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. 2026 Mar 5;26(2):ieag009. doi: 10.1093/jisesa/ieag009

Investigating the host plant range of Contarinia nasturtii (Diptera: Cecidomyiidae), reveals novel hosts and lack of host plant resistance within a Brassica napus diversity panel

Carina L Lopez 1,✉, Juliana J Soroka 2, Sally L Vail 3, Meghan A Vankosky 4, Boyd A Mori 5,✉
Editor: Xiao-Wei Wang
PMCID: PMC12962850  PMID: 41787641

Abstract

Swede midge, Contarinia nasturtii Kieffer (Diptera: Cecidomyiidae), is an invasive pest of canola (Brassica napus Linnaeus, Brassica juncea Linnaeus, Brassica rapa Linnaeus) and other Brassicaceae crops that causes significant damage in eastern North America. Contarinia nasturtii has the potential to invade the Canadian Prairies, which represents North America’s largest canola growing region. This study examined host plant range, female oviposition preference, and larval development of C. nasturtii on selected weed, cultivated, and model Brassicaceae species. We also examined potential host plant resistance using a diverse panel of B. napus lines and developed a novel measure of larval performance using the proportion of third instar larvae as a proxy for larval development. All tested weed species, except Descurainia sophia Linnaeus, supported C. nasturtii development and 5 new host plants (Lepidium densiflorum Schrad., Neslia paniculata (Linnaeus) Desv., Diplotaxis muralis (Linnaeus) DC., Camelina sativa (Linnaeus) Crantz, and Erysimum cheiranthoides Linnaeus) were identified. Notably, we provide the first evidence that Arabidopsis thaliana Linnaeus can be a host for C. nasturtii, establishing a novel model system for gall midge–plant interaction studies. Evaluation of B. napus lines found slight variation in oviposition but no strong resistance, suggesting the need to investigate resistance sources outside of B. napus. These findings expand our knowledge on the host range of C. nasturtii, introduce A. thaliana as a tractable experimental model system, and underscore the need for investigation of host plant resistance and the development of integrated pest management strategies for C. nasturtii to mitigate threats to North American canola production.

Keywords: Arabidopsis, cruciferous weeds, oviposition preference, gall midge, insect–plant interactions

Introduction

Swede midge, Contarinia nasturtii Kieffer (Diptera: Cecidomyiidae), is a gall-forming insect pest of cruciferous (Brassicaceae) plants (Barnes 1946). Larval feeding damages meristematic tissues, resulting in swollen petioles, crumpled leaves, and the formation of galls on buds and flowers (Readshaw 1961, Bardner et al. 1971). Although the exact larval feeding mechanism is unknown, larval salivary secretions are involved in extra-oral digestion and plant manipulation (Hallett and Heal 2001, Mori et al. 2023, 2024). After feeding, larvae drop to the soil to pupate and emerge as adults within 2 wk or enter diapause to overwinter. Females then use olfactory cues to locate and select suitable hosts for oviposition (Boddum et al. 2018). In brassicaceous crops, C. nasturtii can cause significant damage, with yield losses to canola in Ontario, Canada exceeding 50% (Hallett 2017).

Native to Europe, C. nasturtii was first discovered in North America in southern Ontario, Canada, in 2000 (Hallett and Heal 2001). This pest has since established in Québec, Prince Edward Island, and Nova Scotia, in Canada (Hallett and Sears 2013), and parts of the United States, reaching as far west as Minnesota (Tong et al. 2025). Although initially ­suspected as present in Saskatchewan and Manitoba (Canadian Food Inspection Agency 2009), a recent monitoring program confirms that C. nasturtii has not yet been established in these provinces nor other locations in the Canadian Prairies (Vankosky et al. 2023). Climate projections and bioclimatic models indicate that C. nasturtii has the potential to become established in all Canadian provinces (Olfert et al. 2006) and predict that swede midge will spread over the next 60  yr from its current range into the Canadian Prairies, northern Canada, and midwestern United States (Mika et al. 2008). This predicted westward movement toward the Canadian Prairies would approach the largest canola-growing region in Canada, with potential for severe economic impacts.

