Deciphering the Morpho-Biochemical Determinants of Host Preferences and Population Dynamics in Lipaphis erysimi (Kaltenbach) Across Diverse Brassica juncea Germplasm

The mustard aphid ( L. erysimi Kalt.) is the most significant biotic factor that restricts the production of B. juncea in various agroecological situations. In this study, the key morpho-physicochemical components conferring resistance and their role in aphid preference and population build-up were investigated under open field and control conditions. Twenty diverse B. juncea germplasm were screened for aphid infestation under natural and artificial conditions in two consecutive Rabi seasons (2023-24 and 2024-25). Significant variation was noticed for aphid population build-up, multiplication rate, and resistance index among the germplasm. KRANTI, PM-25 and PDL-1 germplasm gave significantly less aphid infestation, while TARAMERA, PM-29 and LACMA-SP523N5 were highly susceptible and preferred to aphid infestation. Biochemical parameters showed a strong association with resistance: Sugars exhibited a significant positive correlation with aphid population, indicating a stronger host preference. Phenols and glucosinolates exhibit a significant negative correlation with aphid population and act as defensive compounds. High activities of peroxidase (PO), phenylalanine ammonia-lyase (PAL), and tyrosine ammonia-lyase (TAL) also contribute to resistance induction by activating secondary metabolite defence pathways. Wax content under morphological characters showed a significant negative association with aphid multiplication rate and limited or no effect of plant height, stem diameter, and shoot length on the infestation level. Multivariate analyses indicated that biochemical compounds (sugars, phenols and glucosinolates) explain the major share of variance for aphid resistance, whereas morphological parameters also play a secondary role for the resistance to the mustard aphid. Germplasm was clustered in three classes, viz., resistant, moderately resistant and susceptible, according to their defence characteristics. The findings of this study suggest that screening of different B. Juncea germplasm for determining resistance, may be a more viable option to minimise the crop losses caused by L. erysimi. Moreover, the findings of the present study could also help to formulate the breeding programmes aimed at developing cultivars resistant to this notorious pest.