Defoliation of Norway Spruce by Spruce Budworm (Lepidoptera: Tortricidae) and Protection Using Bacillus thuringiensis

Abstract

Norway spruce (Picea abies (L.) Karst.) has been widely planted beyond its natural range due to its fast growth rate and valuable wood. In Québec, over 200 million seedlings have been planted since 1964. Several of these plantations are now facing a new potential threat, i.e., spruce budworm (Choristoneura fumiferana (Clem.)) infestations. Despite contrasting results, Norway and white spruce (P. glauca [Moench] Voss) apparently sustain a similar degree of budworm defoliation. The main study objective is to quantify defoliation in Norway spruce caused by spruce budworm. We also evaluate the efficacy of Bacillus thuringiensis Berliner spp. kurstaki (Btk) in protecting this exotic host tree. Annual defoliation was assessed in plantations of Norway, white, and black spruce (P. mariana [Mill.] BSP) between 2018 and 2022 in the Bas-Saint-Laurent region. Additional surveys were conducted in Norway and white spruce plantations in the Gaspésie and Côte-Nord to evaluate Btk efficacy. We show that both species exhibit similar defoliation levels, though Norway spruce sometimes sustains greater damage (e.g., 35% vs. 10% in 2019). Btk formulations showed low efficacy in protecting Norway spruce foliage (≥49.32% defoliation in treated plantations). Further studies are needed to understand factors influencing Btk efficacy on this host.

1. Introduction

Norway spruce (Picea abies [L.] Karst.) is a species native to central and northern Europe but also occurs east of the Urals [1]. This conifer is one of the most economically and ecologically important tree species in Europe, covering approximately 30 million ha [2]. Norway spruce has been widely planted outside of its natural distribution range due to its fast growth rate and valuable wood [3]. In North America, this conifer was the most frequently planted exotic tree species from the end of the 19th century to the beginning of the 20th century [4,5,6], with its plantations being established mainly in the northeastern United States with 120,000 acres (around 49,000 ha) by 1936 [6], and in Ontario, the Maritime Provinces [6], and Québec [5]. In Québec, the first commercial Norway spruce plantations were established at the beginning of the 20th century [5]. Since 1964, more than 200 million seedlings have been planted, reaching a peak between 1980 and 2000 [7]. Its current use in reforestation has declined sharply because of environmental concerns, its high susceptibility to the white pine weevil (Pissodes strobi Peck), and a lack of markets for its wood, e.g., ref. [8].

Currently, several Norway spruce plantations in the province are reaching commercial maturity and have now been exposed to a new potential threat, i.e., spruce budworm (Choristoneura fumiferana [Clem.]) infestations [9]. Periodically, populations of this native defoliator reach outbreak proportions, leading to a substantial decline in vigor and growth rate and increased mortality of affected trees [10]. The current outbreak in the Province of Québec covered over 14 million ha in 2024 [11], including Norway spruce plantations [12]. Spruce budworm hosts include balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), red spruce (P. rubens Sarg.), and black spruce (P. mariana [Mill.] BSP). A clear hierarchy of host species susceptibility exists, with balsam fir being the most susceptible, followed by white spruce, red spruce, and black spruce, which is the least susceptible [13].

