The eastern spruce budworm (Choristoneura fumiferana (Clem.)) is the most pervasive native pest of the North American boreal forest (Maclean, 2016). Large-scale outbreaks in eastern Canada occur in 30 to 40-year cycles (Jardon et al., 2003) and have severe ecological and socioeconomic consequences (Chang et al., 2012; Maclean, 2016). Spruce budworm larvae preferentially feed on new foliage of balsam fir (Abies balsamea (Mill.)) and spruces (Picea sp.). Outbreaks can lead to high-density populations (>17×10⁶ large larvae per hectare) (Ludwig et al., 1978) which defoliate extensive areas of forest (e.g., approximately 52 million hectares in 1975) (Sleep et al., 2009). Multi-annual defoliation leads to high average tree mortalities (e.g., 85% in balsam fir stands), and growth reduction (e.g., of up to 90%), over extensive areas of forest (Maclean, 1980). In turn, this impacts forest regeneration and succession dynamics (Bouchard et al., 2007), increases CO₂ emissions (Dymond et al., 2010; Maclean, 2019) and the likelihood of forest fires (James et al., 2017), and costs hundreds of millions of dollars in revenue loss and mitigation efforts (Natural Resources Canada, 2021).

Recent observations of the dynamics of rising budworm populations support a theory that attributed the rapid spread, extensive coverage, and widespread synchronization of spruce budworm outbreaks to dispersal events from high-density (i.e., outbreak) areas to low-density ones (Stedinger, 1984; Régnière et al., 2019a). At low-density sites, budworm population size is controlled by larvae mortality when failing to find food, and by predator and parasitoid-induced mortality, so that a limited number of adults contributes to the next generation (Stedinger, 1984). Immigrant arrivals can facilitate the transition to an outbreak density through the large influx of gravid moths causing a rapid population increase that, in turn, reduces stochasticity in larval survival caused during foliage searching, and cannot be suppressed by local enemies (Stedinger, 1984). Despite continuous research on this system for more than 50 years (Royama et al., 2017) and progress in control strategies (Maclean, 2019), gaps in our understanding of dispersal patterns and drivers make outbreaks a major challenge to predict and manage (Johns et al., 2019).

Understanding spruce budworm dispersal is challenging because dispersal events are irregular (Greenbank et al., 1980). Unlike classic roundtrip migrations or seasonal migrations that involve large numbers of actively-flying insects following the same routes every year [e.g., Monarch butterflies (Urquhart and Urquhart, 1978, 1979), Painted Ladies (Menchetti et al., 2019), Bogong moths (Warrant et al., 2016)], spruce budworm moths are mostly passive fliers and use wind currents for long-distance transport (Greenbank et al., 1980; Boulanger et al., 2017). Spruce budworm larvae have limited mobility and stay in the same tree, or adjacent ones, to their site of eclosion (Sanders, 1991), but adults may disperse large distances. After pupation, and for the 10- to 15-day life span of adult moths (Morris and Miller, 1954; Rhainds and Heard, 2015), whenever there is no rain and temperatures are above 14°C (Sanders et al., 1978; Greenbank et al., 1980), individuals will fly upward at dusk. These vertical flights can go up to an altitude of 400 m, with most individuals concentrated between 150 m and 300 m, and last for a few hours, after which the moths settle below canopy level for the rest of the day (Greenbank et al., 1980). During vertical flying, dispersal events may occur when moths are carried away by wind currents, and in some instances, hundreds of thousands of individuals can be transported hundreds of kilometers away by this process (Greenbank et al., 1980). These wind-facilitated dispersal events are relatively infrequent, occurring a few times per source site each year, are directional, which means that simple modeling approaches that assume an isomorphic radiation outside of a source area are inadequate, and could simultaneously involve large extents of forest as source and arrival locations. Such processes, in turn, make it challenging to predict the source of take-off, the direction of travel, or the landing area of dispersal flights. Additionally, this complex dispersal limits the viability of tracking approaches such as tagging [note that this approach is useful for monitoring local movement, i.e., distances below 100 m (e.g., Sanders, 1983)] while the small size of this insect makes radio telemetry unfeasible. Finally, the lack of genetic structure across Eastern Canada’s local “subpopulations” (Lumley et al., 2020) makes genetic-based monitoring approaches unviable. Consequently, there is currently no efficient tool to monitor spruce budworm dispersal, hampering our understanding of processes and conditions driving mass take-off and landing, and ultimately limiting our ability to predict and manage the spread of outbreaks (Johns et al., 2019).

