What Happened to the Southern Pine Beetle?
Ronald F. Billings and Herbert A. (Joe) Pase III
Texas A&M Forest Service
The southern pine beetle (SPB), Dendroctonus frontalis, has earned its reputation as the most destructive pest of commercial pine forests in East Texas. From 1958 through 1997, SPB outbreaks occurred every 6-9 years, with peaks in 1962, 1968, 1976, 1985, and 1993 (Fig. 1). The worst outbreak on record in East Texas occurred in 1985, when over 50,000 acres of pine forests were killed. Only 390 infestations were reported in 1996 and 30 in 1997. Beginning in 1998 and continuing to date, no SPB infestations have been reported on state and private lands in East Texas. Previous SPB outbreaks in Texas and other southern states have been associated with numerous factors. These include above average rainfall, periods of drought, mild temperatures during winter months, abundance of overstocked, overmature, and/or unmanaged pine stands, abundant lightning strikes during April and May, and lack of natural enemies, among other factors.
Fortunately, it takes more than one favorable factor to cause an SPB outbreak. For example, the winter of 1997-1998 was very mild which has been known to contribute to increased SPB activity. Also, a severe windstorm in February of 1998 damaged millions of board feet of timber across several counties in East Texas, but no SPB outbreak occurred that year. Hurricane Rita in 2005 and Hurricane Ike in 2008 adversely impacted the state’s pine forests. Many people thought these wind storms would trigger a southern pine beetle outbreak. And if that weren’t enough, severe drought during the summers of 1998, 2006, 2008 and 2009 would surely cause an increase in SPB populations.
Historically, storm-damaged timber has not been associated with SPB outbreaks. Wind-thrown trees are typically attacked by Ips engraver beetles, rather than SPB. Although hot, dry weather has been known to contribute to SPB population increases in other parts of the South, in East Texas, high temperatures have led more to the collapse SPB outbreaks, not their initiation.
Pheromone traps deployed in March and April of each year for predicting SPB activity have consistently indicated low activity levels of SPB since 1997. In the end, private forest landowners in East Texas have enjoyed 12 consecutive years with no SPB-caused losses. So, what has happened to the southern pine beetle in East Texas? A likely explanation for the lack of SPB outbreaks in recent years is that numerous environmental and host factors have combined to keep this native insect at extremely low levels. First, weather conditions have not been conducive to an outbreak. The SPB outbreaks in the 1970s-1990s coincided with years of above average rainfall, which inundated southeast Texas counties for long periods and predisposed pines to beetle attacks over wide areas. The frequent droughts East Texas has experienced in recent years have not had the same effect.
Second, with the exception of the National Forests, the vast acreages of natural, mature pine stands that supported previous SPB outbreaks have been largely converted to young, intensively-managed plantations that are less susceptible to SPB outbreaks. The SPB Prevention Project has further reduced available food for the beetle by encouraging private landowners to thin the overcrowded and stressed pine stands that favor SPB infestations. As of July 2010, more than 86,000 acres of beetle-prone pine stands on private lands in East Texas have been scheduled for thinning and 3 of every 4 of these acres has already been thinned under the SPB Prevention Project.
The frequent hurricanes and droughts East Texas has suffered in recent years have fostered increased populations of Ips engraver beetles and associated predatory insects. One of these is the clerid or checkered beetle, Thanasimus dubius. This predator feeds on Ips and other small bark beetles, but actually prefers SPB. Indeed, many researchers and pest management specialists considered clerid beetles to be a major natural control factor that may regulate SPB population cycles. For example, Dr. Matt Ayres at Dartmouth University and his collaborators have recently proposed that natural controls may be more responsible than other factors for the absence of SPB activity (S. Martinson, T. Ylioja, B. Sullivan, R. Billings, and M. Ayres, Alternate attractors in populations of southern pine beetle, submitted for publication in Ecology, December 2009). Their analysis of SPB and clerid trap catches from across the South since 1988 and additional field tests support the conclusion that clerid beetles are maintaining SPB populations at low levels.
When no SPB are available, as has been the case for more than a decade in East Texas, the clerids don’t starve. They simply switch over and do quite well feeding on the ubiquitous engraver beetles (Ips avulsus, Ips grandicollis or Ips calligraphus). Thus, these predators far outnumber their SPB prey under current conditions, presumably making it difficult for SPB to rebound to high levels. Evidence for the abundance of predators has been documented in recent years with the SPB pheromone traps that TFS and USFS deploy each spring to forecast SPB outbreaks in East Texas. Four dozen traps deployed in 16 counties have caught 4,000-15,000 clerid beetles per year since 1998 and few or no SPB. Assuming that clerids are providing biological control of SPB populations, landowners should not begrudge the loss of scattered pine trees to engraver beetles during periods of drought. By providing ample food, Ips populations are responsible for maintaining high clerid populations, which in turn may be keeping SPB in check.
Another consideration is that chemical insecticides, widely used to control SPB infestations throughout the 1960s, were voluntarily discontinued around 1970. The standard approach of felling infested trees and spraying the entire boles with BHC or lindane in diesel oil was costly (even with diesel selling for $0.15/gallon) and of dubious effectiveness. One research study (D. L. Williamson and J. P.Vité, Impact of insecticidal control on the southern pine beetle population in east Texas, J. Economic Entomology 64: 1440-1444, 1971) came to the conclusion that chemical insecticides applied on a large scale were responsible for killing more clerid beetle predators than SPB, due to the predator’s slower life cycle. This fact was proposed as the reason why the SPB outbreak in the 1960s persisted year after year for more than a decade, despite intensive direct control efforts.
Since 1970, SPB spots have been controlled almost exclusively by non-toxic, mechanical methods: salvage removal of infested trees and cut-and-leave (a control method in which infested trees and a buffer of uninfested trees are felled and left in place to disrupt further infestation expansion). Predator populations survive in much higher numbers using the cut-and-leave control method.
For whatever the reason(s), SPB populations continue to remain low. No SPB infestations were detected in 2009 throughout East Texas, Louisiana, Arkansas, or Oklahoma. SPB activity throughout the South was the lowest on record last year, with fewer than 400 infestations reported in 16 states combined. Despite this favorable situation, thinning overly-dense young pine stands remains a recommended forestry practice. Thinning releases crop trees from competition, allowing them to attain saw-timber size in fewer years. This means more profits for the landowner, since the price mills pay for sawtimber-sized trees is much higher than for pulpwood. Thinning also serves to reduce the threat of losses from wildfires and other bark beetle pests. And no one knows when the next SPB outbreak will occur. Federal cost shares are available through the SPB Prevention Project to encourage landowners to thin overly dense pine stands. Contact your nearest Texas A&M Forest Service office or the TFS web page for details.
RFB, HAP/July 2010