The host range of C. nasturtii includes many important crop and weed species found on the Canadian Prairies. Crop species on the Prairies most notably include canola (Brassica napus L., Brassica juncea L., and Brassica rapa L.) (Brassicales: Brassicaceae) and vegetable crops such as broccoli, cauliflower, Brussels sprouts, and cabbage (Brassica oleracea L.), as well as other crucifer crops (eg Camelina sativa (L.) Crantz and Sinapis alba L.) (Hallett 2007). Many crucifer weed species are also found on the Prairies and could serve as alternative host plants for C. nasturtii (Chen et al. 2009b). Weed species on the Canadian Prairies that are known hosts for C. nasturtii include Thlaspi arvense L., Sinapis arvensis L., and Camelina microcarpa Andrz. ex DC. (Barnes 1946, Stokes 1953a, 1953b, Hallett 2007), although C. microcarpa has since been reclassified as a non-host (Hallett 2007). Numerous other brassicaceous weed species can be found on the Canadian Prairies (Leeson 2016), including Lepidium densiflorum Schrad., Neslia paniculata (L.) Desv., and Diplotaxis muralis (L.) DC., for all of which the host status is unknown, as well as previously reported non-hosts Descurainia sophia (L.) Webb ex Prantl and Erysimum cheiranthoides L. (Hallett 2007). With a broad range of potential host plants for C. nasturtii on the Prairies, investigation into these plants and potential management strategies is warranted.

Current management of C. nasturtii relies primarily on insecticides (Hallett et al. 2009, Evans and Hallett 2016, Williams and Hallett 2018, Muzzatti 2019), but control can still be difficult (Hallett 2017), and options for organic production are lacking (Hodgdon et al. 2024). Cultural practices such as crop rotation and altered seeding date show promise but require further study and have yet to be fully developed and incorporated into an integrated pest management (IPM) program (Chen et al. 2009a, 2011). Although the parasitoid Synopeas myles Walker has potential as a biological control agent (Abram et al. 2012a, 2012b, Ferland 2020), no biological control programs are currently available, and further research into biocontrol and natural enemies is needed (Corlay Herrera 2006, Hallett 2017). Host plant resistance has also received limited attention, and past screening for host plant resistance in 8 broccoli cultivars did not find any variation for resistance (Jones and Chen 2018). Hallett (2007) tested the relative resistance and susceptibility of various brassicaceous plants to C. nasturtii damage, found varying levels of susceptibility in broccoli cultivars and brassicaceous weed species, and discussed the need for further investigation into resistance, notably with wild crucifer weeds. Chen et al. (2009b) tested the occurrence of C. nasturtii on various brassicaceous weed species and found that the 6 tested crucifer weed species were less suitable for oviposition than cauliflower.

When looking for host plant resistance, it is often beneficial to search within diversity collections and then within crop wild relatives. Diverse breeding populations represent a wealth of phenotypic and genotypic diversity, where genetic mapping can be used to exploit the available genetic data to screen for potential host plant resistance (Yu and Buckler 2006). Wild relatives and species within diversity collections may contain more plant secondary metabolites than commercial cultivars and these metabolites can play a role in host plant resistance via plant defense and insect attraction/repellency. Tissues and seeds of brassicaceous crop cultivars differ in both their glucosinolate and erucic acid content where erucic acid may play a role in seed predation and deter herbivores that feed on cotyledons (de Jong et al. 2016). Many Brassicaceae plants use glucosinolates, a group of secondary metabolites, and breakdown products in their defense responses (Del Carmen Martínez-Ballesta et al. 2013). Recently, a nested-association mapping (NAM) population of B. napus was established from global breeding collections (Perumal et al. 2025) and provides valuable genotypic and phenotypic diversity that warrants further investigation to screen for potential C. nasturtii resistance.

Understanding C. nasturtii oviposition preference, acceptance, and larval development on different Brassicaceae species is crucial to advance pest management strategies. Here, we evaluate C. nasturtii preference and performance across a diverse set of Brassicaceae, including weed species and a diverse B. napus panel, to identify potential alternative hosts on the Canadian Prairies and assess potential sources of host plant resistance.

Materials and Methods

Insects

Contarinia nasturtii used in all experiments were maintained in a colony at Agriculture and Agri-Food Canada (AAFC), Saskatoon, Saskatchewan, Canada. The colony was established in 2016 from insects collected in southern Ontario, Canada (43.6409°, −80.3979°) and reared on B. napus (var. AC Excel) in a walk-in growth room under standard conditions (21 ± 2 °C, ∼70% RH, 16:8 h L:D) (Mori et al. 2023). All experiments were conducted in the colony growth room using midges <24 h old.

