Introduction
Globally, wheat, Triticum aestivum Linnaeus (Poaceae), is a staple crop that supplies approximately 20% of human daily protein (Shiferaw et al. Reference Shiferaw, Smale, Braun, Duveiller, Reynolds and Muricho2013). Canada produces more than 30 million metric tonnes of wheat annually, with over 90% of production in the prairie provinces of Alberta, Saskatchewan, and Manitoba (Food and Agriculture Organisation of the United Nations 2023; Statistics Canada 2024; United States Department of Agriculture, Foreign Agricultural Service 2025). Worldwide, up to 40% of all crop production is lost due to insects, diseases, weeds, and other crop pests (Food and Agriculture Organisation of the United Nations 2022). On the Canadian Prairies, inadequate pest management can lead to significant economic losses in wheat production. Implementing effective integrated pest management tactics is therefore essential to reduce insect-mediated yield losses.
The wheat midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), is a significant pest of wheat in Canada, including common wheat, T. aestivum, durum wheat, T. durum Desfontaines, and spelt wheat, T. spelta Linnaeus (reviewed in Dufton et al. Reference Dufton, Laird, Floate and Otani2021). Although capable of feeding on other Poaceae crops, including rye, Secale cereale Linnaeus, barley, Hordeum vulgare Linnaeus, and oats, Avena sativa Linnaeus (Wright and Doane Reference Wright and Doane1987; Wise et al. Reference Wise, Lamb and Smith2001), the insect causes economic damage only in wheat. Larval feeding results in shrivelled, cracked, and deformed kernels, ultimately reducing yield and grain quality (Olfert et al. Reference Olfert, Mukerji and Doane1985; Lamb et al. Reference Lamb, Tucker, Wise and Smith2000). The Canadian Grain Commission sets a strict tolerance of less than 2% S. mosellana and wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), damage for No. 1 grade Canada western red spring wheat (Canadian Grain Commission 2022). Careful monitoring of wheat crops for S. mosellana presence and abundance is critical to maintain grain quality and to minimise economic losses.
Adult S. mosellana are small (< 3 mm) and characterised by their bright orange colouration (Olfert et al. Reference Olfert, Mukerji and Doane1985). They are univoltine in Canada, overwintering as cocooned second-instar larvae in obligatory diapause (Reeher Reference Reeher1945; Doane and Olfert Reference Doane and Olfert2008). Diapause termination and pupation require adequate precipitation (> 20 mm) in May and June (Elliott et al. Reference Elliott, Mann and Olfert2009). Adults emerge over five weeks in late June or early July (Reeher Reference Reeher1945; Elliott et al. Reference Elliott, Mann and Olfert2009). After mating near emergence sites (typically in previous-year wheat crops), females disperse to current-year wheat crops and oviposit on developing wheat heads (Smith et al. Reference Smith, Wise and Lamb2004). Oviposition typically coincides with the end of head emergence through anthesis (BBCH scale growth stage 61–69), when wheat is most susceptible to S. mosellana damage (Ding and Lamb Reference Ding and Lamb1999; Meier Reference Meier and Meier2018). Female S. mosellana are challenging to detect during the day because they remain in humid areas, typically in the crop canopy or close to the ground. Most egg-laying occurs at dusk when temperatures are cooler, but still warmer than 10–11 °C, and winds are calm (Pivnick and Labbé Reference Pivnick and Labbé1993). Larvae emerge within 4–7 days, feed on developing kernels for 2–3 weeks, then drop to the soil surface and spin a larval cocoon (Doane and Olfert Reference Doane and Olfert2008). Due to their ability to survive prolonged diapause (i.e., up to 11 years), significant, simultaneous emergence can occur when conditions become optimal – that is, when soil temperatures are above 13 °C and more than 20 mm of precipitation falls in May or early June – causing substantial economic losses (Wise and Lamb Reference Wise and Lamb2004; Knodel and Ganehiarachchi Reference Knodel and Ganehiarachchi2008; Elliott et al. Reference Elliott, Mann and Olfert2009). A major S. mosellana outbreak occurred in 1983 in northeastern Saskatchewan, which resulted in a loss of approximately $Cdn 30 million (not adjusted for inflation) in yield (Olfert et al. Reference Olfert, Mukerji and Doane1985). Similar outbreaks have occurred globally, highlighting the persistent challenge of managing S. mosellana despite considerable research worldwide (reviewed in Dufton et al. Reference Dufton, Laird, Floate and Otani2021).
A comprehensive S. mosellana integrated pest management programme has been developed in Canada that aims to reduce reliance on insecticides while improving sustainability and economic viability. This programme incorporates regional survey maps, midge-tolerant wheat varieties, scouting techniques with economic thresholds, biological control agents, and chemical insecticides (reviewed in Dufton et al. Reference Dufton, Laird, Floate and Otani2021 and Senevirathna et al. Reference Senevirathna, Guelly and Mori2023). Although the use of midge-tolerant wheat varieties is increasing – they comprised 28% and 31% of insured wheat grown in Canada in 2023 and 2024 – insecticides are often the primary means of management (Canadian Grain Commission 2024). The annual variation in S. mosellana infestations further complicates decisions on using midge-tolerant wheat varieties and insecticide applications. Currently, producers rely on time-consuming in-field counts of ovipositing females to determine if insecticide applications are necessary. These counts can be challenging because adult midges are small, short-lived, and often hidden in the crop canopy, leading to poor monitoring. The economic thresholds in hard red spring wheat in Canada are one adult S. mosellana per 4–5 heads of wheat to prevent yield loss and one adult per 8–10 heads of wheat to prevent quality loss (Alberta Agriculture and Irrigation 2024). In-field counts are central to management decisions, but the practical challenges – for example, scouting at dusk and the small size of the midges – associated with their implementation present a significant issue in western Canada (Alberta Agriculture and Irrigation 2024).
