Thursday, July 12, 2012

Hemlock Trees Threatened by Exotic Insect

Eastern hemlock infested with hemlock woolly adelgid.
Samford University recently sponsored a symposium titled “Preserving the Eastern Hemlock”.  If one had not heard of the new threat to the hemlocks in Alabama, this might seem like a strange topic.  But the hemlocks in Alabama face a very grave threat from the hemlock woolly adelgid.   The hemlock woolly adelgid, or HWA for short, has killed hundreds of thousands of trees in the Eastern U. S.  I’ve seen firsthand what it’s done to the hemlock trees in the Great Smoky Mountains National Park; almost all of them have been killed in many areas of the park.  HWA kills young and old trees alike. 

The eastern hemlock is an important tree in Winston County.  It is abundant in the sheltered, cool coves and deep ravines that are common in all parts of the county.  The hemlock is considered a conifer, however, unlike other cone-bearing trees, it is very shade tolerant.  It can even grow in the shade of hardwoods, causing one forester at the symposium to say “Hemlock is so shade tolerant it can grow in the dark.”  The tree is well adapted to growing in sheltered ravines, and shady stream banks.  Deep canyons and cool conditions become rare in south Alabama; Jefferson County is the southernmost county that has native stands of hemlock trees.
The loss of the hemlock tree in the state would have a number of negative consequences.   Foresters note that there is no other conifer that can replace it in our forests.  No other conifer can tolerate the shade like a hemlock.  A number of species of birds and animals depend on the tree.  Hemlocks provide shade over many small streams, and when the trees die, water temperatures can rise as much as 4 degrees affecting many fish and aquatic insects.  The tree is an important lumber species in some parts of the U. S.  Finally, the appearance of the forests would change, since we would no longer have the same boughs of dark green in our winter forests.

The hemlock woolly adelgid has been in the United States since the 1950’s when it was introduced from Japan.  The insect has needle-like mouth parts which it uses to suck sap from the plant.  This feeding causes the tree to weaken and die in as little as three years.  Some studies suggest that the insect may even inject a toxin into the tree.  Some trees have survived 20 years after attack by HWA, but often entire stands of hemlocks are completely devastated 10 years after infestation.  The HWA has spread south from Virginia, down to Tennessee, and may soon enter Alabama.

The HWA is tiny, about 1/16” long and varies in color from dark reddish brown to purplish black in color.  As it matures it covers itself in wax filaments, or “wool”.  This wool is most conspicuous when the adult is mature and laying eggs.  The wool is white, and infested trees have white tufts at the base of the needles.  The adults lay eggs, which hatch into “crawlers” that disperse over the tree to find feeding sites at the base of the needles.  They have long feeding parts, much longer than their body, which they insert deep into the stems to feed on the tree’s stored reserves.  The crawlers and eggs are the most common way for HWA to disperse and spread into to new areas.  They can be spread by wind, birds, squirrels and other forest animals.  Long distance movement most often occurs by people transporting infested nursery stock.

The good news is that other states that have been infested with the HWA have developed treatments for the HWA that can be used on important trees.  While these treatments are not the complete answer, they do provide a way to save some trees.  Another strategy of using carefully screened predator beetles imported from Asia has proven to help with HWA, and this technique could benefit the entire population of hemlocks. 

In the coming years, the HWA will probably spread to Alabama.  The hope is that the spread will be slow and the damage will be less severe than it has been in other areas.  If you would like more detailed information about the hemlock woolly adelgid, call us at (205) 489-5376.

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