Protection of coniferous stands against spruce budworm in Canada currently relies on large-scale aerial applications of commercial formulations of Bacillus thuringiensis Berliner ssp. kurstaki (Btk) [14]. This soil-borne, Gram-positive bacterium produces a proteinaceous crystal that is specifically toxic to Lepidoptera. When ingested by larvae, the crystals cause gut damage, leading to death within a few hours or days due to septicemia [15]. In Québec, operations aim to protect at least 50% of current-year foliage to ensure tree survival. Biological insecticide application begins after one year of moderate to severe defoliation, and the first application is generally synchronized with the bud break of balsam fir, which usually occurs at the peak of the third larval instar [16,17]. A second application may be required if larval densities are high, if staminate flowers are present, or if photosynthetic capacity is expected to fall below 38% [17]. This strategy has generally been effective in maintaining defoliation levels below the critical 50% threshold in balsam fir stands, e.g., ref. [18,19]. However, the literature shows that aerial spraying operations may be less effective in protecting spruce foliage, e.g., ref. [18,20,21,22]. In white spruce, foliar chemistry and shoot phenology have been shown to reduce Btk efficacy, notably due to higher tannin concentrations [18] and delayed bud cap shedding, which may shield larvae during early instars [20,23]. In Norway spruce, the efficacy of insecticide applications (including Btk and chemical sprays) has likewise often been low [21,22]. For example, Btk applications have failed to reduce defoliation of Norway spruce in Europe by the nun moth (Lymantria monacha L.), possibly due to limited spray penetration caused by the dense crown structure and long needles [21]. These findings suggest that both the chemical and structural traits of spruce species may hinder uniform deposition and ingestion of insecticides, thereby reducing treatment efficacy.

Little information is available regarding the extent of defoliation in Norway spruce caused by spruce budworm in Canada. One of the first documented attacks on Norway spruce by the spruce budworm in Québec occurred in Norway spruce and white spruce plantations in the Grand-Mère region [24].

Desaulniers [24] indicated that many spruce trees suffered moderate to severe defoliation, but he did not specify which species was the most greatly affected by the budworm. More recently, it was reported that 83% of the Norway spruce plantations surveyed in 2021 had been defoliated by the spruce budworm, exhibiting lower average defoliation than that observed in white spruce plantations, i.e., 15% and 26% defoliation, respectively [12]. In contrast, surveys conducted to evaluate the efficacy of the protection program against spruce budworm showed that Norway spruce exhibited higher defoliation than white spruce [17]. Despite these contrasting results, Norway spruce and white spruce have been considered as being capable of sustaining similar levels of defoliation caused by spruce budworm [25,26]. Regarding the suitability of Norway spruce foliage for budworm development, a previous study showed that budworm performance on Norway spruce was similar to that measured on white spruce, indicating that these species are comparable in terms of host suitability for spruce budworm [9].

The main goal of this study is to evaluate the extent of defoliation caused by spruce budworm on Norway spruce in order to obtain the necessary information for the development of decision support tools required for spruce budworm management in Norway spruce plantations. We also aim to evaluate the efficacy of aerial spraying of Bacillus thuringiensis spp. kurstaki (hereafter, Btk) in reducing spruce budworm defoliation in Norway spruce plantations to determine the relevance of using this biological insecticide to protect this exotic host tree. This information is needed to fill the existing gap in our understanding of spruce budworm/Norway spruce interactions and may help to renew interest in this high-yielding commercial species.

2. Materials and Methods

2.1. Study Area

The study was located in eastern Québec (Canada), specifically the administrative regions of Bas-Saint-Lawrence, Gaspésie, and Côte-Nord. The study area encompassed two bioclimatic domains: balsam fir/white birch (Betula papyrifera Marsh.) and balsam fir/yellow birch (Betula alleghaniensis Britton) domains. The balsam fir/yellow birch domain is a transition zone between the northern temperate zone and the boreal zone. Mean annual temperature ranges from 1.5 to 2.5 °C, with precipitation between 1250 mm and 1450 mm [27]. The balsam fir/white birch domain is located in the southern part of the boreal forest. Mean annual temperature varies between −1.5 and 1.5 °C, with mean annual precipitation that varies between 1000 and 1300 mm [27].