To improve monitoring of spruce budworm dispersal, a network of 22–30 automated traps has been deployed by the Federal and Provincial governments of Canada (Canadian Forest Service and the provinces of Quebec, Nova Scotia, and Newfoundland and Labrador) across Eastern Canada since 2018. This network is strategically set up to sample populations within outbreak areas and at the margins of the spruce budworm distribution, where density ought to be low but susceptible to the spread of the outbreak (Figure 1). These traps gather information that can be used to distinguish between local activity and immigration events (see Materials and Methods). Crucially, local individuals are expected to fly around dusk (Greenbank et al., 1980), thus, captures occurring between 17:00 to 23:00 are interpreted as potential locals. Captures between 23:00 and 05:00 are interpreted as putative immigrants, as immigrant individuals are expected to have some delay between their local-flying time and the time at which they land in a non-local patch after being carried away by winds. To enhance the accuracy of this approach, other lines of evidence are considered when trying to determine the migration status of moths, these include (1) local phenology, which informs if conditions are adequate for adults to be present in the area; (2) temperature and wind patterns at the time of capture, which informs whether moths are likely to have been flying at a given site and the potential trajectory of travel; and (3) radar data, which can detect when large numbers of moths disperse. Although this multifaceted approach improves the detection of dispersal events, it has some potentially important limitations. On the one hand, short-distance migrants could arrive a few hours after their vertical flight and before the 23:00 cut-off point, which would lead to mixing between presumed locals and short-distance immigrants. On the other hand, after an immigration event, previous-day immigrants would initiate their vertical flight behavior around dusk, and therefore, captures between 17:00 and 23:00 may be composed of a mixture of locals and immigrants. One way in which the accuracy of these methods can be validated, and lead to improved detection of immigration events, is by looking at the isotopic signals of captured individuals.

Isotopes of hydrogen (referred to using the delta notation or δ²H values) and strontium (referred to using the ratio of ⁸⁷Sr to ⁸⁶Sr or ⁸⁷Sr/⁸⁶Sr) are intrinsic markers of geographic origin because they reflect where an organism developed, and thus can identify whether an individual grew up in the area where it was captured (Wassenaar and Hobson, 1998; Hobson et al., 1999; Flockhart et al., 2015). Local isotopic signals are integrated into an organism through the process of tissue development. As an organism grows and forms new tissue, hydrogen and strontium isotopes are incorporated into its body from the local food and water they consumed (Bowen et al., 2005; Bataille et al., 2020). Studies on migratory insects (Flockhart et al., 2015; Reich et al., 2023, this issue; Lindroos et al., 2023, this issue), demonstrate that tissues with low metabolic activity (e.g., wings) will carry and preserve the isotopic signal of the location at which the insect fed on the local vegetation (Lindroos et al., 2023, this issue). As spruce budworm moths do not feed during their short adult life (i.e., 10–15 days), it is likely that wings and other tissues preserve the isotopic signature of the spruce or balsam fir needles the larval stages fed on. Hydrogen isotopes are used in insect population ecology because of the predictable relationship between chitinous wings, which have low metabolic activity (Lindroos et al., 2023, this issue), and precipitation δ²H values (Hobson, 2019). The δ²H values in precipitation vary predictably in space and time with the hydrological cycle—in particular through the influence of evaporation and precipitation processes which, in turn, leads to the differential concentration of heavy relative to light hydrogen in local water (i.e., fractionation) (Bowen and Revenaugh, 2003; Bowen et al., 2005). δ²H values variation in organisms follows those of precipitation but also includes a predictable contribution of physical and metabolic processes that segregate light vs. heavy isotopes further (Bowen and West, 2019). The ⁸⁷Sr/⁸⁶Sr ratio is another tool with strong potential to complement hydrogen isotopes for ecological provenance studies (Bataille et al., 2020). ⁸⁷Sr/⁸⁶Sr ratios in ecosystems usually vary at higher spatial resolution than δ²H values following the chemical composition and age of surface geology, are stable through time, and are corrected for metabolic-induced fractionation (Bataille et al., 2020). As such, ⁸⁷Sr/⁸⁶Sr ratios vary independently from δ²H values and have a strong potential to provide more specific geolocation (Bataille et al., 2020). Recent progress in analytical methods has allowed the successful application of ⁸⁷Sr/⁸⁶Sr ratios in wings to track the migration of other Lepidoptera (Reich et al., 2021). However, spruce budworms are small insects and their wings are too small for a precise ⁸⁷Sr/⁸⁶Sr analysis. While Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios in the wings of migratory insects have been shown to reflect the larval diet and local soils, it is possible that some strontium might cycle through insect bodies (Reich et al., 2023, this issue). However, for spruce budworm, adults do not feed which means that there is no addition of dietary Sr during the adult stage. Additionally, spruce budworm moths live for approximately 10 days following emergence (Greenbank et al., 1980) and up to 4 days after immigration (Rhainds et al., 2022) limiting the possibility of Sr intake from locations different from the larval stage site. Consequently, it is likely that the ⁸⁷Sr/⁸⁶Sr ratios in short-lived spruce budworm reflect the larval stage and can be applied for geolocation.

