Eastern larch beetle
- French common name: Dendroctone du mélèze
- Scientific name: Dendroctonus simplex LeConte
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Coleoptera
- Family: Curculionidae
General information and importance
Eastern larch beetle is a native bark beetle found throughout the range of tamarack (also called eastern larch [Larix laricina]) in Canada and the United States. Eastern larch beetle once was considered a low threat to forests. Occasional outbreaks were brief, and attacks restricted to weakened and moribund trees. However, outbreaks beginning in the 1970s in Atlantic Canada, Quebec, and adjacent northern New England states, and since the 2000s in the northwestern Great Lakes region of both Canada and the United States have been prolonged and caused significant mortality, particularly to healthy, large-diameter trees. This change in population behaviour may be attributed to an increase in mature larch on the landscape combined with recent defoliation events by larch sawfly (Pristophora erichsonii), larch casebearer (Coleophora laricella), or spruce budworm (Choristoneura fumiferana), which stress trees, predisposing them to bark beetle attack. There is also increasing evidence that warmer summer conditions in recent decades have favoured survival of eastern larch beetle and increased the number of broods, and possibly a second generation, to successfully complete their life cycle.
Distribution and hosts
Eastern larch beetle can be found throughout the range of tamarack. In Canada, it can be found from Newfoundland and Labrador, south through the Maritime provinces, west through Quebec and Ontario and extending across the Prairie provinces to northeast British Columbia and Yukon. In the contiguous United States, it ranges from the New England states to West Virginia, and westward through the Great Lakes states as far west as Minnesota. It is also found in central Alaska. Tamarack is the only significant host. Eastern larch beetle does not occur west of the Rocky Mountains and so has not been found attacking western larch (L. occidentalis) in nature, although it may do so where this species is grown within the natural range of the beetle. It will also attack introduced ornamental species including European larch (L. decidua), Siberian larch (L. sibirica), and Japanese larch (L. kaempferi).
Tree parts affected
Symptoms and signs
Entrance holes are small, usually placed under bark scales, and easily overlooked, especially in moribund trees. Reddish-brown boring dust as well as orange, resin-soaked frass may be evident around entrance holes. Resin flow may be obvious from attack sites on the trunks of more vigorous trees. Egg galleries are vertical, winding, and branched, frequently intersecting one another and extending 15 to 40 centimetres. Foliage on targeted tamarack trees yellows by late July, earlier than natural colour change for this needle-bearing deciduous tree. Fading begins at the bottom of the tree and foliage may remain green at top until normal seasonal colour change. This makes detection of new outbreaks by aerial surveys generally difficult, except in the fall when newly attacked trees are bright yellow compared to healthy green trees. Infested trees often fail to produce new foliage the next year. Foliage of those that do soon fades to yellow. Woodpeckers often attack infested trees, removing bark and exposing reddish inner bark and white sapwood.
Eggs are oblong (0.9 by 0.5 millimetres) and white. They are deposited in groups of up to four, 30 to 50 eggs per gallery. Larvae are legless grubs, whitish with a light brown to dark orange head capsule. Final-stage larvae are about 4.5 millimetres long. Pupae are in small chambers at the end of tunnels leading away from egg galleries. Mature adults are dark brown to black with reddish-brown wing covers (elytra) and 3.5 to 5.0 millimetres long.
Life cycle
The principal overwintering stage is the adult. Larvae and pupae may overwinter successfully where warmer temperatures permit, but generally have significantly lower survival. Preferred overwintering locations are low on the tree trunk or in exposed roots, protected by snow cover during winter. Feeding resumes within the tree in the spring. Emergence of adults is synchronized with a peak in flight in late May. Female adults tend to emerge earlier than males, selecting the host and initiating egg galleries. Female adults produce an aggregation pheromone, which attracts males and other females to the successfully attacked tree.
Attacks occur on weakened trees, first on the lower half of the trunk and progressing upward, as a succession of beetles join the infestation. Larger branches and exposed roots may also be attacked. Vigorous trees producing resin can counter beetle infestation, although frequent, successful attacks on relatively healthy trees is reported in chronic outbreaks. Diverse symbiotic microbiota introduced by the beetle help overcome tree defences. More than one pair of adults may use the same entrance hole, but each pair constructs their own egg galleries. In most locations within the range, two sister broods are produced by overwintered adults. The first brood is established in late May to early June. Adults then emerge from their galleries and fly to a different tree to produce a second brood in July and August. A third brood may be produced later in the summer in warmer locations. Survival of later broods depends on seasonal timing and the life stage completed before winter as immature stages are less cold tolerant than adults. Thus, there may be more than one brood but, in most cases, it appears there is only one generation per year.
Adults developing from summer broods are sexually immature and only weakly capable of flight. They walk along the bark surface and re-enter galleries at the base of the trees to overwinter.
Natural enemies, including pathogenic fungi and parasitoids, are significant sources of mortality to immature stages of eastern larch beetle during the active season. Woodpeckers peel bark and feed on all life stages. Freezing temperatures cause high mortality in immature life stages that had insufficient time to reach the adult stage by early winter.
