Friday, July 25, 2014

Eastern Hemlocks and the Threat of Woolly Adelgid


Undersides of flat needles are striped.

Spring growth.

Young cones.

Mature cones.

From the spindly trees folks grow around here, and the way they crowd them and torture them into HEDGES, for all love, you'd never know that the eastern hemlock(Tsuga canadensis) is a majestic member of late-stage successional forest here in New England. 

I have been collecting locations of eastern hemlocks (Tsuga canadensis) on my neighborhood perambulations over the last month or so, and have a nice list of trees I look at regularly.  I was moved to do this less because of its iconic status--the third most prevalent tree in Vermont--than because of the danger eastern hemlock faces.  This tree is endangered by yet another of our many alien invasives: the woolly adelgid.  

Our local trees range from sad and tortured to moderately majestic.

Woolly adelgid (Adelges tsugae) was imported accidentally from Japan, and was first noticed in the US near Richmond, VA in the 1920s.  It is now affecting eastern hemlocks as far north as Massachusetts.  Woolly adelgid is an aphid-like "true bug" (insect order Hemiptera) that sucks the sap and starch from hemlock twigs, reducing their food stores.  This weakens the tree, allowing other insect, disease and drought stresses to overwhelm it.   

Wooly adelgid spreads short distances mainly by wind, and by birds and other animals the sticky egg sacks cling to.  Transport of infected nursery trees can spread the insects more widely.  

Each cottony mass tyically hides an individual and its several hundred eggs.

You will know you have this on your hemlock tree if you see the cottony egg masses on the undersides of twigs.  Affected trees will gradually lose needles, becoming more "transparent" and turning grayish.  Trees here in the north typically die four to ten years after infestation.  If allowed to expand unchecked, the woolly adelgid could doom so many of these beautiful trees that whole forest ecosystems could be irrevocably altered. 

Property owners can spray their smaller infested trees yearly with a non-toxic insecticidal soap or horticultural oil; these smother the insects.  Tree foliage insecticides will keep on killing for several years, but are more toxic.  Trees too large to spray can be treated with soil drenches or other chemicals that are absorbed and transported throughout the tree, but are not safe applied near bodies of water.  For whole forests, several insect species that feed exclusively on wooly adelgid were deployed beginning in 2002 in hopes of bringing the pest population down to manageable levels over the long term.  

A nice video pulls all this (and more) together here.
My own little darling at about ten years old is almost chest-high, and free of adelgid.
There are a dozen or so stands of eastern hemlock around my neighborhood, many consisting of several trees.  (Probably thirty or more trees of all sizes could be counted if including individuals in hedges.)  Of these, three stands have obvious signs of woolly adelgid.  There may well be more--I cannot closely examine trees in other people's yards.  My own was a tiny individual transplanted to the woods behind our house; ten or so years later it is only waist-high, but free of insects.  If landowners keep an eye on their own trees and treat them if needed, I have hopes that the spread of the insect can be slowed--at least locally. 

What can you do?  If you have any hemlocks, watch them.  If you find woolly adelgid, consult with state agencies on what treatment would be best.  If you see the insect on your neighbor's tree, let them know.  Don't move an infected tree to a new location.

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