2.2. Evaluation of Spruce Budworm Defoliation on Host Species

The evaluation of spruce budworm defoliation on host species was conducted in the Bas-Saint-Laurent region, which is located along the southern shore of the lower Saint Lawrence River in the Province of Québec (Figure 1). Sampling was conducted between 2018 and 2022. Sampling sites were selected according to the following criteria: (1) presence of spruce plantations of target species, i.e., ≥80% of Norway spruce, white spruce, or black spruce; (2) plantation age ≥20 years; (3) spruce budworm populations or traces of defoliation had been detected; (4) plantations had not been sprayed with Btk; and (5) site accessibility. In each plantation and each year, a variable number of plots were established using a 2-factor prism, with plots being spaced at least 75 m from one another. This resulted in a total of 327 plots in white spruce plantations, 277 plots in Norway spruce plantations, and 242 plots in black spruce plantations that were surveyed during the study period (

Table 1. Number of plots that were sampled per spruce plantation and year. Only individuals of spruce species (Norway, white, and black spruce) and balsam fir were selected in the plots to compare the vulnerability of Norway spruce to the vulnerability of known spruce budworm hosts.

Plantation	Year
	2018	2019	2020	2021	2022	Total
White spruce	39	34	90	82	82	327
Black spruce	66	26	0	75	75	242
Norway spruce	45	14	72	73	73	277
Total plots per year	150	74	162	230	230	846

). Only individuals of spruce species and balsam fir were selected in the plots to compare the level of damage sustained by Norway spruce relative to known spruce budworm hosts. In each selected plot, 10 host trees were randomly selected, and the tree species were identified. An ocular defoliation estimation method was used to assess the percentage of dead or missing needles. Previous studies have reported that this method provides defoliation estimates that are sufficiently accurate to be used in survey and research applications, e.g., ref. [28]. A 45-cm branch tip from the mid-crown of each selected tree was scanned with binoculars to evaluate current-year defoliation. The following percentage defoliation classes were used: 0, 5, 15, 25, 35, 45, 55, 65, 75, 85, 95, and 100%. Plots were evaluated for at least 2 summers during the study period to evaluate defoliation trends.

2.3. Evaluation of Btk Treatments on Defoliation of Norway and White Spruce Plantation

The evaluation of the efficacy of aerial spraying operations in Norway and white spruce plantations was conducted between 2020 and 2021 in Bas-Saint-Laurent, Gaspésie, and Côte-Nord regions using operational data that were collected by SOPFIM to evaluate the efficacity of the protection program that was deployed in spruce plantations (Figure 2). Sampling plots were selected according to the following criteria: (1) high overwintering second-instar spruce budworm populations (≥20 L2 per 75-cm branch tip, meeting the single-application criteria in spruce species); (2) the spruce plantation was composed mainly of target species (≥80% of either Norway spruce or white spruce); (3) plantation age ≥30 years, with moderate previous defoliation; and (4) easy site access, yielding between 100 to 380 plots per spruce species plantation per year (

Table 2. Number of spruce plantations that were used to evaluate the efficacy of Btk treatments tabulated by spruce species, Btk treatment, and year. The number of Btk applications in each plantation was determined based on L2 density estimates that were obtained from surveys conducted the previous autumn. The treatment thresholds were as follows: no Btk application for <20 L2, a single Btk application for 20–39 L2, and a double Btk application for ≥40 L2 per branch tip.

Year	2020
		Btk treatment
Plantation	Control	Single	Double	Total
Norway spruce	30	48	22	100
White spruce	129	137	98	364
Total	159	185	120	464
Year		2021
		Btk treatment
Plantation	Control	Single	Double	Total
Norway spruce	2	47	68	117
White spruce	5	85	290	380
Total	7	132	358	497

). The number of Btk applications in each plantation was determined based on L2 density estimates obtained from surveys that were conducted the previous autumn. The treatment thresholds were as follows: no Btk application for <20 L2, a single Btk application for 20–39 L2, and a double Btk application for ≥40 L2 per branch tip (Table 2). In this study, branches were cut at mid-crown level from six randomly selected trees per plantation, and overwintering larvae were extracted using the method that was described by Miller et al. [29]. Since L2 densities were used as the criterion for assigning treatments, the number of plantations receiving each treatment may vary from year to year due to fluctuations in population densities (Table 2).