Damage
As with most bark beetles, attacks of eastern larch beetle are often associated with stressed larch forests and follow disturbances such as storms and logging that result in accumulation of coarse debris. Major outbreaks have followed previous damage caused by defoliating insects. However, eastern larch beetle also appears capable of attacking and killing healthy trees without evident prior stress. Aggregate attacks of several females in one tree and the potential for high survival in more than one brood annually frequently leads to tree death after a single year of attack.
Prevention and management
Tamarack is not an intensively managed tree species in forestry, so few management strategies to reduce attacks by eastern larch beetle have been evaluated. Promoting tree health and emphasizing sanitation by promptly removing windthrown, weakened trees, and especially infested trees during the winter before emergence of adults, are likely to reduce populations of this beetle and the tree mortality it causes. Trap trees may be used to indicate and reduce population levels but have limited utility in eastern larch forests that are often on wet soils and difficult to access in the spring before adults emerge.
Eastern larch beetle appears susceptible to naturally occurring pathogenic fungi, suggesting the possibility of a biological control strategy.
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.
Photos
Larval tunnels at the wood surface
Thérèse Arcand
Adults in their tunnel
Thérèse Arcand
Eggs in the adult tunnel
Thérèse Arcand
Mature larva in its tunnel
Thérèse Arcand
Nymph in its case
Thérèse Arcand
Larch trunks partially barked after a severe attack
Robert Blais
Larch trunk barked after a severe attack
Robert Blais
Selected references
Crocker, S.J.; Liknes, G.C.; McKee, F.R.; Albers, J.S.; Aukema, B.H. 2016. Stand-level factors associated with resurging mortality from eastern larch beetle (Dendroctonus simplex LeConte). Forest Ecology and Management 375: 27–34. https://doi.org/10.1016/j.foreco.2016.05.016
Duncan, B. 1987. An illustrated guide to the identification and distribution of the species of Dendroctonus Erichson (Coleoptera: Scolytidae) in British Columbia. Journal of the Entomological Society of British Columbia 84: 101–112. https://journal.entsocbc.ca/index.php/journal/article/view/2277 [Accessed April 2024]
Durand, A.-A.; Constant, P.; Déziel, E.; Guertin, C. 2019. The symbiotic complex of Dendroctonus simplex: Implications in the beetle attack and its life cycle. Bulletin of Entomological Research 109(6): 723–732. https://doi.org/10.1017/S0007485319000051
Langor, D.W.; Raske, A.G. 1987. Emergence, host attack, and overwintering behaviour of the eastern larch beetle, Dendroctonus simplex Leconte (Coleoptera: Scolytidae), in Newfoundland. The Canadian Entomologist 119(11): 975–983. https://doi.org/10.4039/Ent119975-11
Langor, D.W.; Raske, A.G. 1988. Mortality factors and life tables of the eastern larch beetle, Dendroctonus simplex (Coleoptera: Scolytidae), in Newfoundland. Environmental Entomology 17(6): 959–963. https://doi.org/10.1093/ee/17.6.959
Langor, D.W.; Raske, A.G. 1989. The eastern larch beetle, another threat to our forests (Coleoptera: Scolytidae). The Forestry Chronicle 65(4): 276–279. https://doi.org/10.5558/tfc65276-4
McKee, F.R.; Aukema, B.H. 2015. Successful reproduction by the eastern larch beetle (Coleoptera: Curculionidae) in the absence of an overwintering period. The Canadian Entomologist 147(5): 602–610. https://doi.org/10.4039/tce.2014.81
McKee, F.R.; Aukema, B.H. 2016. Seasonal phenology and life-history of Dendroctonus simplex (Coleoptera: Scolytidae) in the Great Lakes region of North America. Environmental Entomology 45(4): 812–828. https://doi.org/10.1093/ee/nvw057
McKee, F.R.; Windmuller-Campione, M.A.; Althoff, E.R.; Reinikainen, M.R.; Dubuque, P.A.; Aukema, B.H. 2022. Eastern larch beetle, a changing climate, and impacts to northern tamarack forests. Pages 261–300 in Kamal J.K. Gandhi; Richard W. Hofstetter; editors. Bark beetle management, ecology, and climate change. Academic Press. https://doi.org/10.1016/B978-0-12-822145-7.00001-5
Seybold, S.J.; Albers, M.A.; Katovich, S.A. 2002. Eastern larch beetle. United States Department of Agriculture, Forest Service. Forest Insect & Disease Leaflet 175. 10 p. https://www.govinfo.gov/content/pkg/GOVPUB-A13-PURL-LPS90199/pdf/GOVPUB-A13-PURL-LPS90199.pdf [Accessed August 2024]
Srei, N.; Lavallée, R.; Guertin, C. 2017. Susceptibility of Dendroctonus simplex to Hyprocreales fungi: Towards the development of a biological control strategy. Journal of Applied Entomology 141(6): 487–495. https://doi.org/10.1111/jen.12361
Ward, S.F.; Aukema, B.H. 2019. Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures, changes in precipitation and interspecific interactions. Ecography 42(5): 1068–1078. https://doi.org/10.1111/ecog.04239