Due to the difficulty of in-field counts, alternative monitoring techniques such as pheromone traps have been developed. Green delta traps, baited with lures containing the synthetic female sex pheromone (2S,7S)-2,7-nonanediyl dibutyrate (1-mg racemic mixture), are commonly used across North America to monitor S. mosellana populations (Gries et al. Reference Gries, Gries, Khaskin, King, Olfert and Kaminski2000; Jorgensen et al. Reference Jorgensen, Otani and Evenden2020; Dufton et al. Reference Dufton, Laird, Floate and Otani2021). Although a positive correlation between males captured in pheromone traps and larval infestations was observed in the United Kingdom and resulted in a decision support tool (Bruce et al. Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007), developing a similar tool in Canada has been difficult. Studies in Canada have yielded conflicting results in which either there is no relationship between males captured in pheromone-baited traps and larval populations (Jorgensen et al. Reference Jorgensen, Otani and Evenden2020) or there is a positive correlation between males captured and seed damage at harvest (Mircioiu Reference Mircioiu2004). Consequently, in western Canada, pheromone traps are used only to identify the activity period of S. mosellana.
Nonpheromone monitoring techniques for S. mosellana include soil core surveys conducted in the autumn to determine the number of overwintering cocoons (Doane et al. Reference Doane, Olfert and Mukerji1987; Doane and Olfert Reference Doane and Olfert2008). Based on annual autumn soil core surveys, Alberta and Saskatchewan produce survey maps to help identify the areas most vulnerable to S. mosellana infestations in the coming growing season (Saskatchewan Agriculture 2024; Alberta Agriculture and Irrigation 2025). Jorgensen et al. (Reference Jorgensen, Otani and Evenden2020) found a correlation between cocoon numbers in soil core samples and the number of adults captured in pheromone traps the following spring, suggesting that soil sampling may indicate regional population levels of S. mosellana. However, soil core sampling of S. mosellana cocoons did not reliably predict infestation in Manitoba (Lamb et al. Reference Lamb, Wise, Olfert, Gavloski and Barker1999) or the United Kingdom (Oakley et al. Reference Oakley, Cumbleton, Corbett, Saunderst, Green, Young and Rodgers1998).
In Canada, the effectiveness of S. mosellana pheromone traps to predict infestation levels and guide management decisions remains uncertain. Conflicting studies highlight the need for further investigation of pheromone trapping as an indicator of economically damaging populations. The present study aimed to determine if pheromone traps can be used to predict when in-field counts might occur or to determine if midge populations are at economic thresholds. Building on previous research that examined trapping efficiency for S. mosellana monitoring, different trap and pheromone lure types were compared. Specifically, the present study compared redesigned pheromone lures to standard lures as attractants in white Jackson traps and the commonly used green delta traps in east–central Alberta wheat crops. It also evaluated redesigned pheromone lures to the current standard lure. Finally, the predictive capacity of pheromone traps is compared to midge population estimates obtained from soil cores, emergence traps, and in-field counts.
Materials and methods
Site locations
Experiments were conducted in 2021–2024 in east–central Alberta near Wainwright, in Alberta, Canada’s Central Parkland natural region (Fig. 1; coordinates listed in Supplementary material, Table S1). All sites are in Alberta’s Black Chernozemic soil zone. Mean annual precipitation in this region is approximately 380 mm, of which approximately 75% occurs during the growing season (Alberta Agriculture and Irrigation, no date). Field site locations were selected based on the annual Alberta S. mosellana survey maps, which indicate areas of concern in the province (Alberta Agriculture and Irrigation 2025). Experiments in all study years were conducted in fields seeded to commercial wheat cultivars (Supplementary material, Table S1).
Figure 1. Location of field sites in each sampling year (2021, 2022, 2023, and 2024) in east–central Alberta, Canada. Map generated using ArcGIS Pro, version 3.4 (Esri 2024).