Plants

The panel of potential alternative brassicaceous hosts (alternative host plant panel, hereafter) was selected based on ranking in the 2014 to 2015 Saskatchewan Weed Survey (Leeson 2016), access to seed, and prior studies (Barnes 1946, Stokes 1953a, 1953b, Hallett 2007) (Supplementary Table S1). In addition, S. alba (yellow mustard) and C. sativa were chosen as they are grown (S. alba) or are being developed (C. sativa) as crops on the Canadian Prairies (Zanetti et al. 2017). The alternative host plant panel included known (B. napus, S. alba) and unknown (C. sativa) crop hosts, as well as known (T. arvense, S. arvensis, C. microcarpa) and unknown (L. densiflorum, N. paniculata, D. muralis) weedy hosts, and 2 previously reported non-hosts (D. sophia, E. cheiranthoides) given their high weed rank in the Prairie region (50, 93, respectively; Leeson 2016) (Barnes 1946, Stokes 1953a, 1953b, Hallett 2007). Given initial preference experiment results (see below), 4 lines of S. alba (var. AC Pennant, S96-1702, S93-0860, and S97-1946) with varying seed glucosinolate and erucic acid levels were also selected alongside B. napus (Table 1) to test for female preference. Additionally, 3 crop species cultivated on the Canadian Prairies (B. napus, S. alba var. AC Pennant, and B. oleracea var. Botrytis), known as hosts, were selected (Barnes 1946, Stokes 1953a, 1953b). The brassicaceous model plant Arabidopsis thaliana L. (Columbia-0; Col-0) was also selected to determine if it could serve as a potential host, although previously considered a non-host (Boddum et al. 2018). A second plant panel was selected from reference and founder lines from a NAM population of spring-type B. napus (Ebersbach et al. 2021) to screen for potential host plant resistance. Fifteen B. napus lines plus 2 additional canola lines (low glucosinolate and low erucic acid) from the AAFC breeding program (NAM parental panel, hereafter) that encompassed a diverse global array of genotypes and phenotypes were selected (Ebersbach et al. 2021) (Supplementary Table S2). Seeds of crop and weed species were obtained from J. Soroka (AAFC) and the Plant Genome Resources Canada (PGRC) collection, and seeds of the 17 NAM lines were obtained from S. Vail (AAFC) (Supplementary Table S2).

Table 1.

Seed glucosinolate and erucic acid levels in Brassica napus and 4 Sinapis alba lines, as described in Soroka and Grenkow (2013)

Species Glucosinolates Erucic acid
Brassica napus AC Excel Low Low
Sinapis alba AC Pennant High glucosinalbin Mid
Sinapis alba S96-1702 High None
Sinapis alba S93-0860 Low High
Sinapis alba S97-1946 Low Low
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For all plants, 3 seeds per pot (10.16 cm2) were planted in soil-less potting mix (Stringam 1971) and placed in a greenhouse with supplemental lighting (20 °C, 16:8 h L:D). Seedlings were thinned to 1 per pot 2 wk after germination, and plant growth was monitored daily. Planting was staggered to have plants at a similar growth stage (ie bud stage) for oviposition, which included a 6-wk vernalization period for C. microcarpa, D. sophia, and L. densiflorum. Plants were used at the budding stage for weed species and A. thaliana, and at the 4 to 5 leaf stage for the NAM lines. Brassica napus var. AC Excel was used as the control for all experiments.

Alternative Hosts on the Prairies

Female Oviposition Preference

To identify the oviposition preference of C. nasturtii females on potential alternative host plants on the Canadian Prairies, 1 no-choice and 3 choice experiments were performed. In the first choice experiment, female oviposition preference was compared within the alternative host plant panel. One plant of each species (n = 11) was randomly positioned in a circle inside each oviposition cage (n = 7) (BugDorm-4F4545, 47.5 cm3, MegaView Science, Taiwan). In all choice experiments, midges (8 female:4 male per plant) were introduced to each cage and allowed 24 h to mate and oviposit. Plants were then dissected, eggs were washed from growing points, and counted following Stratton et al. (2018). Based on the high numbers of eggs laid on B. napus and S. alba in the first experiment (see Results section), a subsequent choice experiment was conducted on 4 S. alba lines and B. napus (control) to determine if known seed glucosinolate and erucic acid levels affected female oviposition preference (Table 1). One plant of each line (n = 5) (B. napus and 4 S. alba, Table 1) was randomly positioned in a circle in each oviposition cage (n = 9). Finally, a third choice experiment was conducted to determine if C. nasturtii has a preference among brassicaceous crops of the Prairies (B. napus var. AC Excel, S. alba var. AC Pennant, and B. oleracea var. Botrytis). One plant of each crop species (n = 3) was randomly positioned in a triangular arrangement approximately 10 cm apart in each oviposition cage (n = 12).

