Agrilus anxius (bronze birch borer): addition to the EPPO Alert List
The NPPO of Norway recently suggested that Agrilus anxius (Coleoptera: Buprestidae – bronze birch borer) could usefully be added to the EPPO Alert List, and Dr Bjørn Økland (Norwegian Forest and Landscape Institute) kindly provided most of the information used to prepare the pest description. This borer is an important pest of landscape birches in North America and causes widespread damage to ornamental birches, especially in the North-Eastern USA and Canada. European species of birch (Betula pendula and B. pubescens) planted in North America are among the most susceptible birches and have demonstrated very high mortality. In the EPPO region, these birch species are widespread and important in the landscape, especially in the northern part of the region and in Russia where they constitute a significant part of the forest cover. Thus, A. anxius has the potential to become a serious forest pest should it become introduced in Europe. In addition, small insects are thought to survive in wood chips as it has been demonstrated for close relatives in the genus Agrilus, in particular for A. planipennis (EPPO A1 List). The increasing import of wood chips from North America to Europe includes large volumes of chips made from birch wood. This increasing trade could constitute a pathway for the entry of A. anxius into the EPPO region.
Agrilus anxius (Coleoptera: Buprestidae) - Bronze birch borer
Why: The NPPO of Norway recently suggested that Agrilus anxius (Coleoptera: Buprestidae – bronze birch borer) could usefully be added to the EPPO Alert List. A. anxius originates from North America where it is considered as a serious pest of birch trees (Betula spp.). A. anxius primarily attacks birches that are weakened or stressed but it is suspected that under certain circumstances (e.g. large populations) it can also attack healthy trees. In North America, tree mortality has been observed more particularly on birches planted for ornamental purposes, but A. anxius is also causing problems in forests.
Where: A. anxius is native to North America and occurs throughout the range of birches in Canada and the USA.
EPPO region: absent.
North America: Canada (Alberta, British Columbia, Manitoba, New Brunswick, Newfoundland, Nova Scotia, Ontario, Quebec, Saskatchewan), USA (Alaska, Colorado, Georgia, Idaho, Illinois, Indiana, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nevada, New Jersey, New York, North Dakota, Ohio, Oregon, Pennsylvania, Tennessee, Utah, Washington, West Virginia, Wisconsin, Wyoming).
On which plants: Betula spp. (birches), including Betula alleghaniensis (yellow birch), B. jacquemontii (white-barked Himalayan birch), B. lenta (sweet birch), B. occidentalis (water birch), B. papyrifera (paper birch), B. pendula (silver or European birch), B. platyphylla (Manchurian birch), B. populifolia (gray birch), B. pubescens (downy birch), B. utilis (Himalayan birch). A. anxius is known to attack all native and introduced birch species (and their numerous crosses) in North America, although susceptibility varies between the birch species. It has been observed that the white-barked birches were generally more susceptible than those without white bark. For example, B. jacquemontii and B. pendula (which are widely planted in North America) are considered to be highly susceptible; B. alleghaniensis, B. lenta, B. papyrifera, B. platyphylla and B. populifera moderately susceptible; and B. nigra (river birch) is rarely attacked by A. anxius. A large block experiment with different birch species in Ohio revealed 100% mortality for B. pendula and B. pubescens which are the most widespread birch species in Northern Europe. Earlier studies carried out in Michigan had also shown that B. pendula was particularly susceptible to the insect.
Damage: Damage is caused by larval feeding on the inner bark and cambium of the tree. Repeated attacks and the construction of numerous winding galleries by larvae disrupt nutrient transport that eventually kills the roots. Insect galleries can also girdle the tree branches and trunks. Initially symptoms of an infestation appear in the upper crown the tree with leaf yellowing and branch dieback. Other evidence of an infestation is the presence of 5 mm wide ‘D’-shaped exit holes. Rust coloured sap oozing and staining can also be observed on the bark, along with swellings and bumps where the tree has healed inside. In many cases, tree mortality is observed within a few years after the appearance of the first symptoms. Birches that are weakened or stressed by drought, old age, insect defoliation, soil compaction, stem or root injury are more susceptible to damage caused by A. anxius. Adults feed on leaves (Alnus, Betula, Populus) but damage is insignificant.
