Southern Pine Beetle

History

Southern Pine Beetle Adults

The southern pine beetle (Dendroctonus frontalis Zimmermann) is serious pest of pines in the Southern United States, Mexico, and Central America. The southern pine beetle kills conifers by boring under the bark and destroying the cambium layer of the tree. Trees are often mass attacked by thousands of individual southern pine beetles. Because populations can build rapidly to outbreak levels, large numbers of trees can be killed and forested ecosystems seriously affected. Infested areas may range from ten trees to several thousand acres. Large areas are often killed before land managers are aware of the beetles presence. The southern pine beetle can be found from northern Nicaragua to Maryland to Arizona. The most continuous populations occur in the southern and southeastern states especially in the range of shortleaf pine. Most species of conifers are susceptible to attack during intense outbreaks. In the Southern Appalachians, shortleaf, Virginia, pitch, table-mountain pine, and eastern white pine are attacked (Price 1994). Homogeneous pine stands are more susceptible than those mixed with hardwood and pine. Outbreaks in Tennessee, North Carolina, South Carolina, Virginia, and Georgia were recorded as early as the late 1700s and early 1800s. Beginning in the 1960s outbreaks were systematically surveyed and recorded. The worst southern pine beetle outbreak in the Southern Appalachians since the 1960s occurred between in 1973 and 1976 (SAMAB 1996). Based on the data collected since 1960, over 36.6 million cords of pine pulp and saw timber worth over $901 million dollars.

Biology

Southern Pine Beetle Galleries

Southern pine beetles normally infest open trunks of trees from the base to the crown, usually attacking first at mid-trunk or in the lower crown. After the first beetles arrive, mass attack often occurs as large numbers of beetles respond to pheromones and host attractants. Adult females construct winding egg galleries (SAMAB 1996). After mating, the female continues to excavate the gallery and lays eggs along both lateral walls of the gallery. Eggs are deposited singly, in discrete cavities (egg niches). Eggs hatch in two to nine days and the larvae enter the cambium layer, then the inner bark, enlarging their galleries as they grow. When mature, larvae bore to the outer dead bark, create a cell and pupate. Generally, emerging adults leave the host tree and aggregate on an adjacent tree or leave the area to find a suitable new host tree. Adult emergence may continue for an extended period of time. Dependent upon temperatures the beetles generally complete metamorphosis-egg, larva, pupa, and adult-in 30 to 40 days from April to September in the Southern Appalachians. As many as seven generations (broods) may be produced annually, but in the northern extremities of its range (North Carolina, Virginia, and Maryland) as few as three generations may occur (Price 1994).

Identification

open hole. The crowns of trees attacked by southern pine beetle during warm, dry weather may fade in color within two weeks. In conjunction with fading crowns and pitch tubes, reddish boring dust (chewed bark) will accumulate in the bark crevices. Trees attacked by southern pine beetle are hosts for several other secondary insects, such as wood borers and ambrosia beetles, both producing characteristic boring dust. Ambrosia beetle boring dust accumulates around the trees base and is a good indication of a dying tree (Price 1994).

Recognizing susceptible stands

Across the South, pine stands which are susceptible to southern pine beetle (SPB) infestations are usually dense, older, slower-growing stands on poorly drained sites or those growing on severely eroded sites of poor quality. However, younger stands also may be at risk. Generally SPB susceptible stands have one or more of these characteristics:

• High stand densities
• A large proportion of pine sawtimber
• Declining radial growth
• A high (>=28 percent) clay content in surface and subsurface soils in Piedmont sites
• Slopes exceed 10 percent
• Poorly drained soils and low-lying areas in Coastal Plain sites
• High percentage of shortleaf, pitch and/or loblolly pine in the stand
• Presence of annosus root rot, fusiform rust, littleleaf disease

Annosum Root Rot

Fusiform Rust

Littleleaf Disease

Aerial Survey

Survey and Detection

For many owners, on-site inspections for determinations of SPB hazard may be most practical. Evaluating stands of similar ages, stocking levels, and on similar sites can help pinpoint potential outbreaks. Information from management plans and inventory or timber cruises also is valuable. Pay particular attention to well-stocked and over-stocked stands nearing thinning age or those which are past optimum thinning age and are beginning to experience reduced growth. Other hazard sites are stands on poorly-drained or eroded clay soils and those on steep slopes. Factors leading to tree stress and to possible outbreaks include:

• Prolonged moisture or drought stress in trees or prolonged flooding.
• Slow tree growth common to overstocked and over-mature stands.
• Sites having poor internal drainage or low soil fertility.
• Diseased and storm-damaged stands.
• Excessive damage to residual stand when cutting or doing other work (more likely to lead to black turpentine beetle or Ips beetle attacks.

During outbreak conditions, rapid assessment can be made from aerial surveys and by ground checking previously identified hazard stands.

Southern pine beetle outbreak spot1

Southern pine beetle outbreak spot2

Southern pine beetle outbreak spot3

Silvicultural treatments to reduce losses in existing stands

Good stand management offers the cheapest, most practical, and longest lasting means of preventing SPB infestations, especially where the beetle epidemics occur frequently. Recommended practices are:

• To reduce stand susceptibility and damage from southern pine beetle, periodically reduce stand density to increase or maintain tree vigor.
  
• Southern pine beetle infestations are often associated with poor tree vigor. Tree vigor is related to site, tree, stand, and environmental conditions.
  
• Poor tree vigor is usually associated with densely stocked stands and declining or slow radial growth.
  
