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Will the frigid winter limit forest pests?

February 27, 2014

University Park, Pa. — This winter has provided icy roads, vehicles that won’t start, school cancellations, high heating bills and lots and lots of snow to shovel or plow. With lots of temperatures well below zero and wind chill factors much lower, this has been the coldest winter in nearly 30 years.

Frigid weather takes a toll on deer, wild turkeys and even trout, although for the most part, this state’s native wildlife can take what Mother Nature dishes out. Some hunters and anglers – as they suffer through the sub-zero cold – are wondering if the extreme temperatures will help control a few of our major forest pests.         

According to foresters and entomologists, the answers are ”yes,” “unlikely” and “maybe” – depending on the species and how the state’s spring weather plays out. Penn State University ornamental entomologist Greg Hoover explained the variables involved.

“When it comes to forest pests, one size does not fit all,” he said. “Pennsylvania's forests are very diverse and how much a specific pest will be affected depends on the insect’s life stage and where they spend the winter.”

The hemlock wooly adelgid is one of the major invasive pests – killing Eastern hemlocks. Adelgids have been spreading west through Pennsylvania since 1967, and have now reached most of the state. Their new discovery in several virgin stands of hemlocks in the western part of the state was announced just last year.

Hemlock – the state tree – is important as cover for ruffed grouse, turkeys and other animals. It produces soft wood that is used for paper pulp and construction. In addition, hemlocks shade many mountain streams – keeping the water cool for trout.

“Adelgids are insects that feed during the winter,” said Department of Natural Resources forester Chris Jones. “They begin dying at temperatures of 5 degrees F and below. Therefore, this cold winter, the first in a long time, should deal the pest a major setback and help to keep the woolly adelgid populations in check.”

Hoover concurs. “It is only the adult female adelgids that overwinter. They are like a tiny water balloon attached to the base of hemlock needles and have little protection against extreme cold and desiccating winds. I expect a major die-off.”            

More good news – both Hoover and Jones expect the emerald ash borer to also be negatively impacted, but not as greatly. A relative newcomer, this exotic insect from Asia did not arrive in Pennsylvania until 2006.

According to Jones, it spread quickly and is also found all over the state. The borer is expected to wipe out most of the state’s ash trees. Some experts are predicting a 10- to 20-percent kill of the borers across the state, but neither Hoover nor Jones made a prediction.

“Emerald ash borers overwinter as larva – under the bark of all species of ash trees,” Hoover said. “There is some insulating value to the bark, and they are protected from the wind. I expect them to be affected, but don’t look for a dramatic reduction.”

Jones added, “This won’t be enough to remove the mortal threat the invasive insect poses to ash trees. Many beetles will survive and reproduce, but even a partial kill-off could buy some time for boroughs, cities and homeowners trying to plan for mitigation of tree loss.”

Gypsy moths spend the winter in their egg form – with egg masses often exposed to the elements. Therefore, Hoover sees better news with respect to the gypsy moth.

“If the eggs were laid at the base of the tree or under a rock, they will be somewhat protected by the snow. However, I expect a high mortality of eggs that were laid on the sides of trees last summer,” Hoover said.

“I worked for the Department of Forestry back in 1984-85, our most recent extremely cold winter. I remember collecting exposed egg masses in March, and we placed them in Petri dishes to monitor their hatching. I can’t recall an exact percentage, but I remember that our experiment showed a very high mortality.”

According to Hoover, gypsy moths currently have the highest concentrations in the northwestern part of Pennsylvania – where some of the lowest temperatures have occurred this winter.

Additional research shows that alternate periods of freezing and thawing in the early spring can prevent gypsy moth eggs from hatching.

We will have to wait to see how that plays out in March and April.

Ticks, a nemesis of hunters, hikers, campers and anglers, could be affected by the extreme cold or the freeze-thaw cycle. Experts suggest that the effect will only be minimal because ticks spend the coldest periods buried deep in the leaf litter. 

A 2012 study done in New York found that subzero temperatures increased tick mortality only slightly, with more than 80 percent survival. Other research suggests a correlation between cold winters and reduced incidences of Lyme disease.

“The black-legged tick – the carrier of Lyme disease  is very hardy,” said Steve Jacobs, Penn State urban and public health entomologist. “They can still be active in the forest at 28 degrees.

Unfortunately, this extremely cold weather will have very little impact on the tick population because they are just not exposed to it. Right now most are under a blanket of snow.”

Summing up our harsh winter’s effect on pests:  “Yes,” expect a sharp decline in hemlock wooly adelgids and gypsy moths next spring; “Maybe,” there could be a small reduction in the emerald ash borer population; “Unlikely,” that black-legged deer ticks will be affected at all.

Experts agree, we will be dealing with all of these pests well into the future, regardless of what kind of weather the state receives during the next few months.

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