Alfalfa plant stand characteristics and year in a Multivariate Correlation and Regression Tree (MRT) analysis accounted for ~9% of the variation (cross validated) in insect pest communities in untreated (no insecticide) areas (Figure 7). This suggests that other drivers, e.g., climatic factors, are more important in driving pest pressure. Year was the most important split due to the lower insect pressure in 2023. In 2024, higher alfalfa weevil and alfalfa plant bug pressure was observed in areas with higher stem density (Figure 7). Increased stem density may support larger populations of alfalfa weevil and alfalfa plant bug as they both lay their eggs in the stems of alfalfa. 

In insecticide treated areas, year, stem density, number of insecticide applications, and insecticide mode of actions explain ~56% of the variation in insect pest communities (cross validated) in a MRT analysis (Figure 8). Diamides and neonicotinoids were primary node splits, with diamide applications typically having higher pressures from Lygus or alfalfa plant bug. The latter aligns with diamides being known to be less effective for true bug insects (Miridae). While sprayed areas generally had lower insect pest pressure, this analysis shows the complexity in pest dynamics. 

A significant proportion of variability in insect pest communities is unaccounted for, particularly for unsprayed areas. This unexplained variation may be due to regional climate conditions, landscape factors, overwintering survival, and other management practices, such as insecticide application timing and irrigation. While insecticides have a direct impact on insects, the associations of insecticides with insect communities is also likely due to producer choices, e.g.,  choosing selective chemicals when insect pressure is high, their tolerance of insect pests, and timing of insecticide applications. More data is needed to parse out insecticide effects given the  variation in management practices confounding results. 

Figure 8 -Dendrogram for Multivariate Regression Tree Analysis  using scaled and centred seasonal average insect pest counts in insecticide treated plots, plant stand characteristics, insecticide sprays, insecticide mode of actions and year.

Insect pest management in alfalfa seed is complex due to the different pests and their timings, with Lygus typically having two generations per growing season and  a second generation for alfalfa plant bug suspected to be more common with climate change [5]. Lygus and alfalfa weevil are often managed together early in the season, but differences in their dynamics may influence the efficacy of insecticide applications. 

Peak alfalfa weevil  populations at 260-280 Growing Degree Days (base 9°C, GDD) was generally observed, however the recommendations to spray at 20 larvae per sweep during the 2nd larval instar peak around 220-240 GDDs was typically not observed (Figure 9). In 2023, only the Enchant region was above recommended thresholds but insecticides in this region were applied on average 147 GDDs later than the upper GDD recommendation. In 2024, applications were on average 65 GDD earlier than the upper GDD recommendation. Temperatures in 2024 were very warm in May and then much cooler in June, which could have prolonged the life span of alfalfa weevils and impacted their fecundity [15]. Fluctuating temperatures over shorter (hours) and longer (weeks) periods maybe influencing the accumulation of growing degree days and insect development, therefore GDDs should be monitored closely to optimize applications.

Figure 9- Alfalfa weevil larvae per 10 sweeps by growing degree day (base  9°C) and region for 2024. Coloured points represent sprayed (purple) and unsprayed (grey) areas, the black dashed line represents the current economic threshold, green rectangles represent growing degree days between 220 to 260 (when insecticide action should be taken), and dashed blue lines represent insecticide spray applications. 

Insecticide applications impact the insect pest communities, with many interactions between insecticide mode of actions, application timing, and late season pest management. Growing degree day models provide a useful tool for informing pest management and could be utilized to optimize pest management. Alfalfa weevil remains a primary pest of concern for alfalfa seed producers in southern Alberta, yet this work demonstrates the importance of Miridae pests and their management. Yield responses to insecticide use were highly dependent on the composition and timing of pest pressure, with the greatest losses occurring under multiple-pest scenarios, particularly when early alfalfa weevil and late-season Miridae pressures coincided. To develop effective action thresholds, it is important to understand how multiple pests collectively contribute to yield loss throughout the growing season. This work highlights the complexity of working with real on-farm data, where pest communities vary widely over a small geographic scale. A large proportion of insect pest communities variation remains unaccounted for and may relate to factors such as overwintering success, fluctuating seasonal temperatures, alternate hosts, surrounding crops, insect competition, beneficial predators and parasitoids, insecticide mode of actions and timing, and resistant populations. Overall, the variability in insect pest communities makes on-farm decision-making challenging and reinforces the need for continuous monitoring and the development of  multi-pest thresholds to maximize yield while protecting pollinator populations. Long-term monitoring is needed to parse out inter-annual and seasonal effects on insect pest populations to better predict pest abundance, and future work should examine insecticide resistance and how insecticide application timing influences the efficacy.