For this issue, I have good news and bad news.
The good news is concerning recent research in controlling the hemlock woolly adelgid (HWA).† This introduced aphid-like creature that has no US predators, feeds on the stored starch reserves in the sap, causing needles to desiccate and drop.† With the death of most buds, new growth is severely retarded and the crown begins to thin as infested branches die.† Ultimate death occurs usually within 3 to 10 years, depending on various factors, including cold winter temperatures, droughts, and general tree health.
Known to exist in the Richmond, VA area since 1951, it was first identified in the Shenandoah National Park by recently retired SNP employee Steve Bair in the fall of 1988. †With the SNP busy fighting the onslaught of the gypsy moth main thrust at that time, resources to deal with HWA were limited.† With few tools available to combat this invader at the time, most of the Parkís hemlocks were dead by 1995.† By 2002, HWA was discovered in the Great Smoky Mountains National Park.† The following information reveals the alternatives that are now available and producing beneficial results in the Smokies at this time.
Hemlocks are a keystone species in the streamside forests of the Appalachian region, providing critical habitat for many forest species. They regulate stream flow and moderate temperatures in stream habitats where trout and other aquatic organisms require specific conditions to survive.† Conifer-dwelling birds, such as Blackburnian warblers and winter wren have been greatly diminished in hemlock-ravaged areas, replaced by robins and goldfinch.† Invasive shade intolerant non-natives, such as garlic mustard, Asian ladyís thumb, tree-of-heaven and Paulownia, are replacing shade tolerant species including large purple-fringed orchids.
The average rate of HWA spread over the past decade has been about 15 miles per year.† The current eastern distribution of HWA ranges through 18 states from Maine to Georgia and west to Tennessee.†
Early efforts to control HWA focused on the use of insecticides and soaps, which, although effective, was not practical in a natural setting and was exceeding expensive.† (Private individuals wishing to protect landscape hemlocks can conduct soil drenching every few years with Dinotefuron or Imidacloprid.† The latter is a commonly used insecticide available at major garden centers under the trade name of Merit, among others.)
By 1991, USDA efforts focused on managing HWA in forests through biological control.† This is using predaceous animals to feed on HWA.† Of course, the issue is making sure the predators feed exclusively on HWA (host-specific).†
Four species of predatory beetles from their native range in Asia have passed rigorous USDA evaluation and are currently utilized in release programs in the U.S. These include three ladybird beetle species, Sasajiscymnus tsugae (formerly Pseudoscymnus tsugae) from Japan, Scymnus sinuanodulus and Scymnus ningshanensis (both from China), and a species of fungus beetle, Laricobius nigrinus, which is native to western North America.
S. tsugae has been the most frequently released predator to date. †S. tsugae were first free-released in Connecticut in 1995, and as of 2007, over 3 million S. tsugae had been released on more than 100 sites in 16 eastern states from South Carolina to Maine. Long-term monitoring of S. tsugae release sites has shown the beetles generally reduce HWA numbers in those areas, and successfully overwinter, reproduce and locally disperse. †Hemlocks trees have shown recovery after 4 to 7 years under good growth conditions.† Unfortunately, the beetles are less successful when mild winters enable HWA populations to thrive, when drought stresses hemlocks, or when HWA populations are high when S. tsugae are initially released.† Better results are documented with low winter temperatures and abnormally moist or cool springs and summers.†
Mass production of S. tsugae is conducted in several eastern US locations to support the release programs.† There is an interesting catch regarding the S. tsugae predaceous ladybird beetle that affects both the mass production efforts and success in the forest environment.† The health and reproductive success of these beetles is contingent on itís food source; the HWA.† However, from mid July through October, the HWA generation enters an aestival diapause period Ė the summer version of winter hibernation.† In this condition, the HWA generation constitutes a less nutritious food source, negatively affecting S. tsugae survival rates and egg production.† Recent research by NC State has found that the balsam woolly adelgid (A. piceae - BWA) may provide a means to overcome this constraint on S. tsugae mass rearing programs.† This† exotic forest pest, responsible for eliminating approximately 95% of mature Fraser fir from the southern Appalachians, does not enter a summer diapause; continuing to actively feed and reproduce throughout the dormant period of the HWA.† Results indicate that the S. tsugae readily accept the BWA, which maintains a nutritious food source throughout the late summer months, thus potentially providing a year round supply of beetles for HWA biological control.† Their results also suggest that the development of S. tsugae laboratory colonies reared on a diet consisting only of BWA may be possible, and that the biological control potential of the predator might be expanded to include management of BWA in Christmas tree plantations.
Scymnus sinuanodulus, Scymnus ningshanensis are ladybird beetles native to China that feed primarily on HWA and were imported to the U.S. in the 1990ís. These predatory beetles share a synchronized life cycle with HWA, being active and reproducing during the fall and winter months, and going dormant during the summer. In 2004, a field release of adult S. sinuanodulus beetles was made in the Southern Appalachians, and in 2006 additional beetles were released in the Monongahela National Forest, WV.
Laricobius nigrinus is a small beetle native to northwest North America found feeding on HWA in hemlock seed orchards in British Columbia. Virginia Tech scientists began importing the beetle in 1997 for study under quarantine, and field releases and mass rearing studies began in 2001. Field evaluations of L. nigrinus are ongoing, but have been promising.
Other potential biological control organisms have been investigated, including an oribatid mite (Diapterobates humeralis) from Japan, and several entomopathogens (insect disease pathogens) from China and North America.
Whether these or newly identified biological controls will effectively control HWA is certainly not clear at this time.† Optimism from encouraging research should be tempered by the many biotic and abiotic variables that influence the results.† Because of the lag time often required for a biological control agent to successfully establish following release, and the rapid rate with which HWA kills hemlocks, control of HWA through biological controls may or may not be successful.† Ultimately, a complex of natural enemies will probably be necessary if HWA populations are to be maintained at acceptable levels.†
The bad news?† With this article, I will end my Appalachian Nature series.† Until this year, Iíve done monthly articles since 2000 and have learned a great deal in their production.† Circumstances are changing my priorities, including the taking of two and three month vacations each year, and the recent purchase of a second home on Middle Mountain near Elkins, WV.† Itís my plan to put all my articles on my website; www.bobpickett.org.
I will also be ending my monthly natural history hikes that Iíve done since 1985 (with some misses for various reasons).
†I look forward to many great adventures in WV.† I may even start leading hikes out there!†