Rolf Schilling

Horticulturist & Plant Records Coordinator

Good Weather = Bad News

Warm weather in January has many of us commenting how wonderful this winter balm makes us feel.  Not all is as well as it may appear. A short walk to our Lily Pond garden, which sits at the end of a slope covered with a hemlock grove, was wonderful in the bright sun on brilliant winterberries and the persistent glaucous foliage of dusty zenobias. Then bad news literally fell on me from above. A naughty squirrel dropped a sprig of hemlock on me as I passed.  A quick look at the foliage revealed dense covering of hemlock wooly adelgids (Adelges tsugae).

     Last year, the ground at Garden in the Woods did not freeze until mid-January.  The week of the winter solstice that year, almost two inches of rain fell. This aberrant weather brought on heavy adelgid infestations on hemlocks in 2007.  These adelgids are an exotic pest from more temperate climes in East Asia, and their rampage on hemlock forests in the Southeast is well documented. Their damage to hemlocks in the Northeast has been limited by one factor: cold winters. Every winter where the thermometer dips below zero significantly, and especially when it does for days on end, the adelgid populations are knocked back for several years.

Life cycle of Adelgids and treatment issues

 A quick check under the microscope showed that these January adelgids are active and feeding.  This normally should not occur during winter months.  Warm weather has allowed what would be next year’s generation to hatch early.  During this stage, they are called ‘crawlers’ and are most vulnerable to treatments.  Dormant, emergent, or horticultural oils are one of the main treatments for hemlock wooly adelgids.  Another is use of systemic poisons, such as Merit ™.   It only takes a few days, however, for the crawlers to become sedentary, when they attach to a leaf axil and begin sucking the sugars from the hemlock’s sap.  Once this occurs, the adelgids develop a waxy coating that protects them from hot, dry weather, but also protects them from pesticides and sprays.  As the females near their reproductive stage, their feeding activity is reduced, making them even less vulnerable to treatments. 

     This means that, having gotten a head start, there will not be ‘crawlers’ in spring, but mature adelgids.  Oil spray treatments will not be very effective this spring under this scenario.  Systemic treatments will be less effective since the adelgids will not need to spend as many days feeding before they reach their reproductive fasting.  They can lay their eggs early and get a jump start on their second generation.  This second generation goes dormant during the heat of summer and comes back out in fall to feed in the cool weather.  Our recent extended late autumn warmth only exacerbates the problem.

Questioning the strategy

Treatment programs throughout the Southeast have not succeeded in restraining adelgid infestations, much less eradicating the pest.  In fact, the entire history of synthetic pesticide use paints a dismal picture: one would be hard pressed to find examples where pesticides have been successfully used to eradicate an insect pest.  Quite the reverse is true.  Insects typically become genetically tolerant of the insecticide over generations.  This requires increased toxic loading into the environment in a losing battle, or the invention of a more potent pesticide.  Unfortunately, more potent pesticides are often more toxic to us and our environment.

     There is a second problem with treatments currently in use.  Spraying oils on a tree in spring during emergence indiscriminately suffocates all larvae, including the insect predators of adelgids.  Treating a tree systemically makes the plant poisonous to every insect that feeds on it, and poisons the predators that feed on those insects.  Thus, treating trees chemically prevents populations of natural adelgid predators from reaching an effective threshold.  This brings up the point that perhaps nature must be left to take its course.

     The history of hemlocks in North America adds weight to this point.  It is known through paleobotany that hemlocks retreated to their northern refugia several millennia ago.  The cause is not known, but this evidence suggests that there may be reason to view the presence of hemlocks in warmer climates as periodic.  It may be true that hemlocks, a tree that prefers cool, moist northeast-facing slopes, may follow patterns of advance and retreat along the Appalachians with changes in climate over time.  Infestations and loss of range during times of climactic warming may be ‘normal’ for this species.  Wooly hemlock adelgid is not a native pest, though, which complicates attempts to understand this changing pattern of hemlock distribution.

New strategy needed

Since current treatment programs have not stopped the death of hemlocks from wooly adelgids, new strategies must be found if we desire to stop the northward retreat of this tree.  One way to stack the odds in favor of hemlocks is to plant them only where they experience optimal growing conditions.  Insects uniformly prefer to attack plants under stress.  Stressed plants are not able to devote as much of their energy to the production of defensive chemicals (sometimes inappropriately referred to as ‘secondary metabolites’ because they based on the initial sugars plants make). 

     At Garden in the Woods, this is certainly evident.  Hemlocks isolated in small clump or alone here continually attacked by adelgids.  Hemlocks in hotter, drier locations suffer the most.  Meanwhile, the hemlocks that inhabit cool slopes facing north and east, and especially those that are part of larger groves, are comparatively clean of adelgid.  The edgewise trees are more infected than those within the grove.

     Hemlocks are trees that prefer to live in groves of their own kind.  They are less often found thriving as scattered among other trees than they are found thriving in a solid group of only hemlocks.  Hemlocks also distinctly prefer cool and moist soils.  You can go a long way to protecting hemlocks by planting them in groves on their preferred soils and exposures.