PERTANIKA JOURNAL OF TROPICAL AGRICULTURAL SCIENCE

The cabbage white butterfly (Pieris rapae L.) is a major pest of cruciferous crops, causing significant yield losses through larval feeding. Although synthetic pyrethroids such as deltamethrin are widely used for control, their efficacy may be compromised by larval behavioural responses that reduce lethal exposure. This study quantified the behavioural and toxic effects of deltamethrin on P. rapae larvae using a standardised one-way no-choice laboratory bioassay. The experiment was performed in a completely randomised design with three replications for each treatment under controlled conditions (20-23 °C; 30-60% relative humidity). Cabbage leaves were subjected to three coverage levels: water-treated control, 50% insecticide coverage, and 100% insecticide coverage at the recommended field rate (0.65 mL/L). Late-instar larvae were exposed to treated leaves. Larval survival, movement, and leaf area damage were recorded at multiple time points up to 72 hours. Complete insecticide coverage significantly reduced larval survival, locomotion, and feeding damage compared with partial coverage and controls, resulting in near-complete mortality by 72 hours. Partial coverage produced moderate but highly variable suppression, indicating inconsistent exposure. Early reductions in larval movement and feeding suggest avoidance of treated surfaces, while progressive declines over time indicate cumulative neurotoxic effects. These findings demonstrate that uniformity in insecticide coverage strongly influences both behavioural responses and control efficacy. Hence, optimising spray coverage is critical for effective P. rapae management and for reducing the risk of resistance development associated with sublethal exposure.