The weekly build-up of the mustard aphid, Lipaphis erysimi (Kaltenbach), was recorded in an unprotected and an insecticide-protected plot of Indian mustard (cv. GDM 4) at Anand, Gujarat, during rabi 2024-25 and 2025-26. Five plants per plot were scored each week on a 0-5 aphid index. In the unprotected plot, the aphid was first recorded 2-5 weeks after sowing (WAS), all sampled plants were infested from 6-8 WAS onwards, and the population peaked at an index of 3.4 in the second week of February 2024-25 (12 WAS) and 2.8 in the first week of February 2025-26 (10 WAS), declining sharply before harvest. In the protected plot, the index never exceeded 1.4. Every spray that cleared the aphid (four of the five aphid-period sprays) was followed by recolonisation, the index returning to 0.8-1.4, with 80-100 per cent of plants infested, within two weeks of the post-spray observation, so that the population had recovered by the time the next spray was due. The seasonal mean index was 1.19 and 1.20 in the unprotected plot against 0.33 and 0.37 in the protected plot, and protection reduced the area under the aphid-index curve by 75.00 and 70.39 per cent in the two seasons. The unprotected plot carried a significantly higher aphid load in both years (paired t test, p ≤ 0.01). The critical period of aphid build-up in middle Gujarat falls from mid-January to the third week of February, and a fixed three-week spray interval permits rapid resurgence, pointing to the need for monitoring-based timing of aphid sprays.