Abstract: Laboratory tests were conducted to evaluate the toxicity of Diflube nzuron to pine caterpillars (Dendrolimus tabulaeformis) and (Dendrolimus spectabilis).Symptoms of poisoning and amount of food taking by the larvae were observed,also the effects on the natural enemies of the pine caterpillar were recorded.The acute oral LC₍₅₀₎ of Diflubenzuron to the 4th and 5th instar larvae is about 2—9ppm.The chief effect of the toxicant is the disruption of the molting process.Effected larvae are normal in food taking in the first 4 days,but the amount of food taken is greatly reduced after the 4th day.The effect of Diflubenzuron to two species of natural enemies of pine caterpillar-Corcelia rasella Boranoff (Tachinidae) and Casinaria nigripes Gravenhorst(Ichneumonidae) is slight.