2.4. Btk Formulation Applications

Foray 76B and Bioprotec HP, Btk strain HD-1 commercial formulations at nominal potency of 20.0 billion international units per liter (BIU/L) (Abbott Laboratories, Chicago, IL, USA; on behalf of Valent Bio-Sciences Corporation, Libertyville, IL, USA) were applied to the experimental plots that are described above. The aircraft that were used, C-GMTG Air tractor 502, 602, 802, and Thrush 510P, were equipped with six Micronair atomizers (Micronair Sprayers Ltd., Bromyard, UK). Micronair atomizers, spinning at 8000 rpm, were located within 75% of the total wingspan. These aircraft were flown at an average speed between 227 and 231 km/h, with 80 m spray widths. Aerial treatments were carried out in the early morning or at dusk under good weather conditions (no rain and maximum wind speed of 16 km/h). The flow rate through the nozzles was calibrated to deliver 1.5 L/ha or 30 BIU/ha. The first aerial application of Btk in spruce plantations was timed to coincide with the peak of the fourth larval instar (mid-June), when spruce shoots are expanding, and bud caps are typically detaching from expanding shoots [17]. However, the bud cap may still be attached to the expanding shoot in white spruce during the first aerial application of Btk [18,20]. The second application was carried out five days later [17].

2.5. Btk Treatment Efficacy

A 45 cm branch tip was collected from the midcrown of three Norway spruce and three white spruce trees in each experimental plot 24 h before the first aerial application of Btk. Numbers of spruce budworm larvae and their larval instars were recorded per branch; an insect development index (IDI) was calculated according to the methods of Dorais and Kettela [30]:

$$IDI={\displaystyle \frac{\left[\left(No.L2\times 2\right)+\left(No.L3\times 3\right)+\left(No.L4\times 4\right)+\left(No.L5\times 5\right)+\left(No.L6\times 6\right)+\left(No.P\times 7\right)+\left(No.M\times 8\right)\right]}{Total individuals per 45-cm branch tip}}$$

where No. L2 is the number of second-instar larvae; No. L3 is the number of third-instar larvae; No. L4 is the number of fourth-instar larvae; No. L5 is the number of fifth-instar larvae; No. L6 is the number of sixth-instar larvae; No. P is the number of pupae; and No. M is the number of moths.

Defoliation was estimated using the Fettes method [30,31] by collecting 30 current-year shoots when 85% of individuals had reached the pupal stage. Pupal stage time was estimated using BIOSIM software [32]. Although initial larval population density was determined to be included as co-variable in the analysis of foliage protection efficacy, budworm mortality following Btk treatments was not estimated due to logistical constraints.

2.6. Statistical Analysis

To compare the extent of defoliation caused by spruce budworm on host species, annual defoliation was averaged by plot, host species, and year and then submitted to repeated measures analyses of variance (RMANOVA) in a completely randomized design PROC MIXED, ref. [33]. The model included fixed effects (host species and year) and all possible interactions between these terms and random effects (plot). Plot nested within the sampling site was used as an error term to consider the repeated-measures effect within year (i.e., plot (sampling site)). Given that the data did not meet assumptions of normality and homogeneity of variance, ANOVA tests were performed on log-transformed data.

Insect development index (IDI) and initial larval density (collected 24 h before the first aerial application) were subjected to analyses of variance PROC MIXED; ref. [33] in a completely randomized design. The model included fixed effects (Btk treatment (control vs. single vs. double application)), host species (Norway spruce vs. white spruce), and year (2020 vs. 2021), and all possible interactions between these terms and random effects (region). Given that initial larval density and IDI data did not meet assumptions of normality and homogeneity of variance, ANOVA tests were performed on log-transformed data. To evaluate the efficacy of Btk treatments in Norway spruce plantations, annual defoliation data were subjected to analysis of covariance with the initial larval density as a covariate in a completely randomized design PROC MIXED; ref. [33]. The model included the same fixed and random effects used in the previous models. The LSMEANS statement [33], which was performed for each effect and interaction, computed least-squares means and multiple comparisons (Tukey–Kramer adjustment).

