Brutal cold could take a bite out of forest-munching pests

The prolonged cold got you down? This winter's extreme temperatures could mean good news for forests in Pennsylvania and New Jersey.

Experts say, with a bit of caution, that the cold has the potential to curb the advance of two or three pests that have ravaged woods during the last two mild winters — although it's too early to tell.

The southern pine beetle and spotted lanternfly have both expanded from the South into our region in recent years.

The southern pine beetle, Dendroctonus frontalis, has impacted 46,000 acres in New Jersey since the early 2000s and is considered the most destructive insect for forests.  In Pennsylvania, it was the culprit in a recent rampage in Nottingham Park in Chester County that destroyed hundreds of acres of pitch pine trees.

Columbia University researchers concluded in a 2017 study that mild winters have allowed the pest, which is the size of a rice grain, to migrate north. The researchers found the temperature of tree bark on the coldest night of the year is critical. The insect feeds on living tissue under the bark. If the bark remains cold enough, the pests will die.  But in recent years, the temperature on the coldest night of the year in New Jersey has been rising.

The researchers found that bark temperature of about 14 degrees Fahrenheit is the limit at which the beetle can survive. But this winter has been exceptionally cold and is about to get colder.

The National Weather Service is forecasting a high of 11 degrees Saturday with an overnight low of 2.  Although tree bark provides insulation, the low temperatures could be enough to kill the bugs. 

"It will have an impact with some insects, but not with others," said Donald Eggen, an entomologist and forest health manager for Pennsylvania's Department of Conservation and Natural Resources.  "The good news is the southern pine beetle is susceptible to these cold temperatures. I'm hoping a lot will die because of the cold. That's a positive note."

Experts don't expect the cold will be the end of the southern pine beetle in this area.  But it could slow them down, giving foresters time to catch up and preserve the trees.

Eggen is also hopeful about the cold's impact on the spotted lanternfly, or Lycorma delicatula, an insect native to Asia that first arrived in the United States in Berks County in 2014 and has spread to neighboring counties. The pest is a potential threat to the state's $13.1 billion annual production of apples, grapes, peaches, and other crops, as well as $16 billion in timber and wood products.

Eggen said the spread of the spotted lanternfly has been swift the last two mild winters, as it lays its eggs in trees over winter.

"We're hoping this winter has an impact, but we don't know," Eggen said. "Anecdotal evidence shows cold will cause mortality. If you get a couple hundred eggs, a good percentage of those eggs will die. So instead of a 100 percent hatch, you'll get a 20 percent hatch."

However, the cold's impact on the overall pest population won't be known for sure until spring.

Experts are also concerned about the emerald ash borer but are doubtful the cold will have much impact.  It can survive in bark temperatures of almost 20 below zero, according to Robert Rabaglia, national entomologist with the USDA Forest Service.

Rabaglia said it's not easy to predict the impact of cold. He noted that the life stage of the insect, the duration of the cold, and other factors determine whether pests survive.  For example, a cold snap could kill off a predator, but not a targeted pest, which would thrive come spring.

But he noted that the polar vortex of 2014 killed off a lot of hemlock woolly adelgid, another invasive species from Asia that has sucked the sap from hemlocks and spruce trees in the Northeast.

Ultimately, Rabaglia said, pest populations are so high, there would have to be a big mortality rate to make an impact.

"The timing is important," he said. "There are a lot of variables, so it's hard to make a sweeping statement either way. I think it will potentially affect some but not others."