Forest Snapshot January 2025

Important Dates

Registration is now open for the Forest Health, Insect and Disease Briefing and the Forest Landowner Conference. This is a three-day event offering forest health updates, woodland stewardship workshops, expert presentations, field tours, and valuable resources for landowners and advocates. For more information and to register for either event, please visit the conference website. 

Forest Health

Hemlock Woolly Adelgid – Winter Mortality: How Cold, For How Long, and When?

One of the main controls on HWA populations in Pennsylvania is winter minimum temperatures. Temperatures during the last several winters have not been sufficient to limit the spread and increase of the HWA population. Even with a significant 3-day cold snap during late December of 2022, HWA winter mortality was not enough to limit populations in the subsequent spring generation.

For temperature to appreciably affect HWA populations, several factors must be considered: how low the temperature is, how long that low temperature lasts, and at what time of the year it occurs. The population must experience at least about 95% mortality to actually be reduced. For example, -29 degrees Fahrenheit overnight in late February is very likely to kill almost 100% of adelgids. An average temperature in the low 20s Fahrenheit from December to March is likely to do the same. Negative single-digit cold snaps for a day or two in the heart of winter may feel like they should be doing some damage, but this is not always the case.

The current winter is shaping up to be more impactful to HWA populations than the previous winter. However, what we really need are the same low temperatures in late winter that we’ve seen mid-winter this season. HWA winter generation mortality surveys will be completed by Bureau of Forestry staff in early to mid-March. 

For more information on HWA, visit the DCNR Adelges tsugae page. For treatment recommendations, visit the Integrated Approach to Hemlock Woolly Adelgid Mitigation publication.

Native Species

The sub-zero temperatures of winter are upon us, with many plants well into dormancy. The only green dotting the forests include the evergreens, like the needled species- pines and hemlocks- and a few broadleaf plants like the rhododendron pictured above. Most broadleaf plants are deciduous, losing their leaves in autumn as self-protection from the cold. The evergreens, however, evolved to have thick leaves with waxy coatings and resins to resist cracking in freezing temperatures. Spend time on a snowy hike, and you might notice rhododendrons (Rhododendron maximum) with drooping, curled leaves. This is called thermonasty, a non-directional plant movement in response to a change in temperature. In the cold Pennsylvania winter, drooping and curling help to reduce water loss, light exposure, and the effects of thawing too quickly.

Wildlife and Insects

Barred Owls have been vocalizing their "who cooks for you all" and more excited monkey-ish sounding calls for several weeks. Their breeding began around mid-January. They, along with many other birds, such as bald eagles and great horned owls, will begin sitting on nests in the next month. 

If you're hiking in these frozen times, you may see small creatures hopping around on the top of the ice. Known as snow fleas, these creatures are Hypogastrura nivicola, a species of springtails (Collembola). While springtails are not true insects (and therefore not true 'fleas'), they do have a jumping mechanism that may have contributed to the misnomer. While snow fleas can walk across terrain, they have a specialized 'tail' that is tucked underneath their abdomen which allows them to 'spring' up into the air as a form of transportation and to avoid predators. They are able to walk around in subzero temperatures because of a special antifreeze-like protein that they can synthesize, which researchers at Queen's University in Canada have learned to create in a lab and hope to use to help transport organs or develop better ice creams. These creatures can be found throughout Pennsylvania, and many Entomology students are planning hikes to try and find them within Stone Valley!