Trap- and lure-type experiments
Trap type
Green delta traps, commonly used for S. mosellana monitoring on the prairies, are less convenient for set-up and sticky card replacement than Jackson traps are. Therefore, an experiment was conducted to determine whether trap type (i.e., green delta traps versus white Jackson traps) influences the number of male S. mosellana captured. Traps were deployed at 10 field sites (Supplementary material, Table S1) when adult S. mosellana were active and wheat was susceptible in 2021 (24 June to 15 July) and 2023 (8 June to 27 July). Activity was determined based on the midges’ known emergence in late June to early July in western Canada (Elliott Reference Elliott1988; Lamb et al. Reference Lamb, Wise, Olfert, Gavloski and Barker1999; Doane and Olfert Reference Doane and Olfert2008). At each site, one white Jackson trap (Solida, Saint-Ferréol-les-Neiges, Quebec, Canada; 112.5-cm2 sticky surface) and one green delta trap (Solida; 340-cm2 sticky surface) were placed 25 m apart and at the field margin (Supplementary material, Fig. S1A). The 25-m distance between traps was selected to reduce the potential for intertrap interference and is either beyond (e.g., Bruce et al. Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007; Chavalle et al. Reference Chavalle, Censier, Martin, Gomez and De Proft2019) or equal to (e.g., Bray et al. Reference Bray, Hall, Harte, Farman, Vankosky and Mori2022) the distance used in similar studies on cecidomyids. Traps were placed in open-field areas, away from physical obstructions, such as trees or buildings, to reduce the potential impact of local landscape features. The trap height was adjusted each week to match the height of the crop canopy (Mircioiu Reference Mircioiu2004; Shrestha and Reddy Reference Shrestha, Reddy, Eigenbrode and Rashed2023). All traps were baited with 1-mg (2S,7S)-2,7-nonanediyl dibutyrate lures (Trécé; Great Lakes IPM, Vestaburg, Michigan, United States of America). Traps were collected and replaced weekly, weather permitting. Sticky surfaces (i.e., sticky liner from Jackson trap, the complete inner surface of green delta trap) were examined under a dissecting microscope (Olympus SZ61; Olympus, Tokyo, Japan) at 10× magnification. Sitodiplosis mosellana males were identified and counted.
Lure type
Previous studies found differences in the number of S. mosellana captured in traps baited with different lure types (Jorgensen et al. Reference Jorgensen, Otani and Evenden2020). Therefore, an experiment was conducted to compare the commercially available rubber septa lure (Trécé; red septa), two experimental polyethylene plastic flex lures (ChemTica; Heredia, Santo Domingo, Costa Rica), and two experimental rubber septa lures (ChemTica; grey septa) for their ability to capture S. mosellana males in 2022 and 2023. Because the flex lures (Scotts; Delta, British Columbia, Canada) used by Jorgensen et al. (Reference Jorgensen, Otani and Evenden2020) were unavailable, comparable ChemTica lures were used instead. In both years, at each site, white Jackson traps were baited with one of the following five lure treatments: polyethylene plastic flex lures from ChemTica loaded with either 1 mg or 100 μg of pheromone, rubber septa lures from Trécé loaded with 1 mg of pheromone, or ChemTica rubber septa lures loaded with either 1 mg or 100 μg of pheromone. Each lure type and pheromone loading amount were considered a separate treatment. A no-lure control trap was also included in 2023. All lures contained a racemic mixture of the S. mosellana female-produced sex pheromone, (2S,7S)-2,7-nonanediyl dibutyrate (Gries et al. Reference Gries, Gries, Khaskin, King, Olfert and Kaminski2000). Lure types were arranged in a randomised complete block design, with each commercial field site treated as a block. Traps were positioned 25 m apart at the field margin, with trap height adjusted weekly to the crop canopy (Supplementary material, Fig. S1A; Mircioiu Reference Mircioiu2004; Shrestha and Reddy Reference Shrestha, Reddy, Eigenbrode and Rashed2023). Traps were placed in unobstructed, open areas to minimise potential influence from surrounding landscape features. Sticky liners were collected and replaced weekly, weather permitting, and examined under a dissecting microscope (Olympus SZ61) at 10× magnification. Sitodiplosis mosellana males were identified and counted.
Residual S. mosellana pheromone on lures
To compare the variability in pheromone released by each lure type (see Lure type section), the amount of active ingredient remaining (i.e., residual pheromone) on pheromone lures was measured over six weeks. Fresh lures (n = 3 for Trécé rubber septa lure (1 mg), ChemTica rubber septa lure (1 mg), and ChemTica flex lure (1 mg)) were aged in a laboratory fume hood (∼22 °C). Unfortunately, insufficient ChemTica 100-µg rubber septa and flex lures were available for analysis, so only the 1-mg lures were tested further. Residual pheromone on lures was extracted from high-performance liquid chromatography–grade hexane (Fisher Scientific, Ottawa, Ontario, Canada) at three-week intervals. For extraction, individual lures were sonicated in 10-mL hexanes in 33-mL glass vials for 30 minutes. Following sonification, lures were removed from the solvent, and the vials were stored at –20 °C until further analysis.
Residual pheromone was quantified on a gas chromatography–flame ionising detector system using the internal standard method (Gries et al. Reference Gries, Gries, Khaskin, King, Olfert and Kaminski2000; Zada et al. Reference Zada, Soroker, Harel, Nakache and Dunkelblum2002; Vacas et al. Reference Vacas, Primo and Navarro-Llopis2017). Individual 2-mL vials were prepared by adding 100 µL aliquots of each extract with up to 750 µL of hexane. An internal standard (25 µL of 3-octyl butyrate (Sigma Aldrich, Oakville, Ontario, Canada) at 4960 ng/µL) was added to each vial ([final internal standard] = 160 ng/µL). Samples (1 µL each) were injected into an Agilent 7890B gas chromatography–flame ionising detector (Agilent Technologies, Santa Clara, California, United States of America) using a PAL Autosampler (Agilent Technologies) at 250 °C with a 1:10 split ratio. Helium was used as the carrier gas at a 1.2-mL/minute flow rate through a VF-5ms capillary column (Agilent Technologies). The oven was initially set to 50 °C and held for 1 minute; then, the temperature was increased to 100 °C at a rate of 25 °C/minute for 5 minutes and held for 2 minutes and finally increased to 280 °C at a rate of 10 °C/minute. A calibration curve was built using an authentic standard of the S. mosellana pheromone (ChemTica Internacional, Heredia, Costa Rica) at concentrations of 500, 250, 125, 62.5, 31.25, 15.6, and 7.8 ng/μL (all containing 25 μL of 3-octyl butyrate as internal standard) to determine the concentration of the residual pheromone remaining in the lures after ageing (Supplementary material, Fig. S2).
