University of Georgia
There was a feast in Georgia's Oconee National Forest last year. Southern pine beetles were munching away on weak, old trees. And the drought may have issued the invitation to dinner, a University of Georgia expert says.
“The drought could have contributed to an increase in beetle populations, but we don’t have any way to tell for sure,” said Keith Douce, a UGA entomologist with the College of Agricultural and Environmental Sciences.
“We’ve been under drought stress for several years,” he said. “Even if you get adequate rainfalls, it takes two or three years for a tree to get back to full health.”
It’s during those weakened years that problems begin. Beetle numbers build up over a few years and reach what Douce calls a “tipping point.”
“A tree can have 10,000 beetles in it,” he said. “They put out pheromones that other beetles detect. And more beetles come over to eat like when grandma’s cooking good food and the neighbors come over because they smell it.”
Once a large number of beetles enter a tree, they put out another chemical signal that the tree is full and other beetles should go somewhere else.
Beetle populations are also affected by temperature. “A lot depends on the beetles and how well they over winter,” Douce said. “That 19 degree temperature we had recently can effect the population by reducing their numbers.”
Georgia has several different types of beetles that attack trees. The most common are the Southern pine beetle, the black turpentine beetle and three species of ips beetles.
“There have been large numbers of ips beetles around the state this year,” Douce said. “We have seen increases before during this dry weather.”
Beetle populations vary over time. “Between 7 and 12 years they increase and decrease,” he said. “We don’t know what causes those cycles, but over time we will see increases and decreases.”
Douce said the increased activity of Southern pine beetles last year was “undoubtedly due to a lot of stress from drought and over maturing of trees that haven’t been harvested.”
When trees get to be 40 to 45 years old, they slow down just like people do. The tree can survive, but it’s less healthy, and beetles are better able to attack it.
“Just like older people are more susceptible to disease and injury, so are trees,” he said.
The life cycle of the tree and the beetles interact. The Southern pine beetle is particularly good at finding trees in decline.
“The drought causes trees to decline,” he explained. “The resin in the tree reduces the organisms that can get into the tree.”
A good, growing, healthy tree produces large amounts of resin to keep the beetles out. The beetles get stuck in the resin and don’t get into the tree.
“As you have drought, the tree produces less resin,” Douce said. “You also get things like lightening strikes that cause a tree to decline, and the beetles can detect that and attack the tree.
“If you go through that cycle two or three times over a summer, you have literally thousands of beetles flying around and attacking trees under stress. Lightening itself can fry the tap root of a tree and it’s a goner. But beetles can find that tree because it’s under stress and they can attack and finish it off.”