Pisum sativum agglutinin has been shown to act as a feeding inhibitor for various insect pests belonging to different orders: Lepidoptera, Coleoptera and Hemiptera. In the present study, its insecticidal activity was assessed through monitoring the growth and development of a dipteran pest Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae). Pea lectin, P. sativum agglutinin (PSA) was purified by single step affinity chromatography on a Sephadex G-100 and the purification was monitored through hemagglutination activity and SDS-PAGE. Insect feeding assays were conducted to determine the effect of pea lectin against first and second instar larvae of melon fruit fly B. cucurbitae. Lectin was incorporated in an artificial diet at a varied range of concentrations, 12.5, 25, 50, 100, 200 and 400 μg/mL. The lectin showed highly significant antimetabolic effects in both first and second instars. Time taken for pupation and development as well as percentage pupation and percentage adult emergence were adversely affected. The activity of three hydrolase enzymes (esterases, acid and alkaline phosphatases), five oxidoreductases (superoxide dismutase, catalase, ascorbate peroxidase, peroxidase, O-demethylase) and one group transfer enzyme (glutathione-S-transferases) was also assessed in second instar larvae fed on lectin treated diet at 100 μg/mL concentration. The P. sativum lectin significantly and deleteriously influenced the activity of all these enzymes at all exposure intervals.