Autumn is the perfect season to explore Appalachian hiking trails and enjoy spectacular changes in foliage color as deciduous trees prepare to drop their leaves in preparation of winter’s chill. On a recent adventure to Savage River State Forest in Garrett County, Maryland, the westernmost county in the Free State, Fagus grandifolia, American beech, displayed its full glory sporting leaves of russet and gold. As I walked trails lined with these forest monarchs, I was surprised to see the usually unblemished smooth gray beech bark covered with cascades of fluffy white wax. Flocculant wax on leaves or bark is usually a bad sign, as it signals an infestation of some type of noxious sucking insect pest. Yes, this was indeed the case for our friends the American beech, who are in a mortal battle with an invasive insect pest from Europe called beech bark scale and a cabal of invasive fungal pathogens in the genus Neonectria. Together these aggressive pests cause a slowly developing but often lethal disease known as Beech Bark Disease, or BBD.
Beech bark scale belongs to a family of sucking insects called Eriococcidae. This rascal was identified as a pest of European beech trees in the mid-1800’s and was introduced into Nova Scotia in the 1890’s. Since then, it has spread throughout the range of American beech trees in eastern North America. Both adult and juvenile (nymph) beech bark scale insects insert tiny sucking mouthparts into nutritious cells beneath the outer bark of the tree. These stylets pierce and kill underlying cells. As thousands of scales feed, tissues beneath the bark die and bark cracks form. This allows the spores of Neonectria to enter the tree, multiply, and produce eruptive bark cankers, further spreading the fungus. The combined injury caused by thousands of scale insects bursting cells and fungi killing tissues beneath the bark eventually girdle the tree, resulting in a spiraling cycle of death that may take several years to kill their host.
White fluffy wax on the trunk of American beech trees is a good indication of an infestation of beech bark scale. Beneath the wax, tiny nymphs dormant throughout late fall and winter will resume feeding and develop into adults in the warmth of spring and summer next year. As thousands of adults and nymphs feed through the bark, their wounds allow fungal pathogens to enter, multiply, and produce a disease called beech bark disease (BBD). Over the span of several years, BBD can be lethal to infected trees.
In a strange evolutionary twist that we have seen in other insects like beech blight aphids and some scale insects, beech bark scales are parthenogenic, meaning female scale insects skip the business of romance and reproduce without the service of males. These nasty girls rule! Inside their fluffy bundles of wax, nymphs pass the winter months and resume development when spring returns. Wingless adults are present in June and July and new batches of eggs are deposited that will later hatch into mobile nymphs called crawlers. Ok, if adults are wingless how is it that they have spread from Nova Scotia in the 1890’s to western Maryland between 2010 and 2015? Tiny crawlers hatch from eggs and become windborne where they join aerial plankton capable of traveling untold distances on currents of wind. These tiny vagabonds may also crawl onto the feet of birds and be transported from one tree to another as birds migrate and stopover on trees along their route. Some evidence points to human-assisted travel as campers move vehicles around the country with nearly invisibly tiny vagabonds hitching a ride.
Is there any good news for our imperiled beech trees? Yes! Very cold temperatures around 30 degrees below zero Fahrenheit that occur in parts of the range of American beech can be lethal to overwintering scale insects. But in a warming world, this may not be all that helpful. Variation in susceptibility of beech trees to infestations provides hope that some trees carry genes conferring resistance to the scale insect and that resistant cultivars of beech can be bred. Lady beetles, including the twice stabbed lady beetle (gotta love that name), feed on adult and immature beech bark scales, and several insecticides are effective in killing these noxious pests. Insecticides can certainly help for specimen trees in managed landscapes but this tactic is not a solution for the vast stands of beech trees in natural settings. Perhaps Mother Nature has another trick up her sleeve to help quell this invader from overseas. Let’s hope so, but in the meantime take a moment to visit and enjoy American beech trees, remarkable sovereigns of the forest.
We thank Dr. Shrewsbury for spotting several beech trees in Savage River State Park. Several enlightening references were used to prepare this episode. They include “Beech bark disease in North America: Over a century of research revisited” by Jonathan A. Calea, Mariann T. Garrison-Johnston, Stephen A. Tealec and John D. Castello, “ Beech Bark Disease” by Esther Kibbe and Enrico Bonello, and “Characterization of mating type genes in heterothallic Neonectria species, with emphasis on N. coccinea, N. ditissima, and N. faginata” by Cameron M. Stauder, Jeff R. Garnas, Eric W. Morrison and Catalina Salgado-Salazar.