Beech Health Update

Paul Hetzler, ISA Certified Arborist

In the early 19th century, a Prussian diplomat asserted that “when France sneezes, the whole of Europe catches a cold.” Things changed, obviously. For a long while it has been an American financial sneeze able to make the world ill. Though China’s economy is projected to soon zip past ours, other countries still put hankies to their faces when the USA coughs.

Canada has been covering up for some time now, but not for the usual reasons. Regardless of trade policy over the past 150 years, the US has exported tree diseases to Canada at a steady pace. To be fair, most weren’t ours to begin with, but we’ve shipped them Dutch elm disease, chestnut blight, butternut canker, and now (probably) oak wilt.

And we’ve been generous to a fault with invasive forest pests, breaking the ice quite early in the relationship with gypsy moths. A few decades ago, the “Big 3” automakers sold American auto-parts jobs to the Chinese, who repaid us well. Like a twisted version of Cracker-Jack, boxes of cheap Chinese auto parts arrived in Detroit with hidden prizes included for free: emerald ash borers and Asian longhorned beetles. Of course we shared.

Just recently, however, Canada sneezed out a new invasive tree scourge, and American foresters should grab some Kleenex. The beech-leaf miner (Orchestes fagi), native to Europe, was first identified in North America in 2012 near Halifax, Nova Scotia. Biologists think it probably arrived around 2007, but as with many invasive pests, it took a few years for the problem to become evident. The beech-leaf miner feeds on all species of Fagus, including exotic landscape trees, but the American beech, a native forest species whose nuts provide food for a wide range of wildlife, is the greatest concern.

The larvae of this weevil devour soft tissues between the upper and lower surface of the leaf, “mining” tunnels which expand to encompass much of the leaf. Natural Resources Canada states that “Early results indicate American beech are dying after successive years of defoliation by the weevil. In Europe, the adults feed on a variety of alternate hosts, including cherry and apple, this has not yet been observed in Nova Scotia.”

The white larvae have a black head, and are less than a half-millimeter at first. When full-size (just prior to pupation), larvae are about 5 mm long. Adults are around 2.5 mm long, black with minute golden hairs, and rather beefy back legs. According to invasiveinsects.ca, “the weevil larvae leaves a characteristic mine pattern useful for identifying its presence. A narrow linear mine begins from the mid-rib of the leaf to the margin where there is a blotch mine [large dead area]. The leaves may turn brown around the edges and wilt.”

So far, the affected region is relatively small, but as scientists from Natural Resources Canada note, “Where the weevil has been established for 5 to 10 years, beech mortality increased from 18% in 2014 to 88% in 2015.”

Forest Entomologist Mark Whitmore, Director of the Whitmore Lab at Cornell University, brought this to my attention in September 2019 after checking on hemlock woolly adelgid (another American gift) infestations in Nova Scotia. In an email he wrote “Beech leaf miner is on nobody’s radar [in the US] at the moment. I’m bringing it up with the US Forest Service, and hopefully more states will begin to look for it. I was very alarmed when I visited a couple weeks ago. It’s a big deal, spreading rapidly in Nova Scotia with mortality.”

At this time, the most important thing anyone can do is to not move firewood. Beech-leaf miner adults overwinter under bud scales and bark, so bringing a souvenir piece of bark or twig home from vacation could be very significant. Also, cover your mouth with the crook of your elbow when sneezing, even if you’re alone in the woods. You never know what scourge you might help prevent.

Paul Hetzler has been an International Society of Arboriculture Certified Arborist since 1996, and is a member of the Society of American Foresters and the Canadian Institute of Forestry.

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