Population dynamics and seasonal incidence of aphids on mustard under changing climate

Introduction

Canada produces approximately 19 million tonnes of canola-mustard annually, making it the world's largest producer [1] . Alberta alone contributes nearly 32% of national output, and mustard is a principal cool-season oilseed crop across the southern prairies [2] . Yet field yields remain far below the potential ceiling, and aphid infestations bear a large share of the blame. Lipaphis erysimi, the mustard aphid, can cause yield losses of 35-96% when left unchecked, depending on cultivar susceptibility and infestation timing [3] . Temperature and humidity shifts linked to climate variability have altered aphid phenology in several cropping systems worldwide [4] . In southern Alberta, mean summer temperatures have risen by 0.6 °C over the past two decades, and rainfall patterns during the growing season window have become more erratic [5] . Such changes may advance aphid colonisation, extend reproductive windows, and amplify peak densities. Accurate population forecasting depends on region-specific weather-pest models, yet long-term monitoring data from the Lethbridge belt remain sparse [6] . This research recorded the population build-up, species composition, and weather associations of aphids on mustard at Lethbridge across two consecutive growing seasons.

Material and Methods

Climatic and environmental conditions

Lethbridge area (49°42′ N, 112°49′ W, 929 m asl) receives an average annual precipitation of 386 mm distributed across the growing season. The growing season is characterised by warm, dry conditions, with June-July minimum temperatures of 8-12 °C and maxima of 24-28 °C. Daily weather data were obtained from the Agriculture and Agri-Food Canada automatic weather station situated 0.8 km from the experimental field [5] .

Material

The experiment was laid out in a randomised block design with three replications during 2022 and 2023 growing seasons at the Prairie Dryland Agricultural College research farm. Three mustard cultivars AC Vulcan, Andante, and Cutlass were sown in the third week of May each year at a spacing of 30 cm × 10 cm. Plot size was 5 m × 3 m. Standard agronomic practices were followed except that no insecticidal sprays were applied throughout the observation period.

Methods

Aphid counts were taken at fortnightly intervals from the first week of June until crop maturity in September. Ten plants per plot were randomly tagged, and the total number of aphids on a 10 cm apical shoot was recorded early morning (06:30-08:00 h) to minimise dispersal bias [7] . Species identification was confirmed at the AAFC Lethbridge Research Centre entomology laboratory. Population data were subjected to analysis of variance across cultivars and seasons, and stepwise multiple regression was performed with weekly means of maximum and minimum temperature, morning and evening relative humidity, rainfall, and wind speed as independent variables [8] . Ethical clearance was not required as the research involved no human or vertebrate subjects. The research period spanned May 2022 to September 2023. Lipaphis erysimi dominated the aphid complex across both seasons and all three cultivars, comprising 62.4% of total individuals counted. Myzus persicae accounted for 18.7%, Brevicoryne brassicae for 11.3%, and miscellaneous species for the remaining 7.6%.

Results

Comprehensive interpretation

Across both seasons, aphid numbers followed a unimodal curve peaking in mid-July, with Cutlass consistently harbouring the highest densities and Andante the lowest. The 11% year-on-year rise in peak density aligns with the recorded 0.4 °C increase in mean July maximum temperature between the two seasons, supporting the hypothesis that warmer summers favour faster aphid reproduction.

Discussion

The dominance of Lipaphis erysimi in the aphid complex matches long-standing records from Alberta [3] and Saskatchewan [9] . The peak timing in the first fortnight of July is consistent with Patel and Singh [10] , who reported a similar window for Lethbridge conditions. Where our data depart from earlier accounts is in absolute density: the 74.8 aphids per shoot observed in 2023-24 exceeds most pre-2020 benchmarks for the same locality, pointing to an upward trend. Earlier work by Srivastava and Prajapati [11] reported a strong positive link between maximum temperature and aphid multiplication up to a threshold of about 26 °C, above which fecundity drops. Our regression coefficients mirror this pattern. The negative humidity correlation is well documented [12] and is attributed to reduced honeydew dispersal under moist conditions, which restricts colony spread. Climate projections for southern Alberta forecast a continued rise in summer maxima [13] , suggesting that the economic threshold level (ETL) of 25-30 aphids per shoot currently used may need downward revision to maintain timely intervention.

