Ponderosa pine showing signs of stress


CHEYENNE  – Across eastern Wyoming, from Cheyenne to Newcastle, people have noticed high levels of damage to ponderosa pine trees. Damage varies in extent and intensity from partial yellowing of needles to tree mortality. At this time, experts do not fully understand all of the factors contributing to the condition of the trees, but Wyoming State Forestry Division is working with the U.S. Forest Service and the University of Wyoming Extension on the issue.

Two factors which explain some of the effects people are seeing on ponderosa pine trees:

• Winter damage caused by drying out of needles and the fluctuation of cold temperatures

• Twig bark beetles

At the beginning of last October 2019, Wyoming experienced abnormally high fluctuations between daytime high and nighttime low temperatures. This combined with a late spring freeze in early April likely damaged branch tips and more susceptible areas of the trees. In May of this year, high amounts of winter desiccation were observed. Signs include needles yellowing from the tips back, often on the windward side of the tree. It is caused by the trees losing water due to warm temperatures, high winds or not having access to adequate moisture in the soil.

Throughout the summer in multiple areas, we have found increased evidence of twig beetles in ponderosa pine trees affected by winter damage. Twig beetles are a variety of small native bark beetle species that primarily affect small branches in stressed or weakened trees. Twig beetles bore into the bark killing the ends of branches. The result is a complete reddish yellowing color in affected areas. This can appear as spotted discoloring throughout the tree and examples can be seen in the pictures.

Twig beetles have multiple generations per year and have the ability to increase exponentially when conditions are favorable. Favorable conditions, in this case, are the abundance of stressed trees from winter damage providing high amounts of food and breeding habitat for the twig beetles.

Treatments at this time are limited and only practical for high-value trees. Ensure yard trees are watered when winter temperatures are above freezing and no snow melt is present. Insecticides are available for twig beetle control, but only applicable in severe cases. Wyoming State Forestry is continuing to work on the issue and will be providing more information as it becomes available.

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