3. Results

3.1. Annual Defoliation on Host Species

Annual defoliation varied according to host species, year, and the interaction between these two parameters (

Table 3. Summary of repeated measures analysis of variance using PROC MIXED showing the effects of host species and year on annual defoliation *. Numbers in bold indicate statistically significant effects at p = 0.05.

Source of Variation	F	df	p
Host species	84.95	38,730	<0.0001
Year	469.56	31,253	<0.0001
Host species × Year	17.36	91,204	<0.0001

* Data were log-transformed.

). In fact, annual defoliation is constant neither from year to year nor among the different host species. In most of the years, balsam fir was the most defoliated species, followed by Norway spruce and white spruce (Figure 3). Black spruce exhibited the lowest defoliation in every year of the study period (Figure 3). Norway spruce exhibited higher defoliation than did white spruce in 2019, 2020, and 2021. In 2019, Norway spruce sustained 25% more defoliation than white spruce and was the most defoliated host species.

3.2. Evaluation of Treatment Efficacy

Initial larval density was statistically different only between host species and Btk treatments (

Table 4. Summary of analysis of variance using PROC MIXED showing the effects of Btk treatment, host species, and year on initial larval density and insect development index *. Numbers in bold indicate statistically significant effects at p = 0.05.

	Initial Larval Density			Insect Development Index (IDI)
Source of Variation	F	df	p	F	df	p
Btk treatment	46.73	2688	<0.0001	0.65	2692	0.52
Host species	5.25	1657	0.0223	0.02	1693	0.87
Host species × Btk treatment	1.90	2691	0.15	2.26	2691	0.105
Year	1.42	1692	0.23	0.01	1692	0.93
Btk treatment × year	0.54	2689	0.58	4.09	2692	0.0172
Host species × year	0.90	1677	0.34	3.21	1693	0.07
Btk treatment × Host × Year	0.26	2677	0.76	5.76	2692	0.0033

* Data were log-transformed.

). White spruce had about 5 more larvae per branch than did Norway spruce prior to the beginning of treatment application. Plantations treated with two applications of Btk had about 15 and 20 more larvae per branch than those plantations that had received a single application and untreated plantations, respectively (Figure 4). A significant interaction between Btk treatments and years and among the three fixed effects that were tested was detected for the insect development index (IDI) (Table 4). In fact, IDI values at the time of the first aerial application of Btk were usually higher than the target IDI (4) in both types of spruce plantations (

Table 5. Insect development index (IDI), which was tabulated by host species, Btk treatment, and year (LSMEANS ± SEM).

Year		Host Species
	Btk Treatment	White Spruce	Norway Spruce
2020	Control	4.59 ± 0.22	5.29 ± 0.38
	Single application	4.51 ± 0.12	4.51 ± 0.13
	Double application	4.52 ± 0.12	4.47 ± 0.15
2021	Control	4.84 ± 0.28	3.29 ± 0.39
	Single application	4.66 ± 0.12	5.06 ± 0.13
	Double application	4.74 ± 0.11	4.77 ± 0.13

). Furthermore, Btk treatments were not particularly effective in reducing budworm defoliation in either spruce species, as no significant effect was detected by the statistical analysis (

Table 6. Summary of analysis of variance using PROC MIXED showing the effects of Btk treatment, host species, and year on annual defoliation *. Numbers in bold indicate statistically significant effects at p = 0.05.

Source of Variation	F	dl	p
Initial larval density	140.47	1691	<0.0001
Btk treatment	0.84	2691	0.4337
Host species	2.05	1685	0.1524
Host species × Btk treatment	2.68	2691	0.0695
Year	8.94	1692	0.0029
Btk treatment × year	2.28	2691	0.1029
Host species × year	3.03	1689	0.082
Btk treatment × Host × Year	2.36	2688	0.0947

* Data were log-transformed.