Chemical identification and retention times of the pheromone and internal standard were confirmed on an Agilent 5977B gas chromatography/mass selective detector (Agilent Technologies) using the methods and conditions described above.
Seasonal trapping experiments
Pheromone trapping
To determine if the number of male S. mosellana captured in pheromone-baited traps was related to the numbers of overwintering cocoons, of emerging males and females, and of ovipositing females, two Jackson traps baited with 1-mg Trécé rubber septa lures per site were deployed in 2023 (n = 10) and 2024 (n = 7). Traps were placed 25 m apart at the field margin, with trap height adjusted weekly to crop canopy height (Mircioiu Reference Mircioiu2004; Shrestha and Reddy Reference Shrestha, Reddy, Eigenbrode and Rashed2023). All traps were placed in open areas, away from obstructions to minimise potential landscape effects. Trap sticky liners were collected and replaced weekly, weather permitting, during the adult activity period and examined under a dissecting microscope (Olympus S761) at 10× magnification. Sitodiplosis mosellana males were identified and counted.
Soil core samples
To determine if overwintering S. mosellana larval cocoons predict the number of S. mosellana males captured in season-long pheromone traps the following year, soil core samples were collected the previous autumn. Samples were collected postharvest at each pheromone trapping site (Supplementary material, Table S1; n = 10 in 2022, n = 9 in 2023). Seventeen soil cores (1.9 cm diameter, 10–15.5 cm deep) were collected per site in a horseshoe pattern, spaced every 10 m (3–4 cores per sampling location; Supplementary material, Fig. S1B) following the Prairie Pest Monitoring Network protocol (Wist and Vankosky Reference Wist and Vankosky2019). Soil from all cores was collected in a bucket and stored at 4 °C in plastic bags until samples were wet sieved. Samples were washed through three sizes of sieves (0.63-cm, 0.16-cm, and 0.05-cm mesh; Doane et al. Reference Doane, Olfert and Mukerji1987, Reference Doane, Mukerji and Olfert2000), and the remaining material was examined for S. mosellana cocoons and larvae under a dissecting microscope (Olympus S761) at 10× magnification.
Emergence traps
Emergence traps were used in 2023 and 2024 to determine if emerging S. mosellana adult populations were related to the number of overwintering cocoons (from soil core samples) and the number of males captured in pheromone traps. Emergence monitoring traps were installed in the field on 8 June and monitored until 27 July in 2023 (Supplementary material, Table S1; n = 10 previous-year wheat fields, n = 10 current-year wheat fields) and installed in the field on 19 June and monitored until 30 July in 2024 (Supplementary material, Table S1; n = 9 previous-year wheat fields, n = 7 current-year wheat fields). Emergence traps were constructed from white two-gallon (7.5-L) pails (ULINE, Edmonton, Alberta, Canada) with the bottoms cut-off. Ventilation holes (6.35 cm in diameter) lined with fine mesh were drilled into each side of the pail (×3) and in the lid (×1). A yellow sticky card (21 cm × 10 cm) was secured to a metal rod with a binder clip under the lid to capture emerging S. mosellana. Five emergence traps per field were placed 10 m apart between the first two rows of stubble (∼15 cm from the field edge), with the bottom 8.5 cm of each pail buried underground (Supplementary material, Fig. S1C). Emergence traps were placed in open areas of the fields, away from obstructions to minimise landscape effects. Sticky cards were replaced weekly, weather permitting, during the adult activity period and examined under a dissecting microscope (Olympus S761) at 10× magnification. Sitodiplosis mosellana adults were identified and counted.
In-field counts
In-field counts were completed at dusk to estimate the numbers of ovipositing females, which was compared to the number of male S. mosellana captured the previous week in pheromone-baited traps positioned at the same sites. Due to time and travel distance constraints, a subset of all field sites was randomly selected for inspection each year. Counts were completed on 5 July 2023 (n = 4) and 11 July 2024 (n = 7) between 20:30 and 23:00 hours, local time (Elliott and Mann Reference Elliott and Mann1996). At five randomly selected locations in each field, ovipositing females were counted on 4–5 wheat heads (Elliott et al. Reference Elliott, Olfert and Hartley2011).
Precipitation
To determine the effect of precipitation on S. mosellana males captured in pheromone-baited traps, the number of males was compared with precipitation data from surrounding weather stations. Daily precipitation data from four weather stations surrounding the sampling site locations (Holden AGDM station, Kinsella Research station, Viking AGCM station, and Wainwright Canadian Forces Base Airfield 21) were retrieved from the Alberta Climate Information Service (Alberta Agriculture and Irrigation, no date). Daily precipitation data were averaged across weather stations and dates. The wheat developmental stage (BBCH scale) was also recorded weekly at each field (Meier Reference Meier and Meier2018).
