Beech bark disease is the result of an insect-fungus complex caused by a non-native scale insect, Cryptococcus fagisuga, and Neonectria canker fungi. The effects of the disease are severe cankering on beech trees, deformation of the stem, and eventual tree death (OMNRF, 2014).
Cryptococcus fagisuga: yellow, soft-bodied scale insect about 0.5 to 1 mm long as an adult. The females are legless and wingless, and use their 2 mm long stylets to attach to beech trees. During the nymph stage, the insects secrete a white woolly wax to cover their bodies, which can make infested beech trees look like they are covered in wool (Global Invasive Species Database, 2011). The scale insect feeds on beech bark, creating feeding punctures that produce cracks in the bark through which canker fungus can enter. The fungus then causes small cankers that appear on the bark surface, from which small orange-red fruiting bodies are produced in late summer and fall (OMNRF, 2014).
Photo: Taylor Scarr
Beech trees infested with beech scale appear as if they are covered in wool
There are three phases of beech bark disease that represent different ecosystem states: the advancing front, the killing front, and the aftermath forest.
Advancing front: begins when the non-native beech scale insect, Cryptococcus fagisuga, arrives and rapidly proliferates. Ecosystem change is minimal during this phase, but the high-density presence of the scale insect makes the stand susceptible for infestation by bark-killing Neonectria fungi.
Killing front: refers to stands of trees with high populations of beech scale, severe attacks by Neonectria, and 50-85% mortality of large beech trees within 10 years of infestation by the scale insect and fungi.
Aftermath forest: stands that have survived the first wave of beech mortality. There is an increased prevalence of understory suckers growing from the roots of the attacked parent plant. Beech root suckers choke out natural regrowth or regeneration of desired species such as maple or birch, and subsequently become infect with BBD (Loo 2009).
Beech bark disease is found on American beech (Fagus grandifolia) and on European beech (Fagus sylvatica). The disease, which results from the interaction between a scale insect and canker fungi, is found in mixed-hardwood forests.
Trees are at an increased risk for BBD when there is a nutrient imbalance in the forest. Weather conditions, such as temperature or autumn rainfall, also affect beech scale populations and therefore BBD. Some beech trees may exhibit a resistance to the scale insect.
Photo: Rob Routledge, Sault College, Bugwood.org
Foliage of the American beech (Fagus grandifolia), a species susceptible to beech bark disease infection.
Beech scale, Cryptococcus fagisuga, is native to Europe and was introduced to Halifax, Nova Scotia in the late 1800s (Loo 2009). The scale insect continues to move throughout the natural range of beech trees and is now found throughout the maritime provinces, several states in the eastern U.S. as well as Quebec and Ontario.
The scale insects are spread by wind, animals, and through human movement of beech wood that has intact bark. Beech bark disease results when beech scale is combined with Neonectria fungal spores. Fungal spores, spread by wind and rain splash, move into the trees after they are attacked by the scale insect (OMNRF, 2014).
Map: Cale et al., 2017
Map depicting the spread of beech scale, Cryptococcus fagisuga, from its introduction until 2015. Cross hatching indicates the reported range of Neonetria spp.
Beech bark disease attacks beech trees in North America and is caused by the combined effects of the non-native scale insect, Cryptococcus fagisuga, and Neonectria fungi.
The scale insect feeds on the beech tree sap, opening wounds in the tree for the fungus to start colonizing the bark, cambium layer, and sapwood of the tree (OFAH/OMNR Invading Species Awareness Program, 2012).
Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org
Beech bark disease weakens host trees, and makes them susceptible to "beech snap", where the trunk of a beech tree breaks. This is a safety hazard to humans, and contributes to ecological changes in the forest.
Beech bark disease diminishes the quality of beech wood, reducing the marketability of infected trees. The reduced supply of healthy beech trees can have a negative impact on the hardwood forest industry (OFAH/OMNR Invading Species Awareness Program, 2012). Beech bark disease also affects desirable timber species (e.g. sugar maple) as regeneration of these species is inhibited by the heavy beech understory that typically follows mortality (Cale et al., 2017).
Beech trees are valuable nut-producing trees, providing an important source of mast for many forest-dwelling birds and animals. Therefore, the loss of beech trees from the landscape would mean a lost food source for wildlife. Black bears, in particular, rely heavily on beech nuts for food within northern hardwood forests (Loo 2009).
Photo: Rob Routledge, Sault College, Bugwood.org
The loss of beech trees can also cause a shift in forest structure, resulting from loss of canopy cover, the increase of root suckers from dead trees, and an increase in coarse woody debris from dead trees on the forest floor (Loo 2009). BBD results in a shift toward younger, smaller forests. In addition BBD may cause long-term changes to species composition in a forest. Young beech saplings proliferate in the understories of BBD impacted forests adversely affecting biodiversity.
Since beech bark disease is still progressively moving through North American forests, the long-term ecological impacts of the invasive are yet to be fully understood (Loo 2009).
Healthy beech trees can grow to become beautiful, large hardwood trees. The loss of beech trees could mean a decrease in the aesthetic value of hardwood forests within Ontario. In the maritime provinces, where beech bark disease has been around for decades, the majority of beech trees are gnarled and disfigured by the bark disease, making them quite displeasing to look at.
Breeding programs have developed to combat BBD by creating beech-scale resistant American beech trees (Cale et al., 2017).
Respond & Control
Regular application of paraffin and lye-based blends is effective at controlling beech scale on ornamental trees (Cale et al., 2017).
The effectiveness of using systemic insecticides such as imidacloprid to control beech trees is uncertain (Cale et al., 2017). Imidacloprid alone seems to be ineffective at controlling beech scale on trees even with annual reapplication but a study by Roberts (2013) shows that annual imidacloprid injections and buprofezin spraying prevents been scale infestation (Cale et al., 2017). Chemical approaches to control BBD are costly and likely to be non-feasible for use in forest settings.
The herbicides glyphosate and triclopyr are effective methods to control beech regeneration and inhibit sucker production for two growing seasons after beech harvests and BBD-induced mortality (Cale et al., 2017).
Parasitic fungi, predaceous mites and insects have been studied as possible biological controls for beech scale while investigations for biological control of Neonectria spp. have focussed on parasitic fungi (Cale et al., 2017).
A mortality risk classification that considers tree diameter, crown damage, branch stubs, sparse crown foliage and decay fungi presence, can be used by forest managers to determine which tree to harvest in stands with impending disease-induced mortality (Cale et al., 2017). In addition, selection cuttings that remove trees that are infested and infected by beech scale or Neonectria spp. may reduce future mortality by limiting the spread of these agents to healthy beech.
Trees with apparent resistance to beech scale should be retained to improve species diversity, sustain an important wildlife resource and improve beech wood quality. Trees that wall off their cankers (form layers beneath the canker to prevent them from reaching and killing the cambium) are considered to be tolerant and are also potential candidates for retention.
Beech understory root sprouts can be killed by girdling or by brush saw treatments.
Beech Bark Disease in Ontario: A Primer and Management Recommendations
John McLaughlin and Sylvia Greifenhagen,
Ontario Forest Research Institute, Ontario Ministry of Natural Resources and Forestry
Forest Health Care: Beech Bark Disease
(City of Toronto Urban Forestry Branch, 2010)