Female Oviposition Acceptance

To determine female C. nasturtii acceptance in the absence of alternative hosts and to aid in determining host plant range, an additional no-choice experiment was performed with the alternative host plant panel. One plant was placed in each oviposition cage (n = 7 per species) (BugDorm-E2222, 24.5 cm3, MegaView Science, Taiwan), then adult C. nasturtii (8 female:4 male per plant) were introduced and allowed to oviposit for 48 h. Although not included in the choice experiment since C. nasturtii would not come in contact with this species in the wild on the Canadian Prairies, A. thaliana, previously classed as a non-host (Boddum et al. 2018), was also tested (n = 15) to determine if it could serve as a potential host. Plants were then dissected, and eggs were counted following the methods described above.

Larval Performance

To determine larval performance on the alternative host plant panel, a no-choice experiment was conducted. As above, 1 plant was placed in each cage (n = 10 per species) (BugDorm-E2222), then adult C. nasturtii (8 female:4 male per plant) were introduced and allowed to oviposit for 48 h. After 48 h, plants were transferred individually into separate C. nasturtii-free cages (BugDorm-E2222) and allowed to develop for 10 days, after which the total number of larvae on each plant was counted. The number of larvae on each plant was used as a proxy to indicate egg survivorship. Following Abram et al. (2012a), larvae were examined for the presence of a spatula and classified as first or second instar if they did not have a spatula, and classified as third instar if a spatula was present (Mamaev and Krivosheina 2023). The percentage of third instar larvae 10 days after oviposition was used as a proxy for development time, where a lower percentage of third instar larvae 10 days post-oviposition found on a particular plant species compared to the control indicates slower development.

Screening for Host Plant Resistance Variation in a B. napus NAM Parental Panel

To determine female oviposition acceptance and larval performance on the NAM parental panel, 2 no-choice experiments were performed. In the first experiment, female C. nasturtii acceptance was explored within the panel. One plant was placed in each oviposition cage (n = 7 per line) (BugDorm-E2222), then adult C. nasturtii (8 female:4 male per plant) were introduced and allowed to oviposit for 48 h. Plants were then dissected, and eggs were counted following the methods described above. To determine larval performance on the NAM parental panel, a second no-choice experiment was conducted. As above, 1 plant was placed in each cage (n = 6 per line) (BugDorm-E2222), then adult C. nasturtii (8 female:4 male per plant) were introduced and allowed to oviposit for 48 h. After 48 h, plants were transferred individually into separate C. nasturtii-free cages (BugDorm-E2222) and allowed to develop for 10 days, after which the total number of larvae on each plant was counted. As above, the percentage of third instar larvae 10 days after oviposition was used as a proxy for development time.

Statistical Analysis

All statistical analyses were conducted in R, version 4.4.0 (R Core Team 2024). Normality of all data was checked visually prior to model selection. Due to overdispersion in count data (eggs and larvae), models were fit using a negative binomial distribution. The fit of each model was examined using the package DHARMa (Hartig 2024).

To compare oviposition preference (choice experiments), oviposition acceptance (no-choice experiments), and larval performance (no-choice experiments) among the plants included in the alternative host plant and NAM parental panels, a generalized linear mixed effects model with a negative binomial distribution was fit with the package glmmTMB (Brooks et al. 2017). For oviposition acceptance and preference, the number of eggs was specified as the response variable, plant species as a fixed effect, and oviposition cage as a random effect (block). Only plant species with successful oviposition were included in the oviposition analyses. For larval performance, the number of developing larvae was specified as the response variable, plant species as a fixed effect, and oviposition cage as a random effect (block). Only plant species with larvae present were included in the analyses of the total number of larvae, and only species that had a minimum of 3 plants with developing larvae were included in the third instar larval development analyses. Model significance was assessed using a Wald χ2 test implemented with the Anova function in the car package (Fox and Weisberg 2019). When significance was found (P < 0.05) in oviposition choice experiments, Tukey’s honestly significant difference (HSD) test (multcomp package) (Hothorn et al. 2008) was used to compare eggs laid among all plant species. When significance was found in no-choice experiments (eggs and larvae), a Dunnett’s test was used for pairwise comparisons between B. napus (control) and each plant species (alternative host plant panel) or each line (NAM parental panel) individually using the emmeans package (Lenth 2025).

To examine the differences in the percentage of third instar larvae (no-choice experiments) between each plant species (alternative host plant panel) or line (NAM parental panel) and B. napus (control), a non-parametric Wilcoxon rank sum test (wilcox.test function) was performed and the resulting P-values adjusted (p.adjust function) using the Benjamini–Hochberg method (Benjamini and Hochberg 1995).

