A third pest is now attacking American beech (Fagus grandifolia) in North America, the beech leaf-mining weevil (BLMW), Orchestes fagi L. (see also pages on beech bark disease [BBD] and beech leaf disease [BLD]). The beech leaf-mining weevil is native to Europe, where it commonly attacks European beech (Fagus sylvatica L.) but without causing significant damage (Sweeney et al. 2020).
The beech leaf-mining weevil was discovered infesting American beech in Halifax and Cape Breton Island, Nova Scotia, Canada in 2012. Anecdotal reports of defoliated beech in the Halifax area as early as 2006 suggest it actually became established 5–10 years earlier (Sweeney et al. 2020).
Early surveys (in 2012) confirmed that O. fagi was established in six Nova Scotia counties concentrated mainly within 50 km of Halifax, and in Cape Breton, which is about 300 km from Halifax. No weevils were found at many sites between these two locations. This discontinuous distribution suggests the weevil has either been moved by human assistance there have been more than one introduction from Europe (Sweeney et al. 2012; includes a map of initial introduction sites and photograph of leaf damage [symptoms]).
As of 2017, the weevil had been detected at sites up to 200 km west of Halifax (Sweeney et al. 2020). By 2021, citizen reports indicated the weevil had been established more widely in Nova Scotia and on Prince Edward Island (J. Sweeney, pers. comm.)
Beech trees in the affected area have already experienced beech bark disease; the “killing front” / intense mortality occurred in the early 1900’s (Sweeney et al. 2020). So the issues is to what extent the beech leaf-mining weevil exacerbates beech mortality during the current “aftermath” phase.
Study plots established in 2014 at three sites in the Halifax area with nil (Mt. Uniacke), low (Oakfield) or moderate (Sandy Lake) levels of weevil infestations, and monitored from 2015-2019, demonstrated increases in beech weevil infestation and cumulative beech mortality at each site. Cumulative beech mortality increased from 35% to 48% at Mount Uniacke, from 10% to 70% at Oakfield, and from 24% to 94% at Sandy Lake site (Sweeney et al. 2020). Of the 217 beech trees alive in the study plots in 2014, only 80 remained alive in 2019, an overall mortality rate of 64% in 5 years. Contrary to predictions, severity of BBD was unrelated to cumulative beech mortality, except at Mt. Uniacke, which had the lowest level of weevil infestation and fewest years of weevil infestation. In other words, in the sites with moderate to heavy weevil infestations, beech were dying regardless of their apparent susceptibility to BBD (Sweeney et al. 2020).
Mortality rates were lower in urban areas than in forests, even though weevil defoliation had been apparent to residents as early as 2011–2012. Scientists speculate that urban trees were more open grown so faced less competition for resources (Sweeney et al. 2020).
There is a high risk of the weevil being spread by human movement of firewood and saw logs (Sweeney et al. 2020) because it is small (2.2 – 2.8 mm) and spends 8 or 9 months (July – May) in cracks in the bark of tree trunks (boles). A study in Nova Scotia (Morrison et al. 2017) found adult weevils under the bark of not just of American beech, but also red maple (Acer rubrum) and red spruce (Picea rubens). Experts suspect that weevils will overwinter in niches of the bark of any kind of tree. Weevils are attracted to rough bark on beech trees infected by BBD, which is ubiquitous in northeastern North America. Few weevils were found on branches, under moss on the trees, or in leaf litter. They conclude that any woody stems growing in areas infested by O. fagi are likely to harbor adults.
Morrison et al. (2017) express concern that movement of logs with bark is likely to transport the weevil to new areas. The three species in the study are commonly used as firewood or are harvested for timber. Firewood is a common heating fuel in Atlantic Canada (Morrison et al. 2017) and Northeastern U.S.
Since defoliation by insects increases the susceptibility of American beech to decay fungi, the weevil might increase mortality rates in forests that are surviving in the “aftermath” stage of BBD (Sweeney et al. 2020).
Leaf miner activity does not appear to have reduced beech trees’ annual radial incremental growth. This might be because beech defoliated by O. fagi respond with a second flush of foliage in mid-summer (Sweeney et al. 2020).
Residents of the Halifax area indicated that they first noticed defoliation in approximately 2011–2012. By 2016, about 1/3 of American beech on residential properties were dead; most of the living trees appeared unhealthy with thin crowns and obvious weevil damage. By fall 2018, the percentage of beech that had died had increased to 44%. The staff at two municipal parks apparently removed more dead and unhealthy trees than did homeowners, probably because of the safety threat to visitors. (Sweeney et al. 2020.
Weevil defoliation has resulted in considerable mortality of American beech in Nova Scotia in just a decade. Tree mortality has been significantly greater in stands that had experienced more consecutive years of moderate to high weevil infestations than in a stand where infestation levels were relatively low until 2018. Surprisingly, beech mortality was independent of BBD cankering at the two study sites where the weevil had been present sufficiently long to make an impact. Rather than directly causing tree mortality, it is likely that successive years of weevil defoliation has weakened trees and made them more susceptible to mortality from root rot fungi (Sweeney et al. 2020).
Sweeney et al. (2020) suggest several possible reasons why O. fagi is causing more damage on American beech than on European beech. First, it appears that European beech leaves are tougher so they might be more difficult for early-instar O. fagi larvae to mine. Second, weevil larvae might have less competition for food and space in American beech leaves, which are larger than European leaves. Third, studies suggest parasitism levels are higher in Europe than in North America (Sweeney et al. 2020). Surveys in Europe in 2018 and 2019 identified some possible biocontrol agents, but the Canadian Forest Service has insufficient funds to continue the work (J. Sweeney, pers. comm.)
Sweeney et al. (2020) predict the weevil will continue to spread throughout the range of American beech through both natural dispersal and human-assisted movement, and might have considerable impact on the species and wildlife that are highly dependent upon it.
Morrison, A., J. Sweeney, C. Hughes, R.C. Johns. 2017. Hitching a ride: firewood as a potential pathway for range expansion of an exotic beech leaf-mining weevil, Orchestes fagi (Coleoptera: Curculionidae). Can. Entomol. 149: 129–137 (2017)
Sweeney, J. R.S. Anderson, R.P. Webster and R. Neville. 2012. First Records of Orchestes Fagi (L.) (Coleoptera: Curculionidae: Curculioninae) in North America, with a Checklist of the North
American Rhamphini. The Coleopterists Bulletin, 66(4):297-304. 2012.
Sweeney J.D., Hughes, C., Zhang, H., Hillier, N.K., Morrison, A. and Johns R. (2020) Impact of the Invasive Beech Leaf-Mining Weevil, Orchestes fagi, on Am. Beech in NS, Canada. Frontiers in Forests and Global Change | www.frontiersin.org 1 April 2020 | Volume 3 | Article 46
Photo Credit: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org