European elm bark beetle
- French common name: Scolyte européen de l’orme
- Other common names: smaller European elm bark beetle, lesser European elm bark
- Scientific name: Scolytus multistriatus (Marsham)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Coleoptera
- Family: Curculionidae
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Partial list of synonyms:
- Eccoptogaster multistriatus Marsh
- Ips multistriatus Marsham
General information and importance
European elm bark beetle was introduced to North America from Europe on elm logs infested with live larvae. It was first discovered in Massachusetts, United States, in 1909 and in Ontario, Canada, in 1946. It is now distributed throughout Canada from Nova Scotia to British Columbia and in most of the contiguous United States. The beetle causes little direct damage to healthy trees. Adults locate recently dead and moribund trees where they bore through the outer bark to lay their eggs. The larvae tunnel under the bark, feeding on decaying phloem and accelerating decomposition of the dying tree. The greatest threat from the beetle is that it is a prominent vector of the pathogens (Ophiostoma ulmi and O. novo-ulmi) causing Dutch elm disease. The pathogens are most likely Asian in origin and reached both Europe and North America in the 20th century. The more virulent O. novo-ulmi, which was introduced more recently than O. ulmi, has become the dominant pathogen that causes Dutch elm disease in Canada. Dutch elm disease is a vascular wilt with catastrophic impacts in Europe and North America. Spores produced by either species of pathogenic fungus stick to adult beetles emerging from infected trees. The spores are inoculated into healthy trees through small scars caused by adult beetles feeding on twigs of healthy trees. Infected, dying trees are then selected by the adults for their brood, and the next generation of beetle adults become contaminated vectors as well. It is the pathogen and not the bark beetle that causes death of the tree.
In North America, European elm bark beetle is one of three bark beetles known to vector either pathogen causing Dutch elm disease. It has displaced native elm bark beetle (Hylurgopinus rufipes) to become the main vector in the southern portions of their shared ranges. It now co-occurs with the more recently introduced banded elm bark beetle (S. schevyrewi) throughout much of its range, particularly in western regions.
Distribution and hosts
This insect is native throughout Europe including western Russia and Turkey. It has invaded Canada south of the boreal forest region from Nova Scotia to British Columbia and most of the contiguous United States displacing native elm bark beetle in much of their shared range. The insect is also considered introduced to Lebanon, Iran, Egypt, and Algeria, as well as Australia, New Zealand, and possibly temperate South America.
The main host in Canada and the United States is white or American elm (Ulmus americana) but all native and introduced elm species in North America are attacked by the beetle including rock elm (U. thomasii), slippery elm (U. rubra), Siberian elm (U. pumila), Chinese elm (U. parvifolia), and as well as the closely related Japanese Zelkova (Zelkova serrata). European elm bark beetle is known to breed in a wide variety of broadleaf hosts, including alder (Alnus), willow (Salix), poplar (Populus), oak (Quercus), and some fruit trees. None of these latter species are susceptible to Dutch elm disease. North American records of this insect west of the native range of white elm are likely from beetle populations that must persist on amenity elms and other tree species. Canadian records of this insect outside the range of native elms, especially multiple records from British Columbia, were found in traps used in routine bark beetle surveys and so do not have associated host records.
Tree parts affected
Adults of European elm bark beetle feed inconspicuously in the crotches of small twigs of healthy elm trees. This is the common site of pathogen infection. Adults bore into the bark on the lower portions of moribund and recently dead trees and lay their eggs in galleries in the inner bark. Hatching larvae tunnel through the phloem.
Symptoms and signs
Adult beetles are 2.2 to 3.9 millimetres long, with a black thorax and reddish-brown wing coverings (elytra). Red sawdust on the bark surface around holes in the lower portion of the trunks and larger branches of moribund trees is evidence of colonization. Peeling back the bark reveals single branched egg galleries 2.5 to 5.5 centimetres long and following the grain of the wood. The egg galleries are distinctive among bark beetles in being horizontal or V-shaped across the grain of wood. Feeding tunnels by the larvae of European elm bark beetle radiate perpendicular to egg galleries, meandering across the grain of the wood, rarely intersecting, and forming a fan-shaped pattern. In contrast, feeding tunnels of larvae of the native elm bark beetle are at right angles to the egg galleries, parallel to the grain of wood. Round holes in the bark caused by emerging adults of European elm bark beetle may be clustered in a “shot-hole” pattern.
Life cycle
European elm bark beetle has two generations per year in Canada, and three farther south in the United States. They overwinter as larvae, pupae, or sometimes adults under the bark of the tree in which they developed. Adults emerging in the spring feed on twigs of healthy trees. Damage is slight, but if the beetles are carrying spores of the fungus causing Dutch elm disease, healthy trees can become infected through the feeding scars. Healthy trees are exploited for maturation feeding but rarely are colonized for breeding. Mature females identify moribund trees in which to lay their eggs using a blend of volatile organic compounds released by dying trees. Colonizing females then release an aggregation pheromone attracting males and other females, resulting in mass attacks, which overcome already weakened tree defences. European elm bark beetles are monogamous. The female forms a nuptial chamber under the bark where she is joined by a male and mated. The mated female excavates an egg gallery in the phloem parallel to the wood grain. Eggs are laid in niches along the sides of the egg gallery. Larval feeding tunnels are at right angles to the egg gallery across the grain of the wood. They rarely intersect and get larger as the larvae progresses through five life stages, pupating in a chamber in the outer sapwood. Adults emerge directly from these chambers and fly to healthy trees to complete their maturation by feeding on live twigs. They then seek a weakened tree to initiate the next generation.