Eastern Canada is an excellent location to apply insect isotope geolocation techniques. The climate of eastern Canada is varied with strong precipitation and temperature gradients going from the coast to more in-land locations leading to large δ²H value variations across the breeding range of eastern spruce budworm. Similarly, the geology of Eastern Canada is one of the most varied in the world with Paleozoic carbonate units juxtaposed with Precambrian cratons and Grenville orogeny lithologies (Canada and Wheeler, 1996) leading to large ⁸⁷Sr/⁸⁶Sr ratios variability across the breeding range of eastern spruce budworms. Consequently, these geolocation tools are highly applicable across this study area. Combining hydrogen and strontium isotopes have shown great promise in increasing the specificity of geographic assignments of animals (Reich et al., 2021). Dual hydrogen-strontium isotope geolocation has a high potential to facilitate the identification of local vs. immigrant insects (i.e., nominal assignment) as well as estimating the location of origin of immigrant individuals (i.e., continuous geographic assignment) (Wunder, 2010; Ma et al., 2020; Magozzi et al., 2021). As a first step toward modeling the dispersal dynamics of spruce budworm, we test the applicability of δ²H values and ⁸⁷Sr/⁸⁶Sr ratios as tools to evaluate whether immigration events have occurred at a given site, and distinguish between local and migrant individuals of eastern spruce budworm moths in six automated trap sites across the current outbreak and potential expansion distribution of the species (Figure 1). These automated trap sites are chosen based on the current spread of the spruce budworm outbreak. Three of our sites are in locations undergoing outbreak dynamics and thus have high population densities (Forestville, St. Modeste and Baldwin in Quebec, sampled in 2019), and the three other sites are in areas susceptible to the spread of an outbreak but remained at endemic levels with low-density populations at the time of sampling (Arisaig and Inverness in Nova Scotia 2020, and Zinc Mine in Newfoundland in 2019). First, we use a subset of individuals captured at these sites, presumed to be locals, and individuals following an immigration event that are presumed to be immigrants. We hypothesize that selected individuals will differ in their δ²H values and ⁸⁷Sr/⁸⁶Sr ratios given that locals and immigrants likely fed and formed their tissues in different localities of eastern Canada. Second, we discuss currently used flight-behavior approaches to determine the origin of individuals, contrast them to isotopic approaches, and make suggestions for better understanding dispersal of spruce budworm moths.

The eastern spruce budworm is the most severe pest of the boreal forest and causes millions of dollars in lost revenue to the Canadian economy during outbreak periods (Maclean, 2019). A key component of outbreak dynamics and spread is dispersal from areas of high spruce budworm density (i.e., areas undergoing an outbreak) to areas of low density [i.e., undergoing endemic dynamics where spruce budworm populations numbers are kept down through low larval survival rates while foliage searching and through the action of predators and parasitoids (Stedinger, 1984)]. Spruce budworm moth dispersal has been challenging to monitor and manage, despite major improvements in the understanding of the ecology and pest control strategies of the species (Johns et al., 2019; Maclean, 2019). In this study, we use hydrogen and strontium isotopes to improve an approach currently used to determine migration events, which combines a network of automated traps that record the date and time of capture in discrete blocks, and evidence of local activity and dispersal from radar data, weather patterns and phenological records (see Materials and Methods).

