<p><i>Phyllotreta</i> spp. flea beetles (Coleoptera: Chrysomelidae: Galerucinae: Alticini) have recently intensified as major early-season pests of mustard in South and Southeast Asia, including Bangladesh. However, effective management is constrained by two gaps; the species associated with mustard in Bangladesh, their relative abundance, crop stage-specific damages remain limited, and the influence of sowing time on infestation is unclear, particularly under delayed planting caused by prolonged monsoon rain and <i>aman</i> rice harvests. To address these gaps, a two-year field study was conducted during the 2023–24 and 2024–25 mustard seasons to document <i>Phyllotreta</i> species and assess the impact of six sowing dates (mid-October to January) on beetle incidence and foliar damage. Two flea beetle species were consistently recorded on mustard viz the striped flea beetle (<i>P. striolata</i>) and the crucifer flea beetle (<i>P. cruciferae</i>). <i>Phyllotreta striolata</i> was predominant, reaching mean densities of 137.75 beetles per 10 sweeps at the seedling stage, compared with 55.50 for <i>P. cruciferae</i>. Sowing date (DS) strongly influenced infestation dynamics. Crops sown from mid-October to mid-November (DS1-DS3) showed low beetle densities, minimal foliar infestation (9–20%), whereas early-December sowing (DS4) produced moderate damage (45–63%). In contrast, mid-December to January sowings (DS5-DS6) suffered severe outbreaks, with leaf infestation exceeding 80%. These patterns were consistent across both seasons, and the seedling stage proved the most vulnerable phase. These findings highlight the predominance of <i>P. striolata</i> and demonstrate that optimized sowing time can serve as an effective, climate-responsive cultural strategy for reducing <i>Phyllotreta</i> infestations in South Asian mustard production systems.</p> Graphical abstract <p></p>