Results

Alternative Hosts on the Prairies

Female Oviposition Preference

Contarinia nasturtii females showed strong preferences among species when exposed to the complete alternative host plant panel (χ2 = 130.37, df = 8, P < 0.001). Females preferred to oviposit on S. alba over all other species (Fig. 1A). They also had a significant preference for B. napus over all species except D. muralis. No other significant preferences were observed, as few or no eggs (eg E. cheiranthoides, D. sophia) were laid on the remaining species (Fig. 1A). When provided with the 1 B. napus and 4 S. alba varieties, female C. nasturtii preferred to oviposit on B. napus, S. alba var. AC Pennant, and S. alba (S93-0860) compared to S. alba (S96-1702), but not S. alba (S97-1946) (χ2 = 15.287, df = 4, P = 0.004) (Fig. 1B). When exposed to the 3 crop varieties, female C. nasturtii preferred to oviposit on B. napus compared to S. alba and B. oleracea (χ2 = 37.069, df = 2, P < 0.001) (Fig. 1C). No significant difference was found between the number of eggs laid on S. alba and B. oleracea (Fig. 1C).

Fig. 1.

Boxplot with 3 panels (A, B, and C) comparing number of eggs laid across various Brassicaceae species, showing Brassica napus consistently receiving the highest number of eggs compared to most other tested weed species, Sinapis alba lines, and crop species.

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Female oviposition preference bioassays on selected weed and cultivated Brassicaceae species found on the Canadian Prairies. Number of eggs laid on A) selected weed and cultivated Brassicaceae species, B) Brassica napus and selected Sinapis alba lines, and C) 3 crop species cultivated on the Prairies. Circles represent the raw data points. Different letters indicate significant differences among plant species (GLMM followed by Tukey’s HSD, P < 0.05).

Female Oviposition Acceptance

When comparing oviposition acceptance (no choice oviposition bioassay) across the alternative host plant panel and B. napus, female C. nasturtii exhibited preferences among host plant species (χ2 = 38.033, df = 9, P < 0.001) (Fig. 2A). Females did not oviposit on D. sophia. Females oviposited fewer eggs on L. densiflorum (P = 0.0004), E. cheiranthoides (P = 0.0311), C. microcarpa (P = 0.0002), and D. muralis (P = 0.0014) compared to B. napus (Fig. 2A). There were no significant differences in the number of eggs laid on S. alba, C. sativa, T. arvense, S. arvensis, and N. paniculata compared to B. napus (P > 0.05). Eggs were also found on A. thaliana under no-choice conditions (Fig. 2A).

Fig. 2.

Graph with 3 panels (A, B, and C) comparing number of eggs laid, total number of larvae, and mean percentage of third instar larvae across various Brassicaceae species.

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Female oviposition acceptance and larval performance bioassays on selected weed, cultivated, and model Brassicaceae species. A) Number of eggs laid on selected weed and cultivated Brassicaceae species, as well as on Arabidopsis thaliana. B) Number of total larvae counted on each plant species. C) Percentage of total larvae that were in the third instar per plant species, as identified using the larval spatula. Circles represent the raw data points. Different letters indicate significant differences between each plant species (gray) and the control (Brassica napus, light green) (GLMM followed by Dunnett’s test for panels (A) and (B), P < 0.05; Wilcoxon rank sum test for panel (C), P < 0.05 with P-values adjusted using the Benjamini–Hochberg method).

Larval Performance

When comparing the total amount of larvae (no-choice larval development bioassay) across the alternative host plant panel and B. napus, the number of total larvae differed across plant species (χ2 = 44.461, df = 9, P < 0.001) (Fig. 2B;  Supplementary Table S3). Females again did not oviposit on D. sophia, so no larvae were found. Fewer larvae were found on S. arvensis (P = 0.0381), L. densiflorum (P < 0.0001), E. cheiranthoides (P = 0.0053), N. paniculata (P = 0.0272), and C. microcarpa (P = 0.0007) compared to B. napus (Fig. 2B). There were no significant differences in the total number of larvae found on S. alba, C. sativa, T. arvense, and D. muralis compared to B. napus (P > 0.05). When comparing larval development after 10 days (no choice larval development bioassay), the percentage of third instar larvae was significantly lower on L. densiflorum compared to B. napus (W = 99, P = 0.001216) (Fig. 2C). The percentage of third instar larvae found on all other species was not significantly different compared to B. napus (P > 0.05). Although oviposition was quite variable between individual plants (replicate), larvae were found to develop on A. thaliana. The 1 A. thaliana plant with successful oviposition for the larval development portion of the experiment had 16 developing larvae (Fig. 2B and C).

