In the current work, the toxicity and mechanism of phytohaemagglutinin (PHA), lectin isolated from the kidney bean (Phaseolus vulgaris) to the grain aphid (Sitobion avenae F.) were studied. When S. avenae was fed an artificial diet containing the lectin PHA, toxicity assays indicated that fecundity decreased, the pre-reproductive period and generation time were prolonged, and mortality increased. To elucidate the mode of action of PHA, the interaction between the lectin and the insect gut was investigated. Interestingly, DNA fragmentation was observed in extract of gut of treated grain aphid, and this was accompanied by an increase in caspase-3 activity. Results indicate that the detrimental activity of PHA on S. avenae may involve effects on death of the gut epithelial cells.