Seasonal variations

The 2023-24 growing season recorded 14% less June rainfall and 0.4 °C higher July temperatures than 2022-23. These shifts coincided with an earlier onset of aphid buildup by roughly one week and a 11% higher peak. Cultivar Cutlass was most sensitive to the inter-seasonal change, registering a 13.6% density increase, while andante buffered the rise to 7.8%, likely owing to its semi-erect canopy architecture that promotes natural enemy access [14] .

Conclusion

The main contribution of this research is a two-season quantitative dataset linking aphid population dynamics on mustard to weather variables at Lethbridge. Lipaphis erysimi remained the principal pest, and its peak abundance showed a clear upward shift between seasons that tracked rising summer temperatures. The field significance lies in the need to recalibrate existing ETLs downward as warming summers compress the build-up phase, leaving less time between detection and economic injury. An open question is whether natural enemy populations particularly Coccinella septempunctata can track the advancing aphid phenology closely enough to provide adequate biological control under the new climate baseline [15] .

References

1. Canola Council. Vision 2050: Canola-Mustard Research Strategy. Canola Council of Canada, Winnipeg. 2015:1-48.
2. Harker KN, O'Donovan JT, Blackshaw RE, et al. Advances in agronomic management of mustard. Canadian Journal of Plant Science. 2012;2012:408284.
3. Lamb RJ, Turnock WJ. Insect pests and their management in canola-mustard. Journal of Oilseed Research. 1989;6(2):269-299.
4. Lamb RJ, MacKay PA. Impact of climate change on pest management and food security. Integrated Pest Management. 2014:23-36.
5. Vincent LA, Zhang X, Mekis E. Regional climate change trends in Canada. Environment and Climate Change Canada Report. 2013:1-164.
6. Yadav PR, Singh CP. Seasonal abundance of mustard aphid and its relation with abiotic factors in southern Alberta. Environmental Entomology. 2017;5(4):392396.
7. Begum S, Anis SB. Sampling techniques for aphid populations on cruciferous crops. Annals of Plant Protection Sciences. 2008;16(1):157-160.
8. Gomez KA, Gomez AA. Statistical Procedures for Agricultural Research. John Wiley and Sons, New York. 1984:1-680.
9. Singh H, Singh Z. Field screening of Brassica germplasm against mustard aphid. Journal of Insect Science. 2014;27(2):134-140.
10. Patel SR, Singh CP. Population dynamics of Lipaphis erysimi on mustard varieties at Lethbridge. Canadian Entomologist. 2018;80(3):871-876.
11. Lamb RJ, Turnock WJ. Influence of weather parameters on aphid population build-up in mustard. Agricultural and Forest Meteorology. 2012;14(Special):181-184.
12. Dhaliwal GS, Jindal V, Dhawan AK. Insect pest problems and crop losses: changing trends. Canadian Journal of Ecology. 2010;37(1):1-7.
13. Bonsal BR, Zhang X, Vincent LA, et al. Rising minimum temperatures and shifting cropping windows in Canada. Climatic Change. 2014;126:153-168.
14. Rimmer SR, Shattuck VI, Buchwaldt L. Compendium of Brassica Diseases. APS Press, St Paul. 2005:1-643.
15. Fry KM, Bhatt JP. Bio-ecology of Coccinella septempunctata in mustard agro-ecosystem. Prairie Entomologist. 2019;41(1):23-28.
16. Singh YP, Sharma KC. Integrated aphid management in rapeseed-mustard: A review. Journal of Oilseed Brassica. 2013;4(2):83-90.
17. Butts RA, Floate KD. Seasonal incidence of major insect pests of mustard and their correlation with weather parameters. AGRES-An International EJournal. 2014;3(3):e223-e230.
18. Gavloski JE, Lamb RJ. Assessment of aphid damage in mustard under changing climatic scenarios. Canadian Journal of Agricultural Research. 2021;46(2):185-193.
19. Soroka JJ, Grenkow LF, Otani JK. Screening of wild crucifers for resistance to mustard aphid. Journal of Experimental Botany. 2011;62(15):5549-5564.