch pine
production. Although still a significant component of the national
market, the popularity of Scotch pine has declined over the past
several years. Increased sales of both Douglas fir and Fraser fir
have reduced market share in several locations. It remains a favorite
of cutyourown growers in many areas of the central and
eastern United States.
White pine: The name white pine may refer to several
species, including western white pine and southwestern white pine.
However, when used to describe Christmas trees, it commonly refers to
eastern white pine. This species is native to the northeastern United
States, adjacent Canada and higher elevations in the Appalachians. It
is also an important timber species and was a notably valuable lumber
species in the late 1800's. It grows on a wide range of soil types,
including loamy sands to heavier clay loam soils. When planted to
produce Christmas trees, it responds well to traditional shaping and
shearing practices. It can be developed into a tree with thick
foliage and uniform symmetry, and it exhibits excellent needle
retention.
Austrian pine: This European native was a fairly popular
Christmas tree species several years ago, but presently enjoys
limited popularity among consumers. It tolerates a wide variety of
soil and site conditions and is often planted along roadsides and as
an ornamental. When grown as a Christmas tree, it generally is
restricted to cutyour-own operations, most often in locations
where poor sites prohibit production of other species. It is
relatively unpopular among consumers because of its long needles,
stiff branches and generally "coarse" appearance.
Red pine: Red pine is native to states in the north central
and northeastern regions of the U.S. and is a valuable tree for the
production of wood fiber and timber products. It has been locally
planted for Christmas tree production, but does not enjoy widespread
popularity among consumers. It grows best on well-drained loam or
sandy loam soils of low to medium fertility. It has characteristics
similar to those of Austrian pine, although its needles are usually
somewhat shorter and not as stiff.
PEST DAMAGE
Damage from insect and disease pests can lead to dead branches or
terminal leaders, and some pests will kill trees. Loss of needles or
shoots due to disease or defoliating insects will reduce tree growth,
resulting in longer rotation times and economic loss. Aesthetic
injury is probably the most common damage caused by pests. Christmas
tree value is based largely on the appearance of the tree and its
attractiveness to potential customers. Missing or dead foliage, a
crooked stem or tiny white scales on the foliage will reduce the
value of the tree or even make the tree unsalable.
PRINCIPAL CLASSES OF INSECT AND DISEASE INJURY
Needle loss - Defoliation
Needlecast diseases or defoliating insects such as sawflies cause
needle loss. This injury can lead to reduced growth rates and thin,
unattractive canopies. Loss of current-year needles, where nutrients
are concentrated, usually has greater effects on tree health and
appearance than loss of older foliage.
Death of branches, terminal leader or lateral shoots
White pine weevil will kill 2 to 4 years of terminal leader
growth, affecting rotation length and stem shape. Zimmerman pine
moth, Cytospora canker, and gall rust pathogens (e.g. Cronartium
spp. and Endocronartium harknessii) can kill large
branches, affecting tree appearance and sometimes growth rates.
Several shoot-boring insects (e.g. Rhyacionia spp. and
Eucosma gloriola), Pales weevil (Hylobius pales), and
some diseases such as Diplodia kill individual shoots.
Sap-Feeders - aphids, scales, mites
High populations of aphids, scales, mites and other sap-feeders
often seriously reduce the appearance of Christmas trees and can
affect tree health. Some species, such as Cooley's spruce gall
adelgid cause unsightly galls to form on lateral shoots of spruce
trees. Pine tortoise scale (Toumeyella parvicornis) excretes
large amounts of sugary honeydew. A black sooty mold grows on the
honeydew, turning needles and shoots black. White armor covering pine
needle scales gives pine needles a white, flecked appearance. Feeding
by the balsam twig aphid will cause needles of fir trees to twist and
curl tightly.
Tree mortality
Some pests, including pine root collar weevil and white pine
blister rust (Cronartium ribicola), kill Christmas trees.
Pales weevil, and root diseases (e.g. Phytophora spp.) may
kill young seedlings.
ECONOMIC INJURY LEVEL AND ACTION THRESHOLD
In IPM, the Economic Injury Level (EIL) and Action (or Economic)
Threshold (AT) are used to determine if and when pest control
measures are needed. Unfortunately, few thresholds have been
developed for Christmas tree pests. The difficulty of establishing an
EIL or AT for any given pest arises from several factors. These
factors include the difficulty of 1) quantifying aesthetic injury, 2)
determining the economic costs of that injury and 3) relating pest
density to levels of aesthetic injury.
However, the concept of an Action Threshold remains valid
and is incorporated into pest management recommendations wherever
possible. One factor that affects pest management decision-making is
the type of damage caused by the pest. For example, in Scotch pine
fields, the AT for a tree-killing insect such as pine root collar
weevil is lower than for a minor defoliator such as sawflies
(Neodiprion spp.).
