Do Cold Winters Impact Hemlock Wooly Adelgid Populations?

Sarah Johnson from The Nature Conservancy's High Allegheny Hemlock Project shared some encouraging news about overwinter mortality of hemlock wooly adelgid (HWA).  This information was initially provided by Rick Turcotte, Entomologist, US Forest Service, and Tim Frontz, DCNR Forest Pest Management.

Rick shared his analysis of 2 samples of HWA:

~93% mortality of HWA in Clarion River sample

~95% mortality of HWA in Allegheny river sample

Tim Frontz shared the below data about samples from Cook Forest State Park and one Elk County infestation from DCNR lands.  HWA mortality assessments were made on foliage collected on Jan. 15 and Feb. 9, 2014 at Cook Forest State Park, PA.  You can see, the mortality rates were 97% or higher with most at 100%.  That is good news for the hemlocks of the high Allegheny Plateau.

HWA density	County	Live HWA	Dead HWA	% Mortality
HIGH	Elk/ Cameron	9	306	97
LOW	Forest (CF State Park)	0	47	100
LOW	Forest (CF State Park)	0	94	100
LOW	Forest (CF State Park)	0	60	100
LOW	Forest (CF State Park)	1	106	>99

Because of the high fecundity of HWA, an overwinter mortality rate of 91% is necessary to keep the population from increasing.  So mortality rates at 91% mean the infestation will not get any larger, above 91% means a temporary decrease in the infestation size.  The entomologists have also shared a caveat – with such high reproductive rates of HWA, this winter kill would need to be repeated maybe several years in a row, or happen more often (rather than just once every 10 years) to have significant overall impact in the grand scheme of things.

PA DCNR also provided an overview in a recent news release shared below.

DCNR gauging past frigid winter’s effect on forest insect pests

The past winter of seemingly unending snowstorms and frigid temperatures has proved to be a strong ally for state woodland managers battling the No. 1 enemy of Pennsylvania hemlocks, but the reprieve could be short-lived, Department of Conservation and Natural Resources officials said recently.  To read the full story click here.