We found that both δ²H values and ⁸⁷Sr/⁸⁶Sr ratios, individually and when combined, allow to discriminate between putative locals and immigrants (i.e., putative immigrants confirmed to be distinct from the local population isotopic values). The isotope data generally supports the current classification approach based on time of capture (and other lines of evidence), particularly for identifying immigrants, and thus confirm that immigration events did occur by providing evidence that non-local spruce budworm are present at a given site. Among all the putative immigrants analyzed, at least one individual per site shows independent isotope data that differed significantly from that of putative local populations in two out of six populations (Figures 3, 4) and for five of the six populations when comparing dual-isotope signals (Figure 5). Yet, in some instances, the flight behavior and isotope methods do not support each other in identifying locals vs. immigrants. In particular, when immigration events occurred before the collection of putative locals (e.g., up to approximately 1 week, e.g., Forestville, QC and Zinc Mine, NL) or when putative locals were collected immediately before an immigration event (e.g., Baldwin and St. Modeste, QC), the overlapping activity between locals and immigrants may lead to misclassification of trap captures when it is only based on behavior (Figure 2). Our approach using combined δ²H values and ⁸⁷Sr/⁸⁶Sr ratios enhances the resolution of the above methods and allows to distinguish between spruce budworm that completed their larval cycle at the site of capture (i.e., locals) vs. individuals from sites that differed in their isotopic composition (i.e., confirmed immigrants) (Figure 5). In particular, our novel isotopic tool allows to identify instances of potential misclassification and to make inferences about mixing of locals and immigrants in the population. Thus, δ²H values and ⁸⁷Sr/⁸⁶Sr ratios are an effective tool to confirm whether an immigration event has occurred at a given site and, with larger sample sizes, could also help determine the relative composition of locals and immigrants in a given site.

The confidence with which putative immigrants and locals were successfully identified based on capture-time and flight behavior differed among sites, and reflected the nuances of local activity and immigration history at the sites of capture (Figure 2). Arisaig and Inverness (Nova Scotia) are located outside the range of the current spruce budworm outbreak and have low, endemic, population densities (i.e., comparatively low trap capture numbers) and experience less frequent immigration events (Figure 1). At these sites, only one immigration event was detected during the 2020 season and occurred several days (~10 days) after the peak activity of the local population (Figures 2A,B). The timing of the immigration event, close to dusk rather than after 23:00, suggests that these immigrants may have originated relatively close to (i.e., not hundreds of kilometers away) the capture sites, but the differences in δ²H values, in a few instances over 20‰, suggests that these individuals might have come from inland areas further north and away from the coast (Figures 3A,B). The ⁸⁷Sr/⁸⁶Sr ratios are generally higher and corroborate that the immigrants did originate in a site different from the capture (local) site. The higher ⁸⁷Sr/⁸⁶Sr ratios of immigrants at Arisaig (up to 0.717) support a northern origin on the Precambrian North American craton in Quebec, whereas the variable ⁸⁷Sr/⁸⁶Sr ratios of immigrants at Inverness suggest a more regional origin with ⁸⁷Sr/⁸⁶Sr ratios typical of the geologically younger Maritimes (Figures 3A,B). At both sites, dual isotope plots show clear discrimination among immigrant individuals and local values (Figures 5A,B). Furthermore, immigrants at both sites show a broad range of isotopic values, suggesting that they do not come from a single area but instead have different geographical origins covering a likely broad spatial extent consistent with a large dispersal event. The remainder of the sites we sampled reveal a more complex pattern of immigrant and local distributions, highlighting some of the limitations of the flight-behavior approach at high spatial resolutions, and underscoring the value of using isotopes to study spruce budworm dispersal.

Using δ²H values and ⁸⁷Sr/⁸⁶Sr ratios showed that several putative immigrants were indeed immigrants, that is, these individuals had significantly distinct δ²H values and ⁸⁷Sr/⁸⁶Sr ratios from the putative local populations (Figures 3,5). This demonstrates that the combined approaches used to detect immigration (e.g., flight-behavior and timing, radar, phenology, weather) are an effective tool to confirm whether an immigration event has occurred at a given area. Yet, our results also showed a few instances of putative local values falling well outside the range of expected local isotopic variation (e.g., Figures 3C,F, 4D), and several instances of putative immigrants not being different from putative local values.