Screening for Host Plant Resistance Variation in a B. napus NAM Parental Panel

Contarinia nasturtii females demonstrated preferences among the NAM parental panel lines and B. napus (χ2 = 46.419, df = 16, P < 0.001) (Fig. 3A). Female C. nasturtii laid fewer eggs on NAM-17 (P = 0.0192), NAM-76 (P = 0.0273), and NAM-37 (P = 0.0165) compared to B. napus (Fig. 3A). There were no significant differences in the number of eggs laid on any other NAM parental panel lines compared to B. napus (P > 0.05). When comparing the total number of larvae (no choice larval development bioassay) on each plant across the NAM parental panel and B. napus, the number of total larvae was significantly different across species (χ2 = 66.617, df = 16, P < 0.001) (Fig. 3B). There were fewer larvae on NAM-13 (P = 0.0019) compared to B. napus (Fig. 3B). There were no significant differences in the total number of larvae found on any other NAM parental panel lines compared to B. napus (P > 0.05). When comparing larvae development after 10 days (no choice larval development bioassay) on each plant across the NAM parental panel and B. napus (Fig. 3C), the percentage of third instar larvae found on all lines was not significantly different compared to B. napus (P > 0.05).

Fig. 3.

Graph with 3 panels (A, B, and C) comparing number of eggs laid, total number of larvae, and mean percentage of third instar larvae across selected Nested Association Mapping parental lines of Brassica napus.

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Female oviposition acceptance and larval performance bioassays on selected Nested Association Mapping (NAM) population parental lines of Brassica napus. A) Number of eggs counted on each line. B) Number of total larvae counted on each line. C) Percentage of total larvae that were in the third instar per plant population line, as identified using the larval spatula. Circles represent the raw data points. Different letters indicate significant differences between each plant line (gray) and the control (B. napus, light green) (GLMM followed by Dunnett’s test for panels (A) and (B), P < 0.05; Wilcoxon rank sum test for panel (C), P < 0.05 with P-values adjusted using the Benjamini–Hochberg method).

Discussion

Female oviposition preference, acceptance, and larval development varied across the alternative host plant panel. We confirmed the host status of B. napus, S. alba, T. arvense, S. arvensis, and C. microcarpa, which were previously identified as C. nasturtii hosts (Barnes 1946, Stokes 1953a, 1953b, Hallett 2007). We also identified 4 new hosts of C. nasturtii, including C. sativa, L. densiflorum, N. paniculata, and D. muralis. In contrast to Hallett (2007), our study identified E. cheiranthoides as a host. The only non-host identified was D. sophia, corresponding to previous reports (Hallett 2007).

Female oviposition preference is context specific, reflecting the adaptability of C. nasturtii when different host plants are available (Chen et al. 2009b, Boddum et al. 2018). When a preferred host is not available, the combination of variable and selective oviposition choice strategies within a population favors a host plant shift, and other available plants may be used as hosts (Wiklund 1981). Previous studies generally found that brassicaceous field and vegetable crops are preferred over weed species (Hallett 2007, Chen et al. 2009b). Consistent with this, C. nasturtii females preferred the crop species S. alba over all other species in the alternative host plant panel, followed by the crop species B. napus over all species except D. muralis. However, when no alternative hosts were available, females laid eggs and larvae developed on all species except D. sophia, which further emphasizes the context-specific nature of oviposition preference. Importantly, the species comprising the alternative host plant panel in our study can all be found throughout the Canadian Prairies (Leeson 2016, Leeson et al. 2017, 2019). Alternative hosts, including weed species, could act as reservoir hosts for C. nasturtii in the absence of preferred crop hosts (Hallett 2017). Such an extensive host range along with the abundance of preferred and alternative host plants in the Canadian Prairies suggests that, if established, managing C. nasturtii in the Canadian Prairies would be challenging.

