Protecting Eastern hemlock, our next endangered tree species

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In spring and summer 2002, entomologists first collected a small, bright green beetle from ash trees in the Windsor-Detroit area. None had ever seen this insect before. Two months passed before a taxonomist in Slovakia identified it as Agrilus planipennis, an Asian member of the family Buprestidae. This family has about 15,000 species, with brightly coloured adults ("jewel beetles") and larvae that bore through stems, leaves, roots and logs.

The September 2002 issue of the Newsletter of the Michigan Entomological Society notes that the likely pathway for arrival of this species in North America was infested wood crates or pallets carrying imported goods from Asia. Judging from the area of affected ash trees, the beetle had probably been present for at least five years before being detected. Scientists quickly recognized that it might spread throughout the entire range of ash in North America, causing serious economic and environmental damage, and attracting considerable media interest. It needed a common name, and "emerald ash borer" was chosen.

Emerald ash borer is a weak flier that only travels short distances. Long-distance movement of infested ash firewood has aided its spread from southwestern Ontario, and likely caused its jump to Ottawa in 2008. Thousands of metallic green beetles could be seen during the spring on trails in the Greenbelt, and most large ash trees died. The beetle is now spreading from the city to nearby towns such as Arnprior in 2013 and Renfrew in 2015. A County of Renfrew website provides information on how to identify Emerald Ash Borer and slow its spread.

Ash trees have now joined a growing list of native trees in our area affected by invasive species from other continents. The 2011-2016 Forest Management Plan for the Renfrew County Forest identifies white pine blister rust as the most important of these harmful alien species. It is particularly deadly to young seedlings and saplings, and kills up to 95 per cent of the trees in some plantations. The Plan notes that at one time there were extensive stands of American elm in Renfrew County, now all decimated by Dutch elm disease. And it describes Renfrew County as being on the "leading edge" of butternut canker. A 2014 study found that butternut canker was likely introduced through importation of the closely related Japanese walnut, which harbours this fungus but is not killed by it. The deadliness of the canker means that Butternut has disappeared from most of its native range and is now listed as an endangered species. 

Eastern hemlock may be next to fall victim to an invasive alien species. Most Eastern and Carolina hemlocks in the United States have already been wiped out -- a huge loss, considering that hemlock-northern hardwood forests are thought to represent the original dominant forest type over much of this area. It seems probable that Canada's Eastern hemlock stands -- common throughout southern Ontario, including Algonquin Park -- will soon be forever altered. 

The insect that is causing all this damage -- the hemlock woolly adelgid -- was detected in Canada for the first time in Etobicoke in 2012 (probably introduced on infected nursery stock) and was subsequently found in 2013 in the Niagara Gorge. But we may still have a few years to prepare for its arrival in most of hemlock's Canadian range.

Adelgids are plant-juice-sucking bugs in the order Hemiptera, closely related to the aphids. The hemlock woolly adelgid was first detected in Richmond, Virginia in 1951. It was likely imported on exotic ornamental hemlocks from Japan. A series of articles on the website of the Ontario-based group Ancient Forest Exploration and Research describes the slow but seemingly inexorable outward spread of this pest from its point of introduction. Within a decade after it was first discovered in the Great Smoky Mountains National Park in 2002, an estimated 80 per cent of the hemlocks in the park were dead.  

Park staff have managed to save some hemlocks in highly visited areas through applications of systemic neonicotinoid insecticides. But neonicotinoids are water soluble and resist degradation. Neonicotinoid levels in water bodies near chemically treated areas in North America -- such as the corn- and soybean-growing areas of southern Ontario and Quebec -- have steadily risen to levels that exceed guidelines for protecting aquatic life. Streams in the Great Smoky Mountains National Park are now contaminated with neonicotinoids, and hemlock canopies there are toxic to a full range of native insects, including those fed upon by hemlock-loving birds such as the Acadian flycatcher and black-throated green warbler.

It is an agonizing choice whether to apply pesticides, or watch ancient hemlocks succumb to the adelgid. The Ancient Forest group characterizes landscapes where hemlocks are being maintained through ongoing pesticide treatments as "chemically-preserved museums."

Should our much-loved hemlock stands -- such as Shaw Woods, Jackson Park in Peterborough, and heavily visited areas of Algonquin Park -- be converted into chemical museums? 

Establishing populations of predatory insects -- biocontrol -- is an alternative to repeated insecticide treatments. The leading biocontrol candidate, a beetle native to western North America, is achieving only mixed success. The Ancient Forest group urges that "Forward-thinking individuals... consider planting hemlock hedges (using local hemlock stock) which can act as field insectaries." That is, act now to create nurseries for breeding beneficial insect predators that will feed on the adelgid once it arrives.

According to the group, key steps to slow the pest's invasion are to:

i) identify priority stands (e.g., old-growth areas such as Shaw Woods or Jackson Park) where control efforts can be concentrated;

ii) monitor priority hemlock stands to detect the adelgid as early as possible (frequent visits by citizen scientists);

iii) establish nurseries for beneficial predators (such as hemlock hedges) near priority stands; and iv) protect the genetic resources represented by individual large, old trees through trunk injections (not soil drenches) of insecticides so as to conserve them until populations of beneficial predators build up.

Cheap imports from other continents mean ongoing risks of introducing more invasive species: avoid them. Plant only native trees grown in local nurseries. Learn to identify the hemlock woolly adelgid (it's easy -- look for waxy white masses on the underside of twigs in the spring). And visit your local hemlock stands as often as possible.

Ole Hendrickson is a retired forest ecologist and a founding member of the Ottawa River Institute, a non-profit charitable organization based in the Ottawa Valley.

Photo: Joshua Mayer/flickr

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