Between 2005 and 2015, only 18 counties in the southern U.S. experienced southern pine beetle outbreaks–fewer than two outbreak counties per year on average. U.S. Forest Service entomologist Chris Asaro wanted to know why and worked with two other Forest Health Protection scientists, John Nowak and Anthony Elledge, on a new review paper published in Forest Ecology and Management.
The southern pine beetle (Dendroctonus frontalis) is the most destructive native forest insect pest known to the 13-state southern region–most often targeting loblolly pine, shortleaf pine, and Virginia pine plantations, along with natural pine stands. From the early 1960s to 2004, 50 or more counties, on average, would experience major southern pine beetle outbreaks on a five to seven year cycle.
The last major outbreak affected close to a million acres of forest across eight states, with a damage estimate of $1.5 billion.
Across the Piedmont and Coastal Plain, outbreak severity has declined steadily since the mid-1970s and then dramatically since the mid-1990s. Over that same period of time, southern pine plantations grew from 1.8 million acres in the 1950s to 39.5 million acres by 2012, an approximately 20-fold increase.
Why, with over twenty times more plantation area available for the beetles, has the region seen a disproportionately small number of beetle outbreaks over the last decade or two? Previous studies have explored possible explanations focused on weather and climate or controls like natural enemies, competition, or predator-prey relationships.
Asaro and his colleagues thought there should be more consideration for host-oriented hypotheses–genetics, intensive silviculture, and stand fragmentation–that might explain the apparent paradox as well as broaden the conversation and potentially stimulate new research.
Genetically improved pines have outnumbered unimproved trees since 1990, and it may be that selecting for stronger growth characteristics has inadvertently favored better resin flow.
Resin production is the pine tree’s primary defense mechanism against bark beetles–when a beetle starts to bore into a tree’s bark, resinous sap flows out through the hole, encapsulating and trapping the beetle. Under good growing conditions, loblolly and shortleaf pines tend to produce more resin and are better able to resist southern pine beetle attacks.
Current silviculture practices on pine plantations include many more acres with wider tree spacing, thinning, fertilization, and the use of more environmentally benign herbicides compared to earlier decades.
“Thinning is one of the most effective practices for preventing or mitigating southern pine beetle impacts,” says Nowak. “Land managers can thin stands to improve tree condition and also create barriers for beetle population growth and spread.”
The end result is more pine acreage with faster growth rates, shorter rotations, and less competition. All of these stand characteristics are known to prevent or lessen southern pine beetle impacts. At the regional scale, there is a strong correlation between increasing area that’s intensively managed and declining levels of southern pine beetle outbreak.
Despite the large increase in southern pine plantation acreage, it’s important to note that land parcels are smaller, due primarily to widespread urbanization and changes to industrial and private ownership patterns. Recent Forest Health Protection analysis shows that larger stands are more likely to contain a southern pine beetle infestation or “spot.”
Small, localized outbreaks in recent years have primarily occurred on public–federal and state–lands, areas where pine forests may have increased risk due to less intensive management, longer rotations, and fewer genetically improved trees.
Although potentially more susceptible to beetle attacks, these natural pine and mixed hardwood stands provide ecosystem benefits and services beyond timber–clean drinking water, wildlife habitat, and places for recreation.
“It’s likely that multiple variables such as climate, natural enemies, competition, and host-related factors are important and working in combination to affect southern pine beetle behavior,” adds Asaro, coordinator of the USFS Southern Regional Forest Health Monitoring Program. “Sound forest management can minimize problems with major pests and diseases while simultaneously providing economic output and environmental benefits.”
Nowak describes one important research need: an analysis of historic trends in southern pine beetle outbreaks alongside changes in the fragmentation of vulnerable forest types.
The Southern Pine Beetle Prevention Program, managed by Nowak, will continue to monitor outbreaks–however infrequent–and provide assistance with the forest management practices that reduce southern pine beetle hazard.