When the complete alternative host plant panel was used, female C. nasturtii had a significant preference for S. alba over all other species. Given this initial result, female oviposition preference was further tested on 4 lines of S. alba (var. AC Pennant, S96-1702, S93-0860, and S97-1946) with varying seed glucosinolate and erucic acid levels alongside B. napus. This comparative analysis showed that females preferred to oviposit on B. napus, S. alba var. AC Pennant, and S. alba (S93-0860), compared to S. alba (S96-1702), but not S. alba (S97-1946). This lower rate of oviposition on S. alba (S96-1702) is indicative of a lower degree of suitability for C. nasturtii oviposition. Although this line has relatively high seed glucosinolate levels and no erucic acid, it is unclear if these traits, particularly the elevated glucosinolates, are reflected in vegetative tissues. In both B. napus and S. alba, the relationship between seed and tissue glucosinolate levels is inconsistent; previous studies provide conflicting results with regard to the presence or absence of this correlation (Hopkins et al. 1998, Li et al. 1999, Liu et al. 2020). There has been substantial work on glucosinolates as defense compounds and the role they play for different insects (Hopkins et al. 2009), but the resistance mechanisms of glucosinolates and erucic acid together as defensive compounds against C. nasturtii are yet to be determined. Further research into the resistance potential and mechanisms in S. alba should therefore be explored.

When only B. napus, S. alba var. AC Pennant, and B. oleracea var. Botrytis, 3 crop species cultivated on the Canadian Prairies and known hosts (Barnes 1946, Stokes 1953a, 1953b), were used in a subsequent female oviposition preference bioassay, B. napus was preferred over both other species. Previously, Izzo et al. (2025) found that a B. napus variety of kale was more susceptible to C. nasturtii infestation than 3 B. oleracea kale varieties, which further supports that C. nasturtii prefers B. napus hosts when available. Contarinia nasturtii’s preference for B. napus in this experiment is likely due in part to various factors such as odor profiles, nutritional value, and chemical composition of the tested crop species, which can elicit varying responses in plant–insect interactions (Ahuja et al. 2011). Furthermore, oviposition preference for B. napus could in part be attributed to previous feeding experience. Hopkins’ Host Selection principle hypothesizes that adult insects demonstrate a preference for the same host species on which they developed as larvae (Barron 2001), although this was not the case in all other preference experiments. Altogether, our findings indicate that C. nasturtii have a preference for host plants, but when preferred hosts are unavailable, many of the common brassicaceous weed species found on the Canadian Prairies can be utilized as suitable alternative hosts.

Contarinia nasturtii larvae are restricted to developing on the host plant where their egg was deposited. In accordance with optimal oviposition theory (Jaenike 1978), C. nasturtii populations should increase when female preference for host plants results in improved larval performance. To evaluate the impact of host plant on larval performance, we developed a novel measure of larval performance using the proportion of third instar larvae 10 days after oviposition as a proxy for larval development. Results showed fewer third instar larvae 10 days post-oviposition on L. densiflorum compared to B. napus, suggesting that larvae developed slower on L. densiflorum on account of it being a less suitable host for C. nasturtii. A low number of eggs were laid on C. microcarpa in the first female oviposition acceptance bioassay, and only 1 replicate in the subsequent bioassay had developing larvae, which suggests it is also a less suitable host for C. nasturtii development. The absence of oviposition on D. sophia in all experiments indicates this species may not be a suitable host for C. nasturtii, or that the plant stage evaluated (ie bud stage) was not suitable for oviposition and larval development. Anecdotal observations of D. sophia indicate that it is not a host plant at any growth stage, suggesting that the mechanism of host plant resistance appears to be antixenosis (non-preference). Additionally, the morphology of D. sophia is dissimilar to that of the other brassicaceous species tested, which cannot yet be ruled out as a factor contributing to its resistance.

This study is the first to report successful oviposition of C. nasturtii on A. thaliana. Previously, Boddum et al. (2018) found that gravid C. nasturtii females were attracted to A. thaliana ecotype Col-0, but females did not oviposit on 8-wk-old plants at the rosette stage. They also found that females were equally attracted to volatiles of A. thaliana Col-0 and cauliflower (B. oleracea), which suggests A. thaliana could serve as a host. The contrast in oviposition success between this study (bud stage) and Boddum et al. (2018) (rosette stage) is likely due to differences in A. thaliana growth stage susceptibility at the time of exposure to C. nasturtii. The current study concludes that A. thaliana is a host plant for C. nasturtii, which has significant potential to advance our understanding of insect gall formation in a biologically relevant and tractable system. Results of our experiments showed low numbers of C. nasturtii larvae developing on A. thaliana, and further research is needed to circumvent this limitation and optimize the infestation protocol for A. thaliana. Ongoing experiments have successfully infested A. thaliana plants and have also shown C. nasturtii can use A. thaliana as a host plant for multiple generations (unpublished). The vast resources of A. thaliana (Rhee et al. 2003) and the growing genetic resources of C. nasturtii (Mori et al. 2021, 2023, 2024) may allow this new model system to broaden our understanding of gall induction and insect–plant interactions, creating an opportunity for invaluable multifaceted and multidisciplinary research.

