Introduction
The increasing of the population of the cabbage aphid (Brevicoryne brassicae L.) in the rapeseed fields of West Azerbaijan province is attributed to various factors, including recent droughts. Determining the Economic Injury Level (EIL) is essential to prevent excessive pesticide use in the rapeseed fields. This research was conducted to estimate the Economic Injury Level (EIL) using a randomized complete block design with four replications on the Hydromel cultivar under eight treatments during the 2017–2018 years in West Azerbaijan province. The treatments were applied to achieve different levels of natural infestation by the cabbage aphid population and consisted of: Treatment 1 (seven weekly sprays from the beginning of stem elongation, to ensure pest absence); Treatment 2 (six sprays after beginning of stem elongation); Treatment 3 (five sprays after beginning of stem elongation); Treatment 4 (four sprays after beginning og stem elongation); Treatment 5 (three sprays after brining of stem elongation); Treatment 6 (two sprays after beginning of stem elongation); Treatment 7 (one spray after beginning of stem elongation); and Treatment 8 (full pest presence, no insecticide, water spray only). In these treatments, imidacloprid SC35% was applied at a rate of 2 per 1000.The mean aphid count per 10 cm of flowering stem, rapeseed yield, and thousand-seed weight were categorized using Duncan's test across the treatments, and the relationship between the mean aphid count and yield per square meter was calculated. The thousand-seed weight per square meter of rapeseed and the damage per aphid were also determined. The results indicated that the mean Economic Injury Level (EIL) of the cabbage aphid in West Azerbaijan province for the Hydromel cultivar during 2017–2018 and 2018–2019 was 19.4 and 25.2 aphids per 10 centimeters of flowering stem, respectively. The Economic Threshold (ET) of the pest was calculated as 14.5 and 18.9 aphids per 10 centimeters of flowering stem, respectively.
 
Materials and Methods
This study was conducted in Boukan, West Azerbaijan province, using a randomized complete block design (RCBD) with eight treatments and four replications on autumn-sown rapeseed, cultivar Hydromel, during the 2016–2017 and 2017–2018 years. The dimensions of the experimental plots were2 × 10 m, distances between plots and replicates were planted at least 5 and 10 m, respectively. The amount of fertilizer application was based on soil test and the number of irrigation times on the basis of custom The treatments were applied to achieve different levels of natural infestation by the cabbage aphid (Brevicoryne brassicae) population and consisted of: Treatment 1 (seven weekly sprays from the beginning of stem elongation, to ensure pest absence); Treatment 2 (six sprays starting one week after stem elongation); Treatment 3 (five sprays starting two weeks after stem elongation); Treatment 4 (four sprays starting three weeks after stem elongation); Treatment 5 (three sprays starting four weeks after stem elongation); Treatment 6 (two sprays starting five weeks after stem elongation); Treatment 7 (one spray six weeks after stem elongation); and Treatment 8 (full pest presence, no insecticide, water spray only (Obopile, 2006). Five plants were randomly selected from each plot. On each plant, a 10 cm section of the flowering stem was considered, and the number of adult aphids and nymphs on them was counted and recorded one day before the first spray in each treatment (Aslam et al., 2005). Seed yield value and thousand-seed weight were measured for all treatments after harvesting the entire crop of each plot. Data were analyzed using analysis of variance (ANOVA) in SAS software version 9.1 (SAS Institute, 2003) and grouped using Duncan's test. Regression equations were derived for the relationships between crop yield (Y) and the number of aphids per plant (X), and between the number of aphids per plant (X) and percentage of damage (%Y). The Economic Injury Level (EIL), defined as the number of aphids per plant that causes economic damage, was calculated using the formula (Pedigo et al., 1986): EIL = C / (V × I × D × K). Since separating injury (I) and damage (D) for sucking insects like aphids is difficult, the slope of the regression line (b) was considered to represent the damage per insect, i.e., b = I × D (Pedigo and Rice, 2009).
 
Results and Discussion
The Economic Injury Level (EIL) of cabbage aphid in West Azerbaijan during the 2016-2017 and 2017-2018 years was 19.38 and 25.23 aphids per 10 centimeters of flowering stem respectively. The variation in the EIL between two years can be attributed to differences in pest control costs (Filho et al., 2024), the marketing value of the crop (Vayssières et al., 2009) and damage potential of the aphid in each respective year. The findings of this study align with the results reported by Khajehzadeh and Hasanimoghaddam (2009), where the EIL for the mustard aphid in the Behbahan region was 7.53 and 2.49 centimeters of infested flowering stem per square meter during the 2004-2005 and 2005-2006 years, respectively. As noted by these researchers, fluctuations in the EIL can result from several factors, including the pest type and species, climate, crop type and cultivar, growth stage, agronomic practices (such as planting date), and particularly the market value of the product and management costs.
 
Conclussion
 The estimated Economic Injury Levels (EIL) of 19.4 and 25.2 aphids per 10 cm of flowering stem for the Hydromel cultivar in West Azerbaijan province provide a critical quantitative basis for integrated pest management (IPM) in rapeseed. The corresponding Economic Thresholds (ET) of 14.5 and 18.9 aphids offer a practical intervention point to prevent yield loss while minimizing unnecessary insecticide applications. Given the dynamic nature of EIL, which is influenced by market prices, control costs, and environmental conditions, region-specific and cultivar-specific calculations are essential for sustainable pest management. These findings support national policies aimed at reducing pesticide use and promote more economically and ecologically sound decision-making in rapeseed production systems.