Introduction to the Southern Pine Beetle (SPB)
The following is a brief overview of the Southern Pine Beetle
and the context of its status as a forest and shade-tree pest. Additional
and more detailed information is available through the links
provided on the SPB
main page.
What is the Southern Pine Beetle?
The Southern Pine Beetle (SPB) is one of five common species of
pine bark beetles that occur throughout the Southeastern United
States. Pine bark beetles utilize stressed, dying, or recently-dead
pines as hosts. All species tunnel, reproduce, and feed in the
inner bark or phloem (located between the outer bark and the wood);
this activity disables the transport of sugars through the tree. In
addition, several bark beetle species (including the SPB) introduce
a blue-stain fungus into the wood, plugging the tree's water transport
system. Typically, pines successfully colonized by SPB are functionally
dead within one day after attack, despite the fact that the crown
may retain green needles for several days or weeks.
Who Cares?
Why be concerned about beetles that kill trees that are already
weakened and dying anyway? Indeed, bark beetles are generally
scavengers just doing their ecological job of recycling scattered,
stressed-out pines (those weakened by old age, injury, competition
with other plants, etc.) into organic material, and the SPB is
generally restricted to such trees when their populations are
low. Due to their remarkable reproductive capacity and odor-driven
communication system, however, SPB populations can rapidly build
and spread through pine forests or landscapes under certain environmental
conditions, mass-attacking even healthy trees. Groups or "spots"
of SPB-infested trees can enlarge by more than 50 feet per day,
quickly resulting in multiple acres of dead pines. This can result
in substantial economic, aesthetic, recreational, ecological and
other types of losses. During one of Florida's worst SPB years
on record (2001), there were over 2900 infestations and 17,600
acres of killed timber statewide. Furthermore, because the SPB
is no respecter of property boundaries, one landowner's timber
infestation can easily become an adjacent neighbor's residential
shade-tree nightmare, or vice versa.
What conditions contribute to SPB infestations and outbreaks?
Any number of factors that stress, weaken, or injure pines can
increase their susceptibility to SPB attack, population buildup,
and damage. Some of these factors, such as severe droughts or
damaging storm events, are periodical environmental occurrences
over which we typically have no control. Many other stress factors,
however, are within mankind's ability to manipulate, and indeed
have been created by our own mismanagement of the forest over
both the short and long term. Some of these factors include:
• Allowing
pines in plantations, natural stands, and residential landscapes
to exist at very high densities or close spacings (i.e., "overstocked"),
causing tree stress through competition for sunlight, water
and nutrients. Close tree spacing also enhances the spread
of SPB from tree to tree once an infestation is established.
• Exclusion of beneficial, low-intensity fires from
pine forests, resulting in the buildup of competing vegetation
beneath the pines and thus placing additional stress on them.
• Widespread establishment of numerous, large, high-density
monocultures of the SPB's preferred host (loblolly pine).
• Maintaining urban canopies with an over-abundance
of large, senescent, aging loblolly pines.
• Injuring pine root systems and stems during construction
and/or heavy equipment use.
• Changing the nutrient status of the soil beneath
pines due to improper irrigation, fertilization, and/or landscaping.
Although SPB populations naturally fluctuate according to environmentally-driven
cycles, these human-induced factors increase the severity, frequency,
and extent of SPB outbreaks and infestations.
How can one help prevent SPB damage?
The best way to help prevent SPB infestations is to manage pine
forests, woodlots, and residential shade trees in ways that keep
them healthy, vigorous and resistant to attack. These practices
include:
Thinning pine stands to the appropriate density for a given tree
size (i.e., maintain stand at less than 80 ft2 of basal
area per acre).
• Maintaining a distance of at least 20 ft between
mature residential landscape pines.
• Eliminating or minimizing hardwood underbrush and
other competing vegetation from beneath pines through the use
of prescribed burns and/or mechanical removals.
• Converting stands of off-site or highly susceptible
pines to a more resistant pine species (e.g. longleaf pine,
slash pine) on appropriate soils.
• Establishing plantations at low densities (<550
stems/acre) in cases where delayed thinning is preferable.
• Remove/harvest and replace/regenerate, overmature,
declining, stagnant and/or low vigor trees/stands.
• Rapidly salvage or otherwise sanitize areas seriously
damaged or weakened by disease, fire, storms, mechanical/physical
activities, lightning strikes, etc.
• Avoid disturbances or activities that damage or
harm desired pines (e.g., physical wounding, excessive fertilization
and/or irrigation, severe fire, changes in grade (fill or excavation),
root-raking, soil compaction Maintain a thin layer of
acidic pine needle or bark mulch beneath the crowns of landscape
pines.
How should SPB infestations be controlled?
There are many reasons why pine trees die, thus one should not
assume that every circumstance of dying or declining pines is
the result of an SPB infestation. There are other species of
bark beetles that cause symptoms similar to those caused by
the SPB, but infestations by these less-aggressive beetles are
not as critical to control; with such species sometimes the
best approach is to do nothing and let the infestation die out
on its own. Thus, before jumping to the conclusion that SPB
is the culprit, the existence of suspicious pine mortality should
be reported to a Division of Forestry forester or county extension
agent, who can evaluate the situation and recommend a course
of action. (See other links on the SPB main page of this site
regarding identification of SPB, related bark beetles and their
symptoms.)
If dying or declining pines are positively identified as infested
with SPB, the best means of control is to promptly cut and remove
the trees to avoid beetle spread to adjacent living trees. In
forests, a buffer strip of uninfested green trees is also cut
and removed to ensure the infestation has been completely eradicated. Other
control strategies (e.g., cutting and burning/burying infested
trees, felling and leaving infested trees in remote forest areas)
may be warranted in special circumstances where cut-and-remove
is not feasible.
One historical control option that is currently not available
is the cut-and-spray technique. This technique involves felling
infested trees into workable log sections and thoroughly spraying
the bark with an insecticide. The insecticides historically
used for this purpose (e.g., lindane and chlorpyrifos), however,
are no longer registered for such use, and replacement chemicals
have not been developed. Certain restricted-use pesticides (e.g.
synthetic pyrethroids) may be registered for use as a preventive
spray on uninfested landscape trees, but homeowners should carefully
investigate and evaluate the costs and benefits their use. These
products are probably most appropriate for protection of very
high value trees adjacent to SPB-infested trees that cannot
be cut and removed. The Division of Forestry forest entomologist
or a county extension office can be consulted for advice.
Conclusion
In conclusion, it is important to remember that the SPB is a native
insect and a natural part of the pine forests of the Southeast.
When its populations are at low (endemic) levels, it is usually
not much of a problem. Periodic SPB outbreaks have been so destructive
largely because we as a people have created widespread forest
conditions that foster SPB outbreaks. If we really want to lessen
the impact of insects like the SPB, it will be through practicing proactive, prevention-focused management of our forests and shade trees, rather than a reactive, control-focused
response to existing pest problems.
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