Another important factor is how soon the trees will be harvested.
A light infestation of pine needle scale on Scotch pine or Cooley's
adelgid on Douglas-fir is much more critical when trees are within 1
to 2 years of harvest, than if trees are in the middle of their
rotation.
A third factor affecting an AT is how long it will take the tree
to recover from pest damage. For example, shoots killed by European
pine shoot moth (Rhyacionia buoliana) or other shoot borers
are often removed anyway during shearing. However, white pine weevil
attack typically results in the death of 2 to 4 years of growth on
spruce or pine trees. Although tree form can eventually be restored
with corrective pruning, harvest of weevil-damaged trees will be
delayed for at least 2 years.
MANAGEMENT STRATEGIES
Cultural Control
Ideally, pest management should be incorporated into all aspects
of Christmas tree production, from site preparation to harvest. It is
especially important to select a tree species that is well-suited
for the site conditions in the field. For example, Fraser fir
requires relatively high levels of nutrient and moisture
availability. In contrast, Scotch pine tolerates even sandy,
coarse-textured soils where nutrient and moisture availability is
low. Trees growing under stressful conditions are usually more
susceptible to insect and disease pests and recover more slowly from
damage. Irrigation and fertilization may effectively reduce stress
and increase tree vigor. However, these practices will only be
cost-effective for high-value species such as Fraser fir.
The importance of scouting, the practice of inspecting
trees for evidence of pest infestation or damage, cannot be
underestimated. Frequent and regular scouting will enable growers to
detect signs or symptoms of pests before economic damage occurs.
Signs include the physical evidence of pest presence such as
insect frass or cast-off skins, pitch flows, or the fruiting
structures of disease organisms. Symptoms refer to evidence
that the tree has been affected by insect or disease attack. Branch
galls, sparse crowns, or a wilting leader may be evidence of gall
rust, needlecast disease and white pine weevil infestation,
respectively.
Cultural control in Christmas tree fields should include frequent
scouting to identify diseased, damaged or infested trees. Trees that
are heavily infested should be "culled" and removed from the field.
Several pests such as Zimmerman pine moth, an insect that feeds in
the inner bark on stems and branches of pine trees, will complete
their development even after trees are cut. Adult moths can then
return to the field, lay eggs on live trees, and perpetuate the
infestation. Therefore, culled trees should be destroyed within a few
weeks by chipping or burning. Growers may also be able to
mechanically remove and destroy pest-infested leaders, shoots or
galls, before a new generation of the pest can emerge.
Trap logs have been used successfully to manage some insect
pests such as pine shoot beetle (Tomicus piniperda), a
recently discovered exotic bark beetle that is currently a quarantine
pest (see below). Bark beetle larvae feed and develop under the bark
of pine stumps, logs or recently cut trees. Trap logs consisting of 3
ft. long sections of pine logs, are set along the edges and lanes of
Christmas tree fields to attract adult pine shoot beetles. The adult
beetles colonize the trap logs and lay eggs. Growers then have a 4 to
6 week window to collect all trap logs and destroy them by chipping
or burning, effectively eliminating the new generation of
beetles.
Biological Control
Natural enemies often play an important role in reducing
potentially damaging insect populations in Christmas tree fields.
Fields managed on 6 to 12 year rotations are more likely to provide
stable habitat for beneficial arthropods than agricultural systems
where fields are harvested annually. Further, trees are more
structurally complex than most agricultural plants. Trees can provide
beneficial arthropods with resting or oviposition sites, and
protection from adverse weather. Hedgerows along field edges that
include flowering plants may enhance the fecundity or survival of
parasitoids.
Common and important natural enemies in Christmas tree fields
include spiders, flower fly larvae, lacewings and predatory mites.
Ladybird beetle larvae and adults are especially important predators
of aphid and scale insects. In addition, many insect pests are
attacked by specialized parasitoids. Obviously, it is important to
minimize insecticide use and drift to help conserve these natural
enemies.
Biocontrol in Christmas tree production probably holds more
potential than is currently being realized. There is a scarcity of
information on effectiveness of biocontrol agents, and the proper
timing and density for agents that will be released in fields.
To-date, there are few specialized biocontrol agents for Christmas
tree pests available commercially.
Pesticides
Use of insecticides, fungicides, herbicides and rodenticides is
common in Christmas tree production. Application technology varies
widely among growers, depending on the size of the operation. Many
growers use air blast sprayers, but backpack sprayers, boom sprayers
and airplane or helicopter application are not uncommon. Minimizing
pesticide applications makes sense for both economic and
environmental reasons, and makes it easier to integrate pesticides
with other management strategies.
