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U.S. Forest Service
Caring for the land and serving people

United States Department of Agriculture

Forest Health News 2018

DROUGHT causes IPS Bark Beetle Outbreaks

By Chris Asaro, USDA Forest Service

April 2018

Ips Outbreak

Ips Outbreak on the Northern Cheyenne Indian Reservation. Photo by: Joel Egan, USDA Forest Service.

Under normal conditions, Ips bark beetles rarely cause significant, widespread tree health issues other than killing a branch or two high in the tree crown of yellow pines, or sporadically killing a whole tree that is under significant stress. However, the exceptional drought conditions experienced throughout much of the southern Appalachians and Piedmont during 2016 enabled an explosive outbreak. In general, Ips outbreaks are strongly associated with sustained drought because these insects can only attack trees that are in a weakened state. Severe drought conditions built up throughout 2016 and peaked during the fall, in concert with elevated temperatures through November. Such optimal conditions for Ips resulted in numerous spots larger than 10 acres each and one exceeding 50 acres. Damage of this magnitude is typically only seen with the southern pine beetle, Dendroctonus frontalis. Ips spots are just a symptom of widespread tree stress; individual spots tend to be small (2-10 trees) and don’t grow and spread in the manner that southern pine beetle does. Prolonged drought can result in thousands of small spots littered across the landscape. Few, if any, management tactics exist for dealing with Ips other than salvaging (clearcutting) stands that reach a critical threshold of damaged timber. Thinning trees during periods of drought stress and Ips outbreaks is not recommended because it may lead to additional mortality of residual trees.