The first set of unusual outcomes points to likely misclassification of immigrants as putative locals. For example, the δ²H value of the putative local at Zinc Mine with a value of −84‰ (Figure 3C) and the putative local at Forestville with an ⁸⁷Sr/⁸⁶Sr ratio of 0.71440 (Figure 4D) place them as outliers and is well beyond the expectation of local variation that corresponds to a 20 km radius of variation in δ²H values from precipitation or in bioavailable ⁸⁷Sr/⁸⁶Sr ratios (Supplementary Table 5). One possible explanation for this potential misclassification is that following immigration to a given site, surviving immigrants then behave as locals on subsequent days, and lead to the mixing of immigrant and local individuals, an issue that is detected by our isotope tools but which current approaches, including population genetics (e.g., Lumley et al., 2020), have been unable to address.

Putative immigrant individuals that were not significantly isotopically distinct from local groups can be interpreted in several ways. First, they could be ‘true’ locals that were flying during the arrival of immigrants and thus are misclassified individuals. Although local flying activity and the likelihood of encountering the traps are expected to be highest around dusk (Greenbank et al., 1980), it is not known whether the influx of large numbers of immigrants and the likely spike in pheromones from arriving females, could influence local male activity and induce them to become active. Second, given that in our analyses immigrants are defined by comparison to putative locals, limited sample sizes of local individuals and misclassifications - which increase the estimate of the population variance - likely decrease our ability to detect ‘true’ immigrants. For example, removing the outlier putative local from Forestville (i.e., the individual with an ⁸⁷Sr/⁸⁶Sr ratio of 0.71440) changes the putative local mean (± sd) and decreases the standard deviation from 0.71255 (±0.00161) to 0.71163 (±0.00023), making the difference between putative immigrants and locals more conspicuous. Increasing sample size and quality of local individuals (i.e., collections before the arrival of immigrants) will improve the precision of immigrant identification—as is shown by assays in Arisaig and Inverness. Third, individuals that were not significantly different could still be immigrants, but which had originated at sites that have similar isotopic signatures to the local site or had traveled only a short distance. While both δ²H values and ⁸⁷Sr/⁸⁶Sr ratios are redundant independently (Bowen et al., 2005; Bataille et al., 2020), the redundancy decreases substantially when combining these isotope tools because of their independent patterns and scales of variation (Wunder, 2010). Globally, δ²H varies continuously at large spatial scales, with decreasing values as latitude increases and increasing distance away from coastal regions (Bowen and Revenaugh, 2003), and thus non-significant differences in δ²H values could be expected for short-distance migrants coming from climatically similar regions. ⁸⁷Sr/⁸⁶Sr ratios vary at a high spatial resolution, reflecting the age and composition of local geology, and thus, even short-distance travelers could show significant divergence in their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios are highly redundant at regional to global scale (Bataille et al., 2020). Therefore, similar dual hydrogen and strontium isotopes between locals and immigrants is unlikely.

We expected, based on their patterns and scales of variation, that local individuals would show limited variance in both δ²H values and especially ⁸⁷Sr/⁸⁶Sr ratios, since they originate in the same location and are expected to have limited dispersal (Sanders, 1983), relative to immigrant individuals that can have more spatially heterogeneous origins. We indeed find that, overall, ⁸⁷Sr/⁸⁶Sr ratios tended to show smaller variance among locals than immigrant individuals (in particular when we consider potential misclassification of putative locals–e.g. Forestville, QC) (Figure 4). With the exception of Arisaig, we did not detect this same trend in δ²H variance which is likely related to the low scale and continuous variation of this isotope. However, this observation of similar variance between immigrants and locals also underlines the relatively large δ²H variance of local individuals (SD = Arisaig: 1.66‰, Inverness: 7.6‰, Zinc Mine: 8.38‰, Forestville: 3.82‰, St. Modeste: 2.75‰, Baldwin: 8.76‰) well beyond the analytical uncertainty (<2‰). Typical sources of variation in local δ²H values, such as seasonal shifts in precipitation and evaporation, adult feeding, and the formation of new tissue, are unlikely to be relevant to this species. Spruce budworm are short-lived and all individuals at any site have their life-history synchronized, with the emergence of adults—a reflection of developmental rates—occurring within a two-week period (Kucera, 1980; Sanders, 1985). Additionally, spruce budworm does not feed as adults and we sampled wings for δ²H analysis, which have low metabolic activity (Lindroos et al., 2023, this issue), making seasonal variation in precipitation δ²H values unlikely to cause variation in the local moth values. Diet-induced variations at a given site are possible, as local spruce budworm could feed on different trees with distinct water sources (e.g., precipitation, groundwater, lake, river) depending on their rooting depth and position on the landscape. If the local landscape has multiple isotopically distinct reservoirs, this could add further heterogeneity to the δ²H values that get integrated into spruce and balsam fir needles and then transferred to spruce budworm, further increasing local δ²H value variance. Even within one single tree, there are several layers of needles grown at different years which might have δ²H differences. While spruce budworms are known to feed preferentially on fresh needles they can switch to older needles if they emerge too early in the spring to access young needles or when younger needles are not available (Régnière and Nealis, 2008). In any case, our data support the work of Hobson et al. (1999); Hobson (2019) who found that the δ²H values of locally raised monarch butterflies had a standard deviation of approximately 4.5‰ within any given site. Other studies that have analyzed known-origin Lepidoptera also report large intra-site standard deviation, sometimes >5‰ (e.g., Brattström et al., 2008; Satterfield et al., 2018). When considered together, our isotope results suggest that local moth individuals move further than the 100 m suggested by Sanders (1983). Isotope data rather suggest local mobility in the scale of kilometers but precisely determining the range and drivers of local spruce budworm movements requires further investigation.