). In fact, defoliation in spruce trees is similar in all treatments in 2020. In 2021, defoliation in Norway spruce plantations treated with Btk is higher than that observed in unprotected plantations. In contrast, no statistical differences were detected between unprotected and protected white spruce plantations in 2021 (Figure 5).

4. Discussion

Our results show that defoliation caused by spruce budworm in Norway spruce is similar to that in white spruce, but the former may sustain higher defoliation than the latter in certain years. It is also noteworthy that Norway spruce generally suffered less defoliation than did balsam fir, but this host was defoliated more than black spruce. Differences in defoliation among host species may be attributed to species-specific characteristics. For instance, white spruce exhibits more rapid shoot growth, greater development, and higher foliage density per unit area than balsam fir, resulting in reduced defoliation of the former [34]. In contrast, bud break and shoot elongation occur later in black spruce than in balsam fir, e.g., ref. [35], which largely explains the lower susceptibility of black spruce to spruce budworm defoliation compared to balsam fir. As for Norway spruce, although similar factors may contribute to its lower defoliation levels (akin to those observed in white spruce), further research is needed to fully understand the mechanisms underlying this pattern in this exotic species.

These results not only confirm that Norway spruce and white spruce can sustain similar levels of defoliation caused by spruce budworm, as has been observed in the past [24,26], but also that differences in defoliation may be observed in some years [12,17]. All host species exhibited notably low levels of defoliation in 2022, which may be attributed to a significant decline in spruce budworm populations across the province that year. In Bas-Saint-Laurent, L2 densities were particularly low, ranging from 1 to 4 larvae per 45 cm branch tip [17]. This decline led to a substantial reduction in both the total affected area (from 1.6 million ha in 2021 to 0.8 million ha in 2022) and the severity of damage that was observed within those areas, with 96.86% of the affected forest sustaining only light defoliation [11]. These results confirm that Norway spruce plantations are susceptible to spruce budworm attack. Consequently, these plantations may require protection during spruce budworm outbreaks to avoid possible growth losses and eventual tree death. However, to our knowledge, there is little information on the extent of mortality and growth reduction caused by spruce budworm on Norway spruce.

Our results show that Btk treatments exhibited low efficacy in terms of foliage protection in both spruce species that were tested. In white spruce, plantations that were treated with both single and double Btk applications showed a similar level of protection to unprotected plantations in 2020 and 2021. Similarly, Btk treatments failed to significantly reduce defoliation in Norway spruce in both years. These results are consistent with previous studies that have reported more variable efficacy of aerial insecticide applications in protecting spruce foliage in North America [20] and Europe [21,22]. For example, Schönherr [21] reported that sprays of pyrethroid insecticides and Btk failed to reduce nun moth larvae in pure and mixed stands of Norway spruce in Poland, resulting in complete defoliation of most spruce trees. In Denmark, Btk provided adequate protection (76% larval mortality rate) against nun moth in pine stands but failed in spruce stands [22]. Furthermore, a recent study testing the efficacy of three Btk treatments (early application timed to coincide with balsam fir bud break, delayed application six days later, and double application) on white spruce and balsam fir in mixed stands against spruce budworm found a low efficacy of all treatments in terms of foliage protection and larval mortality in the former species [20].

The timing of the first aerial application could be a possible explanation for the low efficacy of Btk treatments that were observed in spruce plantations. In Québec, aerial spray operations for balsam fir protection are usually timed to coincide with the peak of third-instar larvae (IDI around 3.2) [16,17]. However, the timing of the first Btk treatment application in spruce plantations is delayed to ensure that bud caps are detached from expanding shoots [17], thereby minimizing the probability that bud caps protect larvae from acquiring a lethal dose. The potential disadvantage of this late timing (IDI between 4.51 and 4.74 in white spruce and IDI between 4.47 and 5.06 in Norway spruce) is that it may increase the likelihood that larvae cause great foliar damage prior to the first application [23]. This situation may result in low foliage protection and, therefore, failure to achieve the protection target [36]. This response may be particularly true at high larval densities. However, evidence from some studies seems to invalidate this explanation, at least in white spruce. Indeed, aerial applications of Btk may effectively protect white spruce foliage even when the timing of treatment application is delayed [18,19]. In contrast, Fuentealba et al. [20] reported low efficacy of Btk treatments in white spruce even though the first application was carried out earlier than in the aforementioned studies (IDI of 3.7). These results suggest that other factors, such as spruce characteristics, may be more important than the timing of treatment applications in explaining the low efficacy of Btk treatments observed in spruce species in this study.