Statistical analyses
All statistical analyses were conducted using R statistical software, version 4.4.1 (R Core Team 2024), in RStudio, version 4.3.1 (Posit Team 2024). Assumptions of normality and homoscedasticity were analysed visually using quantile–quantile plots and residual plots, and with Shapiro–Wilk and Levene’s tests (Fox and Weisberg Reference Fox and Weisberg2019). Histograms and quantile–quantile plots were used to visualise the data distribution against a normal distribution. Where applicable, linear mixed-effect models were fit with the lme4 package (Bates et al. Reference Bates, Mächler, Bolker and Walker2015). Generalised linear mixed-effects models with negative binomial distributions were chosen when the data had a nonnormal error distribution. Models were selected based on the lowest Akaike information criterion values. For the trap- and lure-type experiments, trap type and lure type were treated as fixed effects, respectively. Field site and sampling week were treated as random effects in each model. For comparisons with midges captured in emergence cages, sampling week was treated as a fixed effect, and field site was treated as a random effect. Year was treated as a random effect for comparisons involving overwintering cocoons and ovipositing females. A generalised linear model with a gamma distribution and log-link functions was used to compare the amount of pheromone extracted from each lure type, with lure type and week treated as fixed effects. To assess the significance of the models, likelihood ratio chi-square tests were conducted on the fitted generalised linear mixed-effects models using the car package (Fox and Weisberg Reference Fox and Weisberg2019). Nonsignificant interaction terms were removed from all models, and pairwise post hoc comparisons of the estimated marginal means were performed using the multcomp and emmeans packages (Hothorn et al. Reference Hothorn, Bretz and Westfall2008; Lenth Reference Lenth2023). Data visualisation was completed using ggplot2 and ggpubr (Wickham Reference Wickham2016; Kassambara Reference Kassambara2023).
Results
Trap- and lure-type experiments
Trap type
In 2021, trap type did not affect the number of S. mosellana males captured (χ2 = 1.2991, df = 1, P = 0.2544). In 2021, the white Jackson traps captured only marginally more males than green delta traps did (Fig. 2A). However, in 2023, white Jackson traps captured significantly more S. mosellana than green delta traps did (χ2 = 89.196, df = 1, P < 0.001; Fig. 2B). Male capture was more variable in the white Jackson traps than in the green delta traps in both years (Fig. 2). The number of males captured in the white Jackson and the green delta traps was also standardised based on the sticky surface area of the traps and compared statistically; however, the standardised capture numbers did not differ from the raw counts in either 2021 or 2023.
Figure 2. Season total number of male S. mosellana in two trap types (delta and Jackson) at crop canopy height. Traps were each baited with the commercially available Trécé rubber septa lure in A, 2021 and B, 2023. Different letters indicate significant differences among trap types within each year (generalised linear mixed-effects models, followed by post hoc Tukey’s test, P = 0.05). Error bars represent standard error (SE).
Lure type
Traps baited with Trécé rubber septa lures captured significantly more S. mosellana than did traps baited with the other lure types tested in 2022 (χ2 = 392.23, df = 4, P < 0.001) and in 2023 (χ2 = 278.25, df = 5, P < 0.001). Trap capture, however, was most variable in traps baited with Trécé rubber septa lures (Fig. 3). Surprisingly, traps baited with the 1-mg Trécé rubber septa lure captured significantly more midges than did those baited with the 1-mg ChemTica septa lure in both 2022 and 2023. All pheromone-baited traps captured significantly more midges than unbaited control traps did in 2023 (Fig. 3). Significantly more S. mosellana were captured in traps baited with rubber septa lures than with flex lures in 2022 (χ2 = 138.26, df = 1, P < 0.0001) and 2023 (χ2 = 152.78, df = 2, P < 0.0001).
Figure 3. Season total number of S. mosellana captured in Jackson traps baited with either rubber septa (Chemtica (1 mg, 100 μg) or Trécé), flex lures (Chemtica (1 mg, 100 μg), or no lures (2023 only) in A, 2022, and B, 2023. Traps were placed along the edge of commercial wheat fields. Different letters indicate significant differences among trap types within each year (generalised linear mixed-effects models, followed by post hoc Tukey’s test, P = 0.05). Error bars represent standard error (SE).
Residual S. mosellana pheromone on lures
The amount of pheromone extracted from each of the 1-mg lures remained relatively stable over time (Table 1). The 1-mg dose of the Trécé and ChemTica rubber septa lures yielded the highest amounts of pheromone, indicating high levels of residual pheromone remaining on the lure over the 6-week ageing experiment. The amount of pheromone in the two rubber septa lures was measured to be higher than the initial loading, which suggests that the initial loading may have been underestimated (Table 1). In contrast, much less pheromone was extracted from the 1-mg ChemTica flex lure than from the rubber septa lures loaded with the same dose (χ2 = 184.25, df = 2, P < 0.001; Table 1). Release rates were estimated from the results presented in Table 1 by calculating the change in pheromone extracted between weeks 3 and 6 of the study. Estimating the release rates in this way, ChemTica flex lures (1 mg), ChemTica rubber septa lures (1 mg), and Trécé rubber septa lures (1 mg) released a mean of 0.0038 µg, 0.0057 µg, and 0.0056 µg of pheromone per day, respectively. Overall, although the amount of pheromone extracted from each lure remained relatively stable over time, the estimated daily release rates varied between the lure types, which may impact lure attractiveness.