Timing of pesticide application can have major effects on
efficacy. Growers are encouraged to use scouting to determine when
the vulnerable stage of the pest is present. Using degree
days, rather than calendar days, can also improve the timing of
pesticide applications. Degree days accumulate rapidly during warm
weather and more slowly when temperatures are cool.
Achieving adequate coverage is another concern when
spraying conifer trees. Dense foliage, particularly on sheared trees,
often makes it difficult to get good coverage. This is especially
important when the target pest occupies the stem (e.g. Zimmerman pine
moth) or older foliage near the stem (e.g. spider mites).
Quarantines - A Challenge for IPM
When exotic (non-native) pests are discovered, federal and state
quarantines may be enacted to limit the spread of the pest.
Quarantines are intended to provide a legal means of reducing the
risk that exotic pests will be carried along when Christmas trees (or
other plants) are shipped to areas not yet infested.
However, quarantines often represent a major challenge to IPM.
Most quarantines restrict out-of-state shipment of trees if even a
single pest is present. For example, state and federal gypsy moth
(Lymantria dispar) regulations currently mandate that
Christmas trees be sprayed at least once with an insecticide during
the summer if the trees will be shipped out of state in the fall.
Mandatory spraying, in combination with tree inspections at harvest
time, was implemented to reduce the risk that gypsy moth egg masses
or other life stages would be accidentally introduced into new areas.
This situation can lead to unnecessary use or overuse of insecticides
by growers who fear penalties or serious economic losses if any egg
masses are found.
In contrast, university scientists and regulatory agencies are
currently working together to develop an IPM program for pine shoot
beetle, another exotic pest. Pine shoot beetle is a bark beetle that
feeds in shoots of live pines and breeds in pine logs, cut pine
trees, and stumps. It was first discovered in North America in 1992
and the infestation is centered in the North Central region of the
U.S.
Under the terms of a proposed PSB Compliance Program, growers
agree to implement cultural practices that will substantially reduce
beetle populations. These practices include burning or chipping the
cut trees and stumps the beetles breed in. Trap logs must be used to
attract breeding adult beetles to prevent a new generation of beetles
from infesting trees. Growers who agree to comply with the prescribed
management protocol can then harvest and ship trees without a
tree-by-tree inspection in fall. The Compliance Program will reduce
the risk of introducing PSB to new areas, while at the same time
promoting good management practices.
CONCLUSIONS
Growing Christmas trees is an intensive agriculture-forestry
enterprise that requires the use of production techniques much like
any other crop. Control of potentially damaging insect and disease
pests is an ongoing part of the production process. IPM concepts such
as scouting, use of thresholds and integration of various control
strategies, are widely used by Christmas tree growers, although these
practices are not as advanced as they are for other crops. Continued
research is needed to develop EILs and Action Thresholds for specific
pests, to identify workable biocontrol options for Christmas tree
pests, and in other areas. A continued emphasis on IPM will ensure
the production of high-quality trees with minimal economic and
environmental impacts.
SELECTED REFERENCES
*Benyus, J.M. 1983. Christmas tree pest manual. USDA Forest
Service, North Central Forest Experiment Station and Northeastern
Area State and Private Forestry. 108 p.
Johnson, W.T. and H.H. Lyon. 1988. Insects that Feed on Trees and
Shrubs. 2nd edition. Cornell University Press. 556 p.
Kachadoorian, R., J.Cummings-Carlson, D.G. McCullough and D.O.
Lantagne. 1995. Pesticides for use in Christmas tree production in
the North Central region. Michigan State University Extension
Bulletin E-2594. 52 p.
Koelling, M.R. and L. J. Dornbush. 1992. Growing Christmas Trees
in Michigan. Michigan State University Extension Bulletin No.
E1172 (Revised), 12p.
Sinclair, W.A., H.H. Lyon and W.T. Johnson. 1987. Diseases of
Trees and Shrubs. Cornell University Press. 575 p.
*Key Reference
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Photograph: Good management practices can result in production of
high-quality Christmas trees like this Fraser Fir grown in
Michigan. |
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Photograph: Workers shear Scotch pine trees in a Christmas tree
plantation. Shearing develops a thick uniform canopy and the
characteristic tapered shape that appeals to Christmas tree
consumers. |
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Photograph: A terminal leader killed by white pine weevil feeding.
Trees may require corrective pruning and 2 to 4 years of growth to
recover from white pine weevil damage. |
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Photograph: These galls on a blue spruce Christmas tree were caused
by Cooley's spruce gall adelgid. Although the galls seldom affect the
health of the tree, they detract from the tree's appearance and
reduce its value.
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