Indeed, tree crown shape and density, as well as morphological characteristics of target foliage, may play a role in achieving protection goals in aerial forestry operations [37]. It has been hypothesized that the dense crown of Norway spruce may prevent adequate deposition, good penetration, and uniform coverage of insecticide droplets on the foliage [21], thereby reducing the probability that larvae consume a lethal dose of insecticide, e.g., ref. [38]. Previous studies have found that crown density may influence deposition [39,40,41,42].

Norway spruce foliage characteristics such as orientation may also play a role in the low efficacy of Btk treatments. The horizontal orientation of foliage is supposed to increase droplet interception because droplets are more likely to be caught by horizontal needles that are in the path of the spray than by vertical needles [37]. For example, Sundaram [37] found that balsam fir, with foliage exhibiting a horizontal orientation, received greater deposition than white spruce, red pine (Pinus resinosa Ait.), and white pine (Pinus strobus L.). Orientation may have important consequences for the efficacy of aerial spraying operations in Norway spruce plantations because, as this host grows larger, needles may begin to droop downward, thereby reducing the likelihood of interception of Btk droplets. As a result, its foliage may receive low Btk deposition on current-year foliage.

The low efficacy of Btk treatments in spruce species suggests that the current approach to protecting boreal forests against spruce budworm, which was originally developed for balsam fir stands, is not well suited to reducing damage in spruce forests and plantations. Consequently, further research is needed to better understand the factors contributing to this reduced efficacy so that Btk-based protection strategies can be adapted specifically to spruce species. In addition to refining existing methods, alternative treatment options should be explored to enhance the protection of Canadian forests. One such promising strategy involves the use of self-DNA inhibition, where extracellular DNA (ex-DNA) from the target pest is applied to disrupt its development or reproduction, e.g., ref. [43].

Another important approach is the identification and incorporation of resistant genotypes into current tree improvement programs. This strategy can significantly reduce insect damage and may enable the reintroduction of host species into areas where pest pressure has previously prevented their establishment, e.g., ref. [44]. In the case of spruce budworm, some white spruce genotypes have been identified as resistant. These trees produce higher levels of hydroxyacetophenones (specifically piceol and pungenol), which act as effective chemical defenses against the budworm. Importantly, the production of high concentrations of these compounds is heritable [45], opening the door to selective breeding for resistance. This promising strategy has not yet been fully explored or integrated into operational forest management. Developing breeding programs that incorporate this naturally occurring resistance could provide a sustainable, long-term, complementary solution to curbing spruce budworm damage in spruce plantations.

5. Conclusions

In conclusion, our results confirm that Norway spruce can sustain defoliation caused by spruce budworm to an extent similar to white spruce. This has important implications because it means that this host species may require protection during outbreaks of this native defoliator. However, we observed that aerial spraying of Btk formulations exhibited low efficacy in protecting the foliage of this spruce species. Consequently, further studies should be conducted to understand factors that would influence the efficacy of aerially sprayed Btk formulations on this host species. Deposition, penetration, and coverage of Btk, together with the effects of foliage characteristics, should be investigated to determine whether current prescriptions are adequate to protect this species or whether modifications are necessary to improve treatment efficacy. Finally, the efficacy of Btk formulations in terms of larval mortality, together with the efficacy of ground applications of Btk in private plantations, should also be evaluated to provide a complete assessment of treatment efficacy.