Eastern Larch Beetle Research Articles

An enhanced lure for eastern populations of the North American spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae).

Regional variation in pheromone production and response has practical implications for the use of semiochemical lures to monitor and control bark beetle populations. We tested 4 lure formulations including 2 new formulations that reflect the pheromone production profiles of western and eastern populations of spruce beetles, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), as well as 2 commercially available formulations (current Rocky Mountain lure and current Atlantic lure), in 2 locations in New Brunswick, Canada. In 2 separate years, the new eastern lure containing seudenol, MCOL, and spruce terpenes captured 4 times (2021) and 11 times (2022) more spruce beetles than the current Atlantic lure that consisted of frontalin, seudenol, and spruce terpenes. In 2021, we also captured more eastern larch beetles, Dendroctonus simplex LeConte (Coleoptera: Curculionidae), with the new eastern lure, whereas in 2022, we captured the most D. simplex with the current Atlantic lure, suggesting that more research is needed on D. simplex pheromone production and response across its range. The bark beetle predator, Thanasimus dubius (Fabr.; Coleoptera: Cleridae), did not respond well to the new eastern blend that lacks frontalin, suggesting that response to frontalin is important in finding prey and might be conserved in predator populations. The reduced trap catch of T. dubius to the enhanced lure is beneficial because it does not inhibit natural population control by removing predators from the community. Our study reveals an improved trap lure for eastern populations of spruce beetles and highlights gaps and research needs in bark beetle pheromone ecology.

Monoterpene Composition of Phloem of Eastern Larch (Larix laricina (Du Roi) K. Koch) in the Great Lakes Region: With What Must the Eastern Larch Beetle (Dendroctonus simplex LeConte) Contend?

The eastern larch beetle (Dendroctonus simplex LeConte) is the only tree-killing bark beetle that colonizes tamarack, or eastern larch, (Larix laricina (Du Roi) K. Koch) in the Great Lakes region. Historically, outbreaks have been intermittent and of short duration, frequently following predisposing factors such as drought or defoliation. However, over the past two decades, this insect has been in a perpetual state of outbreak in parts of the U.S. Great Lakes region, a deviation from historic norms. From 2001–2021, the insect impacted 300,000 ha, or 60% of the tamarack forests in Minnesota. This activity has prompted renewed interest in the beetle’s chemical ecology, including aspects of host semiochemistry. While foliar chemistry has been well documented in L. laricina, characterization of the monoterpene composition of the phloem has been lacking. We collected phloem samples from 56 tamarack trees across 14 locations in Wisconsin and Minnesota and assessed the relative abundances of the major monoterpenes present using gas chromatography-flame ionization detector (GC-FID). Individual terpenoid components identified included α-pinene (39.4%) and ∆-3-carene (30.0%) followed by several other components in small (<8%) amounts. This knowledge provides a basis for future testing of monoterpene synergists or antagonists in pheromone lures targeting eastern larch beetle and/or its natural enemies.

The symbiotic complex of Dendroctonus simplex: implications in the beetle attack and its life cycle.

The eastern larch beetle (Dendroctonus simplex Le Conte) is recognized as a serious destructive forest pest in the upper part of North America. Under epidemic conditions, this beetle can attack healthy trees, causing severe damages to larch stands. Dendroctonus species are considered as holobionts, as they engage in multipartite interactions with microorganisms, such as bacteria, filamentous fungi, and yeasts, which are implicated in physiological processes of the insect, such as nutrition. They also play a key role in the beetle's attack, as they are responsible for the detoxification of the subcortical environment and weaken the tree's defense mechanisms. The eastern larch beetle is associated with bacteria and fungi, but their implication in the success of the beetle remains unknown. Here, we investigated the bacterial and fungal microbiota of this beetle pest throughout its ontogeny (pioneer adults, larvae and pupae) by high-throughput sequencing. A successional microbial assemblage was identified throughout the beetle developmental stages, reflecting the beetle's requirements. These results indicate that a symbiotic association between the eastern larch beetle and some of these microorganisms takes place and that this D. simplex symbiotic complex is helping the insect to colonize its host tree and survive the conditions encountered.

Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures, changes in precipitation and interspecific interactions

Biotic disturbance agents such as insects can be highly responsive to climatic change and have widespread ecological and economic impacts on forests. Quantifying the responses of introduced and native insects to climate, including how dynamics of one agent may mediate those of another, is important for forecasting disturbance and associated impacts on forest structure and function. We investigated drivers of outbreaks by larch casebearer Coleophora laricella, an invasive defoliator, and eastern larch beetle Dendroctonus simplex, a native, tree‐killing bark beetle, on tamarack Larix laricina from 2000 to in Minnesota, USA. We evaluated the utility of temporal, spatial and climatic variables in predicting the presence/absence of outbreaks of each insect in cells of rasterized aerial survey data. The role of defoliation by larch casebearer in outbreaks of eastern larch beetle was also investigated. For both species, the most important predictors of outbreak occurrence were proximity of conspecific outbreaks in space and time. For larch casebearer, outbreak occurrence was positively associated with spring precipitation and warmer growing seasons. Outbreak occurrence of eastern larch beetle was positively associated with warmer and dryer years and was more likely in cells with prior defoliation by larch casebearer. Our results demonstrate that climate can drive large scale outbreaks of introduced and non‐native disturbance agents on a single host species, and that interactions at the tree level between such agents may scale up to manifest across large temporal and spatial scales.

Susceptibility of Dendroctonus simplex to Hypocreales fungi: towards the development of a biological control strategy

AbstractThe use of Hypocreales fungi against bark beetles is an attractive alternative strategy for biological control in forestry. However, fungal inoculation methods remain a major challenge due to the cryptic behaviour of these beetles. Experiments were conducted to evaluate the susceptibility of adult eastern larch beetles, Dendroctonus simplex, to different isolates of Hypocreales fungi and to assess the efficiency of direct inoculation of fungal conidia using an assisted autodissemination device. Among the nine fungal isolates tested, Beauveria bassiana INRS‐242 was selected as the utmost isolate causing a mortality rate of 64.4% of D. simplex adults within 10 days and having a high conidial viability of 81.7%. Using an assisted autodissemination device, a fungal‐coated pouch was a promising contamination chamber. Once it had fallen onto the fungal‐coated pouch, each D. simplex adult loaded an average lethal dose of 2.1 × 109 conidia, which induced a mortality rate of 98.0%. Our results demonstrated the fungal pathogenicity of B. bassiana INRS‐242 against the eastern larch beetles and pointed out the potential of the assisted autodissemination device as an effective method of conidia inoculation. These findings represent the first step to develop a biological control strategy against D. simplex populations.

Seasonal Phenology and Life-History of Dendroctonus simplex (Coleoptera: Curculionidae) in the Great Lakes Region of North America.

The eastern larch beetle, Dendroctonus simplex LeConte (Coleoptera: Curculionidae), is distributed throughout the North American boreal forest sympatric with its primary host, the eastern larch or tamarack, Larix laricina (Du Roi) K. Koch. Outbreaks of eastern larch beetles are typically small and associated with stressed tamaracks. Since 2000, however, an outbreak has killed >90,000 ha of tamarack in Minnesota and surrounding Great Lakes region. Identifying the causes of this epidemic is challenging due to knowledge gaps regarding the insect's biology. We present field data from 2011 to 2014 on degree days associated with spring emergence, dispersal, host colonization, and re-emergence from colonized hosts by mature adult beetles, as well as degree days associated with larval development, and prewinter emergence by adult progeny at study sites in northern Minnesota. After initial host colonization in early spring we found that a second brood was established in early summer by re-emerging parents. In 2012, a third brood was established. Across study years, first broods developed to adults by late summer, with many beetles relocating to the base of the host tree to overwinter. Second broods often reached adulthood and initiated prewinter emergence. The third brood of 2012 overwintered as adults, pupae, and late-instars, resuming development the following spring. Each spring, emergence of adult beetles from all broods established the previous year was highly synchronous. Knowledge of the biology of eastern larch beetles along the southern margin of their range aids in understanding how population dynamics may change with a changing climate.