Table 1. Mean amount of S. mosellana pheromone extracted (µg ± standard error) from lures in 10 mL of solvent after 3 and 6 weeks. Lure types include ChemTica flex lure (CT-flex; 1 mg; n = 3), ChemTica rubber septa lure (CT-septa; 1 mg; n = 3), and Trécé rubber septa lure (Trécé-septa; 1 mg; n = 3). Insufficient ChemTica 100-µg flex and septa lures were available for analysis
Different letters indicate significant differences within each week (generalised linear models, followed by post hoc Tukey’s test, P = 0.05).
Seasonal trapping experiments
The first adult male S. mosellana was captured in pheromone traps within the first week of sampling in 2022, 2023, and 2024. The total number of adult male S. mosellana captured in pheromone traps was higher in 2024 (season-long average of 265 ± 51 (standard error) adults) than in 2021 (60 ± 14 (standard error) adults), 2022 (140 ± 29 (standard error) adults), and 2023 (91 ± 22 (standard error) adults). Peak emergence occurred in mid-July in all sampling years, after most wheat crops had passed the susceptible stage. Cumulative precipitation between May and August was higher in 2022 (∼270 mm) than in 2023 (∼250 mm) and 2024 (∼260 mm). However, all years had greater than 20 mm of precipitation by early June (19 May 2022; 30 May 2023; 7 May 2024), which is conducive to S. mosellana diapause termination (Elliott et al. Reference Elliott, Mann and Olfert2009). Notably, in 2024, approximately 55% of the precipitation occurred in May and June, which was associated with the highest S. mosellana populations present in pheromone traps in all sampling years. In addition, the number of male S. mosellana captured in pheromone traps noticeably increased a few weeks after significant precipitation events (Fig. 4).
Figure 4. Relationship between precipitation and the mean number of adult S. mosellana males collected in two pheromone-baited Jackson traps between 1 May and 1 August in A, 2022, B, 2023, and C, 2024. The grey box represents when the wheat crop was most susceptible to S. mosellana damage. The dotted line represents when precipitation reached 20 mm after 1 May.
Soil core samples
Similar densities of S. mosellana cocoons were found present in soil samples collected in 2022 (144 ± 69 (standard error) cocoons/m2) and 2023 (138 ± 97 (standard error) cocoons/m2). Cocoons were found in four of 10 sites in 2022 and two of 10 sites in 2023. The density of cocoons (i.e., cocoons/m2) was not significantly related to the number of adults captured in emergence traps (normalised per unit trap area (midges/m2); χ2 = 0.6898, df = 1, P = 0.4062) or to the number of adult males captured in pheromone traps (χ2 = 0.8147, df = 1, P = 0.3667).
Emergence traps
Adult S. mosellana were captured in emergence traps at all sites in 2023 and 2024, but most (∼63%) were captured in previous-year wheat fields. In both years, peak capture of adults in emergence traps occurred in mid-July, with S. mosellana emergence occurring earlier in previous-year wheat fields than in current-year wheat fields. The number of adults captured in emergence traps (per m2) in previous-year wheat fields was not related to the number of adult males captured in pheromone traps in 2023 (χ2 = 0.0218, df = 1, P = 0.883) or 2024 (χ2 = 0.0921, df = 1, P = 0.762). The number of adults captured in emergence traps (per m2) in current-year wheat fields, however, was related to the number of males captured in pheromone traps in 2023 (χ2 = 5.2801, df = 1, P = 0.0216) but not in 2024 (χ2 = 0.0548, df = 1, P = 0.815).
In-field counts
In both 2023 and 2024, only one S. mosellana was observed during in-field counts, resulting in an average range of 0 to 0.6 midges per 4–5 wheat heads. Consequently, in most field sites, no S. mosellana were observed during these counts, and the number of ovipositing females was not related to the number of males captured in pheromone traps (in the same week; χ2 = 1.139, df = 1, P = 0.286).
Discussion
The development of S. mosellana is influenced by temperature and precipitation, with larvae terminating diapause in response to increased soil moisture and temperature (Basedow Reference Basedow1977; Basedow and Gillich Reference Basedow and Gillich1982; Elliott et al. Reference Elliott, Mann and Olfert2009; Olfert et al. Reference Olfert, Weiss, Vankosky, Hartley and Doane2020). Dry spring conditions in 2022 and 2023 (33 mm and 24 mm of precipitation from 1 May to 1 June, respectively) likely contributed to low S. mosellana populations and the observed delayed emergence, which was consistent with the Prairie Pest Monitoring Network report over a larger region (Weiss et al. Reference Weiss, Rounce, Giffen, Olfert, Otani and Vankosky2022, Reference Weiss, Rounce, Olfert, Otani and Vankosky2023; Saskatchewan Agriculture 2024; Alberta Agriculture and Irrigation 2025). Conversely, substantial rainfall in 2024 increased the risk of S. mosellana infestation in some areas (Otani et al. Reference Otani, Dufton, Jorgensen and Vankosky2024; Saskatchewan Agriculture 2024; Alberta Agriculture and Irrigation 2025). The study region was initially selected because it was identified as an area of concern for 2021 (Alberta Agriculture and Irrigation 2021), but the risk subsided as the study progressed but increased for 2024 (Saskatchewan Agriculture 2024; Alberta Agriculture and Irrigation 2025). Increased adoption of midge-tolerant wheat varietal blends in recent years – from 28% in 2023 to 31% in 2024 of total insured wheat in Canada (Canadian Grain Commission 2024) – likely also influenced local S. mosellana populations. Therefore, although populations during the sampling years were relatively low compared to those in other studies (Oakley et al. Reference Oakley, Talbot, Dyer, Self, Freer and Angus2005; Bruce et al. Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007; Jorgensen et al. Reference Jorgensen, Otani and Evenden2020), the increased risk observed in 2024 highlights the complex relationship between environmental conditions, regional susceptibility, and wheat varietal blend adoption. However, the low populations of adult S. mosellana captured in the present study could also be a result of failing to capture early season populations before trap set-up. Additional factors, such as S. mosellana’s ability to remain in diapause for multiple years, crop rotation practices that alter host availability, environmental variability, and the midges’ dispersal to suitable wheat fields, further complicate efforts to accurately predict outbreaks.
