[Aim]  To clarify the physiological and biochemical mechanisms responsible for the lethal effects of five biological pesticides on the rice stem borer, Chilo suppressalis. [Methods]  Corrected mortality rates of third instar C. suppressalis larvae after treatment with five biological pesticides, namely Metarhizium anisopliae CQMa421, Beauveria bassiana ZJU435, Bacillus thuringiensis, spinetoram and avermectin, were determined using the diet incorporation method. The toxicity of the most effective pesticide to different larval instars was then measured. Third instar larvae were treated with the LC₅₀ concentration of the pesticide, and dynamic changes in their total protein content, and in the activity of the digestive enzymes α-amylase, sucrase, trehalase, trypsin and lipase, were investigated. [Results]  Avermectin had the strongest toxicity at the recommended dosage, with a corrected mortality rate of 97.78% after 4 days. This was significantly higher than that of other pesticides (P<0.05). Moreover, first instar larvae were the most sensitive to abamectin; LC₅₀ increased with successive larval instars. Treatment with abamectin (LC₅₀) significantly reduced the protein content of larvae (P<0.05), and simultaneously abnormally elevated the activity of all five digestive enzymes monitored (P<0.05). [Conclusion]  Avermectin is highly toxic to young C. suppressalis larvae. Treatment with the LC₅₀ concentration of avermectin elevates the digestive enzyme activity of larvae leading to a nutritional, metabolic imbalance. These results clarify the toxicological mechanism underlying the effects of avermectin on C. suppressalis.