Save our Hemlocks

Conservation of the hemlock is important for many reasons.  Namely these large trees provide lots of stability to landscapes by preventing erosion.  They also create needed habitats for many species of north american wildlife, such as deer which feed on small branches and seek shelter in them during the winter months.  Hemlock is also valued for both ornamental uses and an important source of lumber.  Unlike similar hardwoods, when cured properly hemlock can withstand heat without warping ; this makes it an exceptional choice for saunas and other home spa use.  Research suggests that an alien pest is destroying hemlocks much faster than anticipated as well as spreading at an exponential rate.

The most significant threat to hemlocks in north america is a pest from Asia the hemlock woolly adelgid.  This bug feeds on the sap of hemlock trees and is responsible for the death of about 50% of the hemlocks in the U.S.   The presence of these pests can be identified quite easily by finding the insects egg sacs, which look like small cotton balls which cling to the underside of hemlock branches.  Hemlocks that are infected often appear a light grey/green as apposed to the very dark green which is normal for healthy specimens.

Hemlock woolly adelgid reproduce asexually and commonly have two generations per year, one in the spring and one in the fall.  The larvae spread easily to nearby trees via wind, birds, and other critters.  As these pest feed on sap, they also inject a toxin which causes desiccation that results in the tree to shed its pine needles and not produce any new growth from that area.  Sometimes trees can withstand the adelgids toxins , but most succumb eventually to secondary effects of inhibited growth or other infections.

Infected Hemlock Shown Below

Options :

There are a couple options that can be taken to help save a hemlock.    The adelgid’s can be vigorously washed from branches or infected areas can be removed completely to stop further spread.  This option is great if you home tree has become infected , but on a large scale it creates too much work and doesnt provide too much benefit against the pest which will continue to spread and re-infect trees.  There are also pesticides which can kill all infections relatively quickly , however this also doesnt work for large scale conservation since pesticides can have a large impact on nearby wildlife ; especially if the treated area is near a water source.  Insecticidal soap or horticultural oils are another option which is rarely used on a large scale.

In some areas where the adelgid are just popping up, quarantines are issued to prevent further spread.  Homeowners and nurseries cannot bring in hemlock seedlings or trees from adelgid infested states into these states without an inspection permit certifying that they are pest-free.

Recently, scientists have successfully hybridized a strain of asian hemlock(which is resilient to the adelgid) with our common species found in north america.   These hemlocks retain the same appearance of the species found here,  inhibit adelgid growth , and can withstand their colonization.  By far the best method of conservation is re-population with this new resilient strain.  We have seen this in Northern Canada where deforested areas are repopulated with this new strain which inherits most of the qualities which make it a popular choice in lumber.