Our findings have fundamental and applied implications. First, we provide an independent validation that the combined use of automated traps, radar monitoring, phenology, and weather data is effective at detecting immigration events in spruce budworm, while underscoring the complementarity of using isotopes to validate and enhance the resolution of this trap network. Second, the difficulty in confirming immigration events in spruce budworm means that current approaches to management rely heavily on widespread winter monitoring of larvae at sites with endemic population dynamics, to detect early immigration-driven population increases and to control population growth before it shifts to an outbreak stage (Early Intervention Strategy - Johns et al., 2019; Maclean, 2019). Although this approach is proving to be effective (Maclean et al., 2019), it is also expensive. The combined use of automated traps and isotopes provides a more targeted approach to confirm immigration events, estimate the relative proportion to which they contribute to the local adult population—a proxy for the following year’s reproductive output and population density—and allow cost-effective decision-making of where to invest in subsequent larval monitoring and eradication strategies. Finally, we confirmed that dual hydrogen-strontium isotopic tools can be applied for the geolocation of low-mass insect species with wing material for hydrogen isotope analysis below 150 μg (~ two wings) and body mass for strontium isotopes below 3 mg. We also confirmed that combined δ²H values and ⁸⁷Sr/⁸⁶Sr ratios have enough discriminatory power to investigate the dispersal of these small insect species over small spatial scales (i.e., <100 km). To better understand dispersal dynamics in this system, our next step is to complete analyses on known-origin samples, which will considerably reduce variance around the local population mean, develop spruce budworm moth-calibrated δ²H and ⁸⁷Sr/⁸⁶Sr isoscapes for Eastern Canada, and then perform dual continuous isotope assignment of immigrant moths and reconstruct their origin and dispersal trajectory. The development of these isotope tools to study non-migratory wind-assisted dispersal behavior of small winged insects will open new research avenues to investigate the mobility of many Boreal pest species (e.g., emerald ash borer, mountain pine beetle, spongy moth and Asian long-horned beetle). Dual isotope geolocation can help ascertain if the pest in question has been in a particular region for a long period (i.e., locally reared pest) or if it has recently arrived (i.e., long-distance migrants or invasives). It can also help identify their dispersal routes. Cumulatively, this information is key to develop more efficient early-eradication strategies (e.g., containment and spraying efforts) and to improve the cost-effectiveness and sustainability of forest management decisions by industry and government practitioners.

The raw data supporting the conclusions of this article can be found in the Supplementary material.

FD, J-NC, and CB: conceived and designed the project. FD, J-NC, KP, and MR: collected data. FD, J-NC, KS, and CB: analyzed the data. J-NC and CB: contributed reagents/materials/analysis tools. FD: wrote the first draft with input from coauthors. FD, J-NC, KS, KP, MR, and CB: revised and edited the manuscript. All authors contributed to the article and approved the submitted version.

This study was funded through Healthy Forest Partnership Early Intervention Strategy against Spruce Budworm Phase II Contribution Program awarded to the Invasive Species Centre by Natural Resources Canada. BCP also received funding from the University of Ottawa start-up fund, and NSERC Discovery Grant. MR was supported by the Queen Elizabeth II Graduate Scholarship in Science and Technology (QEII-GSST) and Ontario Graduate Scholarship.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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