Damage
Feeding damage to twigs in the upper crowns of healthy trees is inconspicuous but provides opportunities for infection by the pathogen that causes Dutch elm disease. Brood is produced in trees and large branches that are moribund or already dead. Tree death is caused by the transmitted pathogen and not by the beetle. Dutch elm disease has caused catastrophic mortality to elm, which was once a dominant hardwood tree of eastern North America and favoured amenity tree in urban settings.
Prevention and management
The dramatic impact of Dutch elm disease and the difficulty of controlling the disease directly resulted in many chemical control trials aimed at suppressing elm bark beetles that vector the disease, as well as introductions of biological control agents such as nematodes and insect parasitoids. The efficacy of these interventions is difficult to assess. It takes only a few adult beetles to transmit the pathogen and once present, the rate of infection is magnified rapidly as subsequent generations of beetles are exposed to pathogen spores and there is an increasing number of dying trees available for their brood. The general conclusion is that the reduction of beetle populations typically achieved by these management interventions has made little impact on the transmission of Dutch elm disease.
Monitoring susceptible stands, maintaining vigorous trees, rapid removal of dead, damaged, and infected trees, and preventing movement of infected wood (for example, firewood) are the main methods of prevention and management. Pheromone traps are used to monitor the presence of elm bark beetles and, in some cases, to attempt suppression by mass trapping. Active sanitation is probably the most effective practice. Pruning diseased branches has been effective if maintained diligently, although such an approach is more likely to retard rather than stop progress of the disease. Removal and destruction of dead and dying trees are necessary. Restrictions on movement of firewood or any wood with bark out of infected areas is important because this is a common pathway of transferring both beetles and pathogens.
Infected elm trees in amenity situations can be replaced with non-susceptible species. There appears to be a genetic basis to vulnerability, so collecting seed from young elms that are apparently resistant to Dutch elm disease would be a step toward establishing resistant stock.
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
Pheromones and pesticides are defined as pest control products and are regulated in Canada. Products registered for use against European elm bark beetle under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Chemical pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and according to all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Photos
Female adult in its gallery and young larvae in their own gallery
Thérèse Arcand
Mature larvae
Thérèse Arcand
Appearance of an exit hole at the bark surface
Thérèse Arcand
Side view of an adult
Thérèse Arcand
Gallery on an elm trunk
Thérèse Arcand
Left trunk: smaller European elm bark beetle galleries; right trunk: native elm bark beetle galleries
Service canadien des Forêts
Selected references
Allen, E.A.; Humble, L.M. 2002. Nonindigenous species introductions: a threat to Canada’s forests and forest economy. Canadian Journal of Plant Pathology 24: 103–110. https://www.tandfonline.com/doi/epdf/10.1080/07060660309506983?needAccess=true [Accessed October 2024]
Davis, R.S. 2011. Elm bark beetles and Dutch elm disease. Utah Pests Fact Sheet. ENT-147-11. 3 p. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1895&context=extension_curall [Accessed October 2024]
Martín, J.A.; Domínguez, J.; Solla, A.; Brasier, C.M.; Webber, J.F.; Santini, A.; Martínez-Arias, C.; Bernier, L.; and Gil, L. 2023. Complexities underlying the breeding and deployment of Dutch elm disease resistant elms. New Forests 54: 661–696. https://doi.org/10.1007/s11056-021-09865-y
Martín, J.A.; Sobrino-Plata, J.; Rodríguez-Calcerrada, J.; Collada, C.; Gil, L. 2019. Breeding and scientific advances in the fight against Dutch elm disease: Will they allow the use of elms in forest restoration? New Forests 50: 183–215. https://doi.org/10.1007/s11056-018-9640-x
Santini, A.; Faccoli, M. 2015. Dutch elm disease and elm bark beetles: a century of association. iForest 8(2): 126–134. https://doi.org/10.3832/ifor1231-008
Smith, S.M.; Hulcr, J. 2015. Scolytus and other economically important bark and ambrosia beetles. Pages 495–531 in F.E. Vega; R.W. Hofstetter, editors. Bark beetles: biology and ecology of native and invasive species. Elsevier, Amsterdam. https://doi.org/10.1016/B978-0-12-417156-5.00012-5
Van Driesche, R.G.; LaForest, J.H.; Bargeron, C.T.; Reardon, R.C.; Herhily, M. 2013. Forest pest insects in North America: a photographic guide. USDA-Forest Service, Forest Health Technology Enterprise Team. FHTET 2012-02. 702 p. https://www.fs.usda.gov/foresthealth/technology/pdfs/Forest_Pest_Insects_Photo_Guide_508.pdf [Accessed October 2024]