Trap design and colour affect insect capture in pheromone-baited traps (Athanassiou et al. Reference Athanassiou, Kavallieratos, Gakis, Kyrtsa, Mazomenos and Gravanis2007; Francis et al. Reference Francis, Bloem, Roda, Lapointe, Zhang and Onokpise2007; Reddy et al. Reference Reddy, Balakrishnan, Remolona, Kikuchi and Bamba2011, Reference Reddy, Shrestha, Miller and Oehlschlager2018; Miluch et al. Reference Miluch, Dosdall and Evenden2014; Jorgensen et al. Reference Jorgensen, Otani and Evenden2020; Sukovata et al. Reference Sukovata, Dziuk, Parratt, Bystrowski, Dainton, Polaszek and Moore2020). Jorgensen et al. (Reference Jorgensen, Otani and Evenden2020) captured more S. mosellana males in orange pheromone-baited delta traps than in green traps. In the present study, capture of male S. mosellana in white Jackson traps was similar to or greater than that in similarly baited green delta traps. Jackson traps offer several advantages over green (and orange) delta traps: they are more compact and easier to service and ship than delta traps. Jackson traps are commonly used to monitor several other Cecidomyiidae species, including swede midge, Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae), and canola flower midge, Contarinia brassicola Sinclair (Diptera: Cecidomyiidae) (Andreassen et al. Reference Andreassen, Soroka, Grenkow, Olfert and Hallett2018; Bray et al. Reference Bray, Hall, Harte, Farman, Vankosky and Mori2022).
The Trécé rubber septa lure (1 mg) was the most effective lure among those tested in the present study and remains the only commercially available option. Bruce et al.’s (Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007) study in the United Kingdom found a 1-mg rubber septa lure was the most attractive to S. mosellana adults, but Jorgensen et al.’s (Reference Jorgensen, Otani and Evenden2020) study in northern Alberta, however, found more S. mosellana in traps baited with a low-dose (Scotts; 100 μg of 2,7-nonanediyl dibutyrate) flex lure than with a higher-dose (Trécé; 1 mg of 2,7-nonanediyl dibutyrate) rubber septa lure. In the present study, traps baited with the Trécé rubber septa lure captured significantly more midges than did the traps baited with the polyethylene plastic flex lures. The variation in midges captured by lure type may be influenced by the lure substrate (i.e., material), which can impact pheromone release rate and longevity; these, in turn, influence attractiveness (Howse Reference Howse, Howse, Jones and Stevens1998; Mayer and Mitchell Reference Mayer and Mitchell1999; Mircioiu Reference Mircioiu2004). The higher captures of S. mosellana males in traps baited with Trécé rubber septa lures coincided with a higher estimated release rate of pheromone (0.0056 µg pheromone/day). Although the estimated release rate of the 1-mg ChemTica rubber septa lure was similar (0.0057 µg pheromone/day) to that of the 1-mg Trécé rubber septa lures, fewer midges were captured in traps baited with ChemTica lures. However, a discrepancy occurred between the amount of pheromone reportedly loaded onto both high-dose rubber septa lures (1 mg) and the higher amount measured upon extraction. This finding suggests that the initial loading may have been underestimated. The results of this study contrast with those of Jorgensen et al. (Reference Jorgensen, Otani and Evenden2020), who captured greater numbers of midges in traps baited with lures with lower pheromone release rates. Jorgensen et al. (Reference Jorgensen, Otani and Evenden2020) directly measured pheromone release rates from lures, which could account for the discrepancies between the studies. The release rates estimated in the present study were obtained from pheromone extractions performed on distinct lures at separate time points. Given the destructive nature of pheromone extraction and the limited number of replicates, variability in initial loading and low sample size may have affected the accuracy of these results. Future research incorporating greater replication and continuous monitoring techniques would provide more reliable and accurate assessments of pheromone release dynamics.