Stand-level factors associated with resurging mortality from eastern larch beetle (Dendroctonus simplex LeConte)

The current outbreak of eastern larch beetle (Dendroctonus simplex LeConte) in Minnesota, USA, ongoing since 2000, has been characterized with a severity and duration unprecedented in previous outbreaks within the state. This study investigates the relationship between tamarack mortality due to eastern larch beetle and extant stand and site characteristics. Observations of tree attribute information and site condition collected over the course of the outbreak from nearly 15,000 tamarack trees in three different ecological regions of Minnesota were modeled using linear regression. Increasing tree diameter was the best indicator of tamarack mortality within the northern limits of the state’s tamarack range and during the later portion of the outbreak. Tamarack density was negatively related to tree mortality in the north-central region during the later outbreak, while the density of co-located non-host gymnosperms was positively related to tree mortality in areas of marginal host distribution. A temporal change in the importance of predictor variables suggests that the influence of tree and site characteristics evolved as the outbreak progressed and subsequent mortality increased. A greater understanding of the factors associated with tamarack mortality and how the those factors change over time and space will help to inform management practices and mitigate the impacts of this bark beetle on the tamarack resource in Minnesota.

Population dynamics and epidemiology of four species ofDendroctonus(Coleoptera: Curculionidae): 100 years since J.M. Swaine

AbstractAlmost 100 years have passed since J.M. Swaine, the assistant entomologist in charge of Forest Insect Investigations, wrote, “Canadian bark-beetles: a preliminary classification, with an account of the habits and means of control”. The goal was to “put into the hands of practical foresters information of inestimable practical value… to prevent the continued loss of timber now being destroyed” by “the most insidious enemies of the forest”. In this paper, we celebrate Swaine’s pioneering work by summarising the foundational aspects of his early treatise of 1918: the “general habits” of bark beetles, classifications of their behaviour, causes of population increase, and mitigation tactics. In the founding text, Swaine identified all majorDendroctonusErichson (Coleoptera: Curculionidae) bark beetles found in Canada, although details on life histories were scarce. We summarise current knowledge of the life histories and population dynamics of the spruce beetle,D.rufipennisKirby; the Douglas-fir beetle,D. pseudotsugaeHopkins; the eastern larch beetle,D. simplex; and address the current range expansion of mountain pine beetle,D. ponderosaeHopkins. We review how aspects of global change, such as invasive species, have altered the population dynamics of certain bark beetles. Finally, we conclude with lessons from two of the many past contributors to bark beetle ecology in Canada, J.M. Swaine and H.A. Richmond.

Surveying the endomicrobiome and ectomicrobiome of bark beetles: The case of Dendroctonus simplex

Many bark beetles belonging to the Dendroctonus genus carry bacterial and fungal microbiota, forming a symbiotic complex that helps the insect to colonize the subcortical environment of the host tree. However, the biodiversity of those bacteria at the surface of the cuticle or inside the body parts of bark beetles is not well established. The aim of this study was to characterize the bacterial microbiome associated with the eastern larch beetle, Dendroctonus simplex, using bacterial 16S rRNA gene pyrosequencing. The ecto- and endomicrobiome and the subcortical galleries were investigated. Several bacterial genera were identified, among which Pseudomonas, Serratia and Yersinia are associated with the surface of the beetle cuticle, and genera belonging to Enterobacteriaceae and Gammaproteobacteria with the interior of the insect body. The index of dissimilarity indicates that the bacterial microbiome associated with each environment constitutes exclusive groups. These results suggest the presence of distinct bacterial microbiota on the surface of the cuticle and the interior of D. simplex body. Additionally, the bacterial diversity identified in the galleries is substantially different from the ectomicrobiome, which could indicate a selection by the insect. This study reports for the first time the identification of the eastern larch beetle microbiome.