• Other factors that affect vigor include age, species composition, soil texture, and type, drainage patterns, and stand disturbances associated with cultural practices.
  
• Thinning, especially of weak, less vigorous trees in lower crown classes, that are susceptible to southern pine beetle attack, reduces competition and enhances the vigor of residual trees.
  
• Thinning to reduce southern pine beetle hazard is recommended when stand basal area approaches 120 square feet/acre or when live crown ratios of dominant and co-dominant trees drop to about 40 percent.
  
• Thinning stands back to 60 to 90 square feet/acre basal area reduces the risk of attacks and may also help to slow infestation expansion (spot growth) if an attack does occur.
  
• For greater effectiveness, thinning should be conducted in winter when the beetle is least active.
  
• Any thinning strategy to reduce the risk of southern pine beetle attack should be compatible with management goals and consider such things as site and stand factors, equipment, seasonality, market price for small diameter trees, and product objective. Management of other potential hazard (e.g., fusiform rust, annosus root rot, Ips spp., and black turpentine beetle) that might conflict with recommendations for southern pine beetle must also enter into the decision-making process.

Timing of the First Thinning

Precommercial thinning

• Precommercial thinning is probably justified in dense, natural stands and in plantations established by direct seeding or supplemented with natural regeneration from surrounding stands if there are 1,500 or more well-spaced seedlings per acre

Precommercial Thinning1

Precommercial Thinning2

• The first thinning often is performed as soon as the seedlings are well established, usually between ages 2 and 5 before they have experienced severe intraspecific competition and while they still are small enough to permit thinning with relatively light equipment such as a rotary mower or light chopper.

Precommercial Thinning3

Precommercial Thinning4

• If stocking is fairly uniform, seedlings can be removed in strips. Where stocking is extremely high, cross stripping can be used to further reduce seedling numbers. The best response appears to be obtained with a residual stocking of 500 to 750 trees/acre.

Precommercial Thinning5

Precommercial Thinning6

Commercial thinning

• First commercial thinnings should be made just prior to overcrowding, reduced diameter growth, and heavy mortality and before the live crown ratio is reduced to below 35 percent of total height.

Commercial Thinning1

• The beginning of suppression-caused mortality in 4- to 5-inch-diameter trees serves as a good signal for a first thinning in most stands. Generally this occurs at 13 to 18 years.

Commercial Thinning2

• Thinning guidelines for southern pines frequently suggest removing 30 to 45 percent of the stand basal area.
• The percentage of the basal area to be left tends to increase with increasing site quality because of greater productive capacity. Residual basal areas range from 60 to 90 square feet/acre and tend to be lower on poor than on good sites.

Commercial Thinning3

Commercial Thinning4

Commercial Thinning5

Setting Control Priorities for the Southern Pine Beetle

All southern pine beetle spots (groups of infested trees) do not have the same control priority. The following guidelines should help you set priorities for controlling individual spots.

A. Classify the infested trees according to the stage of attack shown below.

Stage of Attack
Symptom	Stage 1 Fresh attacks	Stage 2 Developing broods	Stage 3 Vacated trees
Foliage	Green	Green; fade to yellow before beetles emerge	Red; needles falling
Pitch tubes	Soft; white to light pink	Hardened; white	Hard; yellow; crumble easily
Checkered beetles	Adults crawling on the bark	Larvae in SPB galleries; pink or red; 1/2 inch long	Larvae and pupae are purple; occur in pockets in the outer bark
Bark	Tight; hard to remove	Loose; peels easily	Very loose; easily removed
Color of wood surface	white, except close to new adult galleries	Light brown with blue or black sections	Dark brown to black; may have sawyer galleries
Exit holes	----	May appear where parent beetles left the tree	Numerous; associated with brood adult emergence
Ambrosia beetle dust	----	White; begins to appear around the base of trees	Abundant at the base of trees

B. Collect spot expansion data:

1. Walk completely around the spot and look for stage 1 trees, which indicate the area of most recent beetle activity. Areas with stage 1 pines are called "Active Heads." Check to see if the spot is expanding in more than one direction. Large spots can have more than one "active head."
2. Determine the number of stage 1 and 2 trees. For large spots that have more than 50 trees, it is not necessary to examine each tree. Just walk the boundaries and estimate the number of these trees in the spot.
3. From a location about 20 feet (6 m) in front of the active head(s), determine the pine basal area (a measure of stand density) in square feet per acre. A 10-factor prism is useful for this purpose.
4. Note whether most trees in the spot are pulpwood (less than 9 inches in diameter) (23 cm) or sawtimber size (more than 9 inches in diameter).
5. If only stage 3 trees are present, control is not necessary.
6. Determine the control priority for the spot. See C, below.

C. Guide to southern pine beetle control priorities (May through October):

	Key to spot growth	Your spots classification	Risk-rating points
A.	Stage 1 trees (fresh attacks)	absent present	0 30
B.	Stage 1 (fresh attacks) and Stage 2 trees (developing brood)	1 to 10 11 to 10 21 to 50 more that 50	0 10 20 40
C.	Pine basal area (ft2/acre) or stand density at active head or heads	less than 80 (low density) 80 to 120 (medium density) more than 120 (high density)	0 10 20
D.	Stand class by average d.b.h. (in inches)	pulpwood (9 inches or less) sawtimber (more than 9 inches)	0 10

Add up the risk rating points that apply to your spot: Score Control priority 70 to 100 High 40 to 60 Medium 0 to 30 Low