Soil core sampling for overwintering S. mosellana in the present study did not predict emerging populations, which is consistent with similar sampling in Saskatchewan (Mircioiu Reference Mircioiu2004), Manitoba (Lamb et al. Reference Lamb, Wise, Olfert, Gavloski and Barker1999), Montana, United States of America (Thompson and Reddy Reference Thompson and Reddy2016), and the United Kingdom (Oakley et al. Reference Oakley, Cumbleton, Corbett, Saunderst, Green, Young and Rodgers1998). Soil samples, however, did indicate S. mosellana population levels in northern Alberta (Jorgensen et al. Reference Jorgensen, Otani and Evenden2020) and Saskatchewan (Olfert et al. Reference Olfert, Elliott and Hartley2009). In Saskatchewan, a threshold of six larvae per 100 cm2 indicates S. mosellana population levels capable of producing economic infestations (Doane et al. Reference Doane, Mukerji and Olfert2000). This threshold was surpassed at some sites in the present study (0–6 larvae per 100 cm2 in 2022; 0–8 larvae per 100 cm2 in 2023), but economic infestations were not detected based on the number of ovipositing females. Because S. mosellana needs precipitation (> 20 mm) to terminate diapause, spring conditions have a significant impact on emergence (Reeher Reference Reeher1945). Furthermore, parasitism by Macroglenes penetrans (Hymenoptera: Pirenidae) can significantly affect S. mosellana population dynamics and may complicate efforts to accurately predict midge population levels (Olfert et al. Reference Olfert, Elliott and Hartley2009). As a result, the ability of soil core sampling to predict S. mosellana populations in the following year would depend on spring soil moisture and temperature. The present study’s findings indicate that soil core sampling is not effective for forecasting S. mosellana populations at low densities but that it may be helpful at high densities, particularly when combined with precipitation data to enhance predictive accuracy. However, other factors, such as S. mosellana’s ability to remain in prolonged diapause and the limitation of sampling only wheat fields each year, further complicate accurate forecasting of regional pest risk and may lead to underestimation of regional risk of S. mosellana infestation.
The present study also found that more S. mosellana emerged in previous-year wheat fields than in current-year wheat fields, which is consistent with Smith et al.‘s (Reference Smith, Wise and Lamb2004) finding that S. mosellana emerge in previous-year wheat fields where they mate before the females ultimately migrate to current-year wheat fields to oviposit. Adult emergence is influenced by precipitation and soil moisture, with larvae potentially undergoing delayed development or extended diapause in response to low precipitation and moisture levels (Basedow Reference Basedow1977; Basedow and Gillich Reference Basedow and Gillich1982; Elliott et al. Reference Elliott, Mann and Olfert2009). In this way, adult emergence of S. mosellana from fields seeded to a nonwheat crop in the previous year is most likely a result of inadequate conditions (i.e., low soil temperatures or insufficient moisture) for pupation in previous years, leading to extended diapause and delayed adult emergence. Nonetheless, approximately 95% of S. mosellana emerge in western Canada after one winter (Wise and Lamb Reference Wise and Lamb2004), typically in late June and early July, which aligns with the emergence timing observed in the present study.
We found no relationship between S. mosellana emergence in previous-year wheat fields and males captured in pheromone traps in current-year wheat fields. However, a relationship in 2023 between current-year wheat field emergence traps and pheromone traps was observed and may have been influenced by weekly variation. These results indicate that male S. mosellana may disperse to current-wheat fields quickly or may remain at their emergence sites. Given that primarily female S. mosellana disperse, males are likely underrepresented in current-year wheat fields (Smith et al. Reference Smith, Wise and Lamb2004), and the number of males captured in pheromone traps may not represent overall population densities. However, the present study did not examine the sex ratio of emerging midges, and it is possible that variation in sex ratios – particularly the proportion of females – could influence population development and affect the interpretation of emergence data.
In-field counts indicated that S. mosellana populations were below the economic threshold to maintain grain yield (1 adult/4–5 wheat heads) and quality (1 adult/8–10 wheat heads) and did not relate to the numbers of male S. mosellana captured in pheromone traps, possibly due to low overall populations. Sitodiplosis mosellana population numbers were lower in this study than in other studies (Oakley et al. Reference Oakley, Talbot, Dyer, Self, Freer and Angus2005; Bruce et al. Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007; Jorgensen et al. Reference Jorgensen, Otani and Evenden2020), and a relationship between the numbers of male S. mosellana captured in pheromone traps and in-field counts may exist at higher populations.
In the United Kingdom, the number of S. mosellana males captured in pheromone traps was correlated to the degree of infestation (% attacked grain), leading to the development of a pheromone-based warning and decision support tool (Oakley et al. Reference Oakley, Talbot, Dyer, Self, Freer and Angus2005; Bruce et al. Reference Bruce, Hooper, Ireland, Jones, Martin and Smart2007). In Canada, using pheromone trapping as a decision support tool may not be feasible due to several challenges, including regional climate variability, especially precipitation, and the extensive wheat acreage in the region. The small size and cryptic behaviour of S. mosellana make scouting difficult, which interferes with accurate in-field counts. Studies in high–midge population years are necessary to determine whether reliable relationships can be found between counts of ovipositing females and the numbers of males captured in pheromone traps. Further research into economic thresholds based on pheromone trapping is recommended to improve understanding of pheromone-baited traps’ potential use for effective S. mosellana monitoring.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.4039/tce.2025.10028.
Acknowledgements
Financial support was provided by the NSERC Industrial Research Chair (545088) and partner organisations (Alberta Grains, Alberta Canola Producers Commission, and Alberta Pulse Growers Commission), as well as by an NSERC Discovery Grant (2021-02479). The authors thank S. Barkley, A. Van Tryp, A. Kittle, and J. Dehaan for their help with data collection and processing, and for their advice. They also thank all of their producer-cooperators and research assistants, especially K. Senevirathna, A. Beaudoin, K. Enstrom, S. Hamilton, N. Laforest, L. Saunders, J. Martin, and T. Frisky.
Competing interests
The authors declare that they have no competing interests.
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