Successful reproduction by the eastern larch beetle (Coleoptera: Curculionidae) in the absence of an overwintering period

AbstractEastern larch beetles, Dendroctonus simplex LeConte (Coleoptera: Curculionidae: Scolytinae), are monophagous, phloem-feeding herbivores of eastern larch (tamarack), Larix laricina (Du Roi) Koch (Pinaceae). Recently dead or moribund trees are preferentially colonised. Outbreaks of eastern larch beetles are generally localised and short-lived, although a large outbreak has been occurring in the Great Lakes region of North America since 2000. The beetle is reported as univoltine, with a single, spring-emergent, reproductive parent generation establishing one to three sibling broods per year. Some progeny emerging during summer or fall re-enter the tree bole close to ground level to overwinter, while remaining brood adults overwinter within pupal chambers in situ. Owing to these behaviours, eastern larch beetles have been suggested to possess an obligate overwintering reproductive diapause. However, studies have not confirmed this hypothesis. We tested the reproductive viability of non-overwintered progeny in three laboratory experiments. Non-overwintered progeny were reproductively viable, suggesting that a portion of the population may exhibit a facultative adult overwintering diapause. Progeny that emerged naturally from the host (i.e., putative fall-emergers) demonstrated reproductive rates almost six-fold those of manually extracted insects demonstrating a propensity to remain in situ (i.e., putative spring-emergers). These results shed new light onto the reproductive behaviour of eastern larch beetles, and suggest that future population dynamics may be influenced by a warming climate.

Influence of temperature on the reproductive success, brood development and brood fitness of the eastern larch beetle Dendroctonus simplexLeConte

Abstract The eastern larch beetle Dendroctonus simplex LeConte colonizes the phloem of tamarack Larix laricina (Du Roi) K. Koch, preferring recently dead or moribund trees weakened by insect defoliation or other factors that predispose trees to beetle attack. Outbreaks of eastern larch beetles are typically localized, of short duration, and collapse when the supply of stressed hosts is exhausted. Although rare, landscape‐level outbreaks of eastern larch beetles can occur if large areas of tamarack become stressed. From 2000 onward, an ongoing outbreak of eastern larch beetles in the Great Lakes Region of North America has resulted in extensive mortality to more than 75 000 ha of tamarack forest in Minnesota, U.S.A. This outbreak has no known biotic predisposing factor, such as extensive defoliation. Trends of recent climate warming, however, are suspected to be a contributing factor. Current efforts to model the effects of climate on eastern larch beetle population dynamics are hampered by an absence of data relating beetle developmental biology to temperature. In a laboratory study, we studied eastern larch beetle reproductive success, larval development and offspring fitness at temperatures in the range 9.9–29.4 °C. Offspring production was similar across temperatures. Successful brood development occurred at 9.9 °C, whereas the minimum and optimal developmental temperatures were calculated to be 7.5 and 27.9 °C, respectively. Offspring size and lipid content were maximized between 20 and 22 °C. Our results indicate a potential trade‐off between temperatures that maximizes eastern larch beetle offspring fitness versus developmental rate. The implications of such a trade‐off are discussed with respect to beetle population dynamics.

Assessing the Threat Posed by Indigenous Exotics: A Case Study of Two North American Bark Beetle Species

ABSTRACT The Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, was detected in 2001 in northern Minnesota outside its natural range and the range of its native hosts, Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco, and western larch, Larix occidentalis Nutt. Consecutive years of detection indicated that D. pseudotsugae may have been established in a new environment and provided a possible example of an indigenous exotic species in North America. Pheromone-baited logs of P. menziesii and tamarack, Larix laricina (Du Roi) K. Koch, were placed at four sites in northern Minnesota in an attempt to detect this indigenous exotic. Bark was removed from one half of logs and checked for D. pseudotsugae galleries. The remaining logs were left intact and reared to collect adult beetles the following spring. No D. pseudotsugae galleries were identified and no adults of the indigenous exotic were located. Along with the eastern larch beetle, Dendroctonus simplex LeConte, native Minnesota populations of Polygra...