Although a NAM population could aid in linking phenotypes in preferred host plants to respective genotypes, C. nasturtii oviposition on the NAM parental panel showed little variation in egg-laying, and larval development did not differ between lines. These results conclude that all tested NAM lines are equally suitable hosts for C. nasturtii. Hervé (2018) emphasized the greater potential of insect resistance in relatives outside of B. napus, particularly S. alba. Sinapis alba has shown some resistance to a variety of insects, including flea beetles (Phyllotreta spp.) (Bodnaryk 1991, Bodnaryk and Lamb 1991, Hopkins et al. 1998, Soroka and Grenkow 2013), cabbage seedpod weevil (Ceutorhynchus obstrictus Marsham) (Kalischuk and Dosdall 2004, Cárcamo et al. 2007, Tansey et al. 2010), and root maggots (Delia spp.) (Dosdall et al. 1994, 2000), through both the mechanisms of antixenosis and antibiosis. In contrast, studies consistently report no insect resistance in B. napus (Dosdall and Kott 2006, Cárcamo et al. 2007, Tansey et al. 2010), which may be attributed to the low genetic diversity present in this plant species (Hasan et al. 2006, Hervé and Cortesero 2016, Hervé 2018). Similarly, our findings did not detect resistance to C. nasturtii with the NAM parental panel, which suggests the need to look at relatives outside of B. napus and to search more broadly for host plant resistance.

Conclusions

Contarinia nasturtii poses a threat to canola production in North America, warranting the exploration of alternative hosts available on the Canadian Prairies and development of management strategies for this invasive species. This research confirms that numerous alternative hosts are available for C. nasturtii and suggests that populations could persist on the Prairies, emphasizing the critical need to prevent population establishment and consider managing alternative hosts as part of an IPM strategy. Notably, this is the first study to identify A. thaliana as a host plant for C. nasturtii, opening a vast area for future research on this novel gall model system.

Supplementary Material

ieag009_Supplementary_Data
ieag009_supplementary_data.xlsx (16.1KB, xlsx)

Acknowledgements

We thank Shane Hladun, Jennifer Holowachuk, Larry Grenkow, and Jonathon Williams for technical expertise.

Contributor Information

Carina L Lopez, Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, AB, Canada.

Juliana J Soroka, Saskatoon Research and Development Centre, Agriculture and Agri-Food Canada, Saskatoon, SK, Canada.

Sally L Vail, Saskatoon Research and Development Centre, Agriculture and Agri-Food Canada, Saskatoon, SK, Canada.

Meghan A Vankosky, Saskatoon Research and Development Centre, Agriculture and Agri-Food Canada, Saskatoon, SK, Canada.

Boyd A Mori, Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, AB, Canada.

Author Contributions

Carina L. Lopez (Formal analysis [equal], Validation [equal], Visualization [equal], Writing—original draft [lead], Writing—review & editing [equal]), Juliana J. Soroka (Conceptualization [equal], Funding acquisition [equal], Methodology [equal], Project administration [equal], Resources [equal], Supervision [supporting], Writing—review & editing [supporting]), Sally L. Vail (Conceptualization [supporting], Funding acquisition [supporting], Methodology [supporting], Resources [supporting], Writing—review & editing [supporting]), Meghan A. Vankosky (Conceptualization [equal], Funding acquisition [equal], Methodology [supporting], Project administration [supporting], Writing—review & editing [supporting]), Boyd A. Mori (Conceptualization [equal], Formal analysis [equal], Funding acquisition [equal], Methodology [equal], Project administration [equal], Resources [equal], Supervision [lead], Visualization [equal], Writing—review & editing [equal])

Supplementary Material

Supplementary material is available at Journal of Insect Science online.

Funding

Financial support was provided by Growing Forward 2, a partnership between the Government of Canada (Agriculture and Agri-Food Canada) and the Government of Saskatchewan (Agricultural Development Fund, Grant 20140159). Further funding was provided by an NSERC Industrial Research Chair (545088) and partner organizations (Alberta Grains, Alberta Canola Producers Commission, Alberta Pulse Growers Commission), and an NSERC Discovery Grant (2021-02479), and the University of Alberta.

Conflicts of Interest

None declared.

Data Availability

Data will be made available on request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ieag009_Supplementary_Data
ieag009_supplementary_data.xlsx (16.1KB, xlsx)

Data Availability Statement

Data will be made available on request.

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