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Hemlock Hybrids Could Reverse Decline in Landscapers’ Favorite

USDA scientists Susan Bentz and Richard Olsen examine bagged branches of hybrid hemlocks inoculated with hemlock woolly adelgid as part of field tests of the hybrids’ tolerance to the Asian pest.

USDA scientists Susan Bentz and Richard Olsen examine bagged branches of hybrid hemlocks inoculated with hemlock woolly adelgid as part of field tests of the hybrids’ tolerance to the Asian pest.

This post is part of the Science Tuesday feature series on the USDA blog. Check back each week as we showcase stories and news from USDA’s rich science and research portfolio.

For nearly 60 years, a relentless Asian insect with a silly-sounding name–the hemlock woolly adelgid, or HWA–has chomped a deadly swath through 17 northeastern states, portions of Canada and the Appalachian Mountains, literally sucking the life out of native hemlock trees.

Fortunately, in the 1990s USDA scientists at the U.S. National Arboretum kicked off a breeding program to develop new hybrid hemlocks that could reverse years of decline of these popular, graceful trees that have long been favorites of landscapers and homeowners.  The breeding program has focused on crossing native hemlocks with other hemlock varieties thought to be resistant to the imported pest.

The two most common hemlock trees in the United States—Tsuga canadensis, commonly known as the eastern hemlock and also called the “Queen of Conifers,” and Tsuga caroliniana, or “Carolina hemlock”—have no natural immunity to HWA. Accidentally introduced from Asia and first observed in Virginia in 1951, HWA has no natural enemies in North America, and so they’re free to feed, often destroying vulnerable hemlocks in just a few years.

The scientists have made several collecting expeditions to Asia to find resistant hemlocks, focusing their efforts mainly on China.  At the end of the expeditions, the ARS scientists brought accessions of three Tsuga species:  T. chinensis, T. dumosa, and T. forrestii.

The scientists crossed native hemlocks with the collected species to develop 140 Tsugu hybrids.  In 2006, they began a multi-year field trial to test each hybrid’s tolerance to HWA.

T. chinensis and its hybrids showed the highest tolerance to HWA, and these trees also offer good vigor and an attractive shape.  Several more years of testing are needed before these new beauties will turn up at local garden centers, but at least the hemlock woolly adelgid’s free ride may soon be coming to an end.

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