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Hemlock Woolly Adelgid (HWA)

The Hemlock Woolly Adelgid (HWA) is an aphid-like insect that feeds mainly upon Hemlock tree species and was introduced into the eastern United States from Japan in 1951 by import of nursery stock. Since its introduction, HWA have spread to 17 states from Georgia to Maine where they cause large scale mortality of Native Eastern and Carolina hemlocks. This tiny parasite spreads across the forest by hitch-hiking on birds.

HWA use sucking mouthparts to pierce through the base of the hemlock needles into the parenchyma cells (nutrient transport cells. Large infestations can quickly deplete trees of their vital nutrients.  Early impact to trees infested with HWA include loss of needles and stunts the growth of new needles and branches. If left untreated, HWA infestations can result in hemlock tree death within 3-5 years of infestation.

In their home environment of Japan, HWA experiences heavy predation, mostly from beetles, that they evolved an impressive bi-annual reproduction cycle. It is precisely these reproductive traits that allow HWA to have such rapid effects on hemlock trees in the U.S. where they have no natural predators.  

During most of the summer they aestivate on the needles (basically a long sleep), once temperatures get colder they develop a white woolly egg mass. This generation is fully developed in the winter, will begin feeding, and reproduce in the spring.

The second generation feeds and produces a new generation that will crawl to a needle to aestivate for the next summer, thus repeating the lifecycle.  In the U.S. all HWA reproduction is asexual, a sexual generation does occur in its native range but targets a different tree species that is not present in the U.S.

Spread of HWA Since its Introduction 1951 - 2015

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