The life cycle of A. anxius can last 1 or 2 years. Adults are small, narrow, metallic coppery beetles of approximately 12 mm long. Females lay eggs (singly or in clusters) in bark crevices, and can lay up to 75 eggs during a lifetime. Larvae are whitish, relatively long (19-25 mm) and flat with a head that is larger than the body, and they immediately bore into the wood after hatching. The insect overwinters as larvae inside the wood. Pupation takes place in shallow cells in the xylem and adults emerge from May to mid-July (depending on the climatic conditions). Pictures can be viewed on the Internet: http://www.forestryimages.org/browse/subthumb.cfm?sub=352
Dissemination: Adults can fly but there is no data on the natural spread of the insect. Over long distances, trade of infested trees and wood can ensure pest dispersal. Except for host preferences, the biology and morphology of A. anxius show similarities with A. planipennis (emerald ash borer – EPPO A1 List). Both insects are small and their larvae make serpentine-like galleries under the bark. The small size of A. anxius and the entry into the wood for pupation make it likely to survive chipping and transport in wood chips (provided the chips are not too small), as this has been demonstrated in experiments for A. planipennis. In Europe, and in particular in the Scandinavian countries, there is an increasing trade of wood chips for biofuel production. It is expected that the yearly import of deciduous wood chips in some Scandinavian countries will reach approximately 1 million tonnes in the coming years, including chips from the main host trees of A. anxius, such as B. papyrifera and B. alleghanensis. In addition, it is very likely that these large amounts of wood chips will be stored outdoors at short distances from birch stands.
Pathway: Betula spp. plants for planting, cut branches?, wood, wood chips from Canada and the USA.
Possible risks: In the EPPO region, birches are stand-forming tree species and are especially common in Northern Europe and Russia. With the exception of the Mediterranean region, they are widely planted for forestry and amenity purposes. For example in the Northern European countries, birches constitute a large part of the forest volume, ranging from 11.6 % in Sweden to 28.2 % in Latvia. In Norway, the area covered by birch forests reaches approximately 30 % of the total forest cover. In Scandinavian forests, the dominance of birch increases with latitude and altitude. Birch wood is also economically important for various building and industry purposes (e.g. plywood, pulpwood, furniture, birch sap).
In North America, the control of A. anxius mainly relies on preventive methods which favour birch growth (e.g. suitable planting sites, sufficient watering). Woodpeckers and some hymenopteran insects (Atanycolus charus (Braconidae), Phasgonophora sulcata (Chalcididae), Spathius simillimus (Braconidae)) are mentioned as natural enemies of A. anxius, but these may not be effective in urban environments. Chemical control targeted at the adults during summer can be used in tree nurseries but is probably not practical in urban and forest environments. The wide geographical distribution of A. anxius in North America, under various climates, strongly suggests that this insect has the potential to establish in the EPPO region. Considering the significant tree mortality observed in North America and the high susceptibility of the most dominant birch species in the EPPO region (i.e. B. pendula and B. pubescens), the introduction and establishment of A. anxius would most probably cause severe outbreaks and damage to Betula species grown in forests, nurseries, parks and gardens.
Acknowledgements: Warm thanks are due to Dr Dr Bjørn Økland (Norwegian Forest and Landscape Institute, Ås, NO) who has kindly provided most of information used to prepare this description.
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EPPO RS 2011/030
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Panel review date: -
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Entry date 2010-02
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Sources
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Miller RO, Bloese PD, Hanover JW, Haack RA (1991) Paper birch and European white birch vary in growth and resistance to bronze birch borer. Journal of the American Society of Horticultural Science 116(3), 580-584.
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INTERNET
The Norwegian Forest and Landscape Institute. http://www.skogoglandskap.no
Russian forests. Forest cover map of the former Soviet Union. http://www.forest.ru/eng/basics/map.html
Atlas of Russia’s Intact Forest Landscapes. Tree species composition. http://www.forest.ru/eng/publications/intact/!tree-russia.html