Toxicity and Repellency of 4–Allylanisole and Monoterpenes from White Spruce and Tamarack to the Spruce Beetle and Eastern Larch Beetle (Coleoptera: Scolytidae)

Monoterpenes found in the oleoresin of white spruce, Picea glauca (Moench) Voss, and tamarack, Larix laricina (DuRoi) K. Koch, and 4-allylanisole (= estragole, methyl chavicol) were bioassayed for their toxicity to adult spruce beetles, Dendroctonus rufipennis (Kirby), and adult eastern larch beetles, D. simplex LeConte. Limonene and 4-allylanisole at 20 ppm were most toxic to spruce beetles at 24 h; whereas, limonene, myrcene, beta phellandrene, and 4-allylanisole were most toxic to eastern larch beetles. Limonene and 4-allylanisole caused 100% mortality to spruce beetle adults exposed to 80 ppm for 24 h. In comparison, limonene, 4-allylanisole, myrcene, and beta phellandrene caused 88-98% mortality to eastern larch beetles exposed to 80 ppm for 24 h. In field tests, limonene, 4-allylanisole, myrcene, and beta phellandrene inhibited the response of the spruce beetle to frontalin; whereas, myrcene and limonene inhibited the response of the eastern larch beetle to seudenol. Limonene, myrcene, and beta phellandrene were found in greater quantities in the wound tissues surrounding the induced reaction zones of white spruce and tamarack than in unwounded tissues which indicates the presence of an induced defensive system. Defoliated tamarack trees responded with increased levels of limonene in the oleoresin during the first 2 yr of successive defoliation, but these levels decreased during the first and 4th yr of successive defoliation. Limonene and 4-allylanisole were the most toxic and inhibitory monoterpenes tested in this study

Arthropods and nematodes co-occurring with the eastern larch beetle,Dendroctonus simplex [Col.: Scolytidae], in Newfoundland

Fifty-four species of insects, 5 of spiders, 5 of mites, 1 harvestman and 1 nematode co-occurred with the eastern larch beetle,Dendroctonus simplex LeConte, on tamarack,Larix laricina (Du Roi) K. Koch, at 2 localities in Newfoundland. The most abundant predators were the flyMedetera gaspensis Bickel and the beetleRhizophagus dimidiatus Mannerheim and the most abundant parasitoids were the HymenopteraSpathius canadensis Ashmead,Rhopalicus tutela (Walker) andRoptrocerus xylophagorum (Ratzburg).D. simplex is recorded as a new host for 9 predator and 3 parasitoid species.

The Eastern Larch Beetle, Another Threat to our Forests (Coleoptera: Scolytidae)

Isolated infestations of the eastern larch beetle, Dendroctonus simplex LeConte, have been reported in North America for over 100 years. Historically, this species has been considered a secondary pest of tamarack, Larix laricina (Du Roi) K. Koch, infesting only weakened or recently felled host material. However, recent widespread outbreaks of this bark beetle in northeastern North America and Alaska suggest that the species can be a primary killer of trees. More than 1.4 million m3 of tamarack was killed in the Atlantic provinces of Canada from 1976 to 1986. No damage estimates are available for Quebec and the United States. Fire, flooding, drought and defoliation by insects are often important agents predisposing tamarack to beetle attack and fostering localized build-up of D. simplex populations which then spread to healthy stands resulting in a widespread outbreak. Key Words: Dendroctonus simplex, bark beetle, outbreaks, damage appraisal, predisposing agents, tamarack.

RESPONSE OF EASTERN LARCH BEETLE (COLEOPTERA: SCOLYTIDAE) IN ALASKA TO ITS NATURAL ATTRACTANT AND TO DOUGLAS-FIR BEETLE PHEROMONES

AbstractIn tests conducted in the McKinley River drainage, Alaska, seudenol combined with α-pinene attracted the most eastern larch beetles, Dendroctonus simplex LeConte. The next most attractive treatments included tamarack log sections infested with either unmated female D. simplex or unmated female Douglas-fir beetles, Dendroctonus pseudotsugae Hopkins, Neither frontalin nor trans- verbenol with α-pinene was attractive. Addition of niethylcyclohexenone (MCH) to the seudenol + α-pinene treatment repressed its attraction by 83%. Sex ratios of simplex responding to logs infested with female simplex or pseudotsugae were equal or favored females, but favored males in the case of seudenol + α-pinene. Addition of MCH shifted the sex ratio in favor of females. Two other scolytid species, Orthotomicus caelatus (Eichhoff) and Ips perturbatus (Eichhoff), were trapped, but were too few to relate to treatments. Two species of clerids were caught on traps: Thanasimus undatulus Say and Thanasimus dubius (L.); the latter is a new Alaskan record.