A tiny insect introduced into the United States from East Asia, the hemlock woolly adelgid feeds at the base of hemlock needles, defoliating and eventually killing trees. Since the insect was first noticed in the 1950s, it has expanded its range at between 4.7 and 12.7 miles a year and currently infests about 45 percent of the range of hemlocks in the United States and 41 percent of all hemlock trees.
"The analysis also showed that the general regional trend in the East over the past 50 years has been one of increasing hemlock volume, even with infestation by the hemlock woolly adelgid," says Randall Morin, NRS FIA research forester and primary author of the update. "Even though the insect has caused substantial negative impacts on hemlock at local scales, analysis of FIA data suggests that infestations have not yet reduced the overall abundance of hemlock, even in states where hemlock woolly adelgid has been active for decades."
The authors caution that the trend of increasing hemlock volume may not last much longer.
"Despite increasing hemlock volume over the last four decades across most of the eastern United States, the regions with long-established populations of hemlock woolly adelgid are also the regions where hemlock is accumulating slowest," says Morin. "Net growth rates decrease as years of infestation increase and mortality rates increase, with mortality starting to equal net growth in areas where hemlock woolly adelgid has been present for 10 to 20 years." As time goes on, the trend of increasing abundance may begin to reverse.
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USDA Forest Service ‑ Southern Research Station