Laboratory experiments were carried out to study the effect of irradiating T. confusum pupae on the susceptibility of their adults to four insecticides. Results indicate that gamma irradiation significantly decreased the tolerance of adults from pupae irradiated 1 and 2 days before eclosion to Duradin. On the contrary, malathion and formothion, belonging to the same chemical group, i.e. organophosphate, were similar in their toxicity to adults, showing that irradiation of pupae afforded some protection to adults from death. However, s. fenvalerate exhibited the same trend noticed for malathion and formothion where irradiation of pupae reduced its effectiveness to adults. In general, adults from pupae irradiated 1 and 2 days before eclosion were less tolerant to tested insecticides than those from pupae irradiated 3 days before eclosion. Studying of tolerance ratio of various insecticides revealed that at 24 hr post-treatment, the most obvious trend is that adults from irradiated pupae, 3 days before eclosion, showed a moderate level of tolerance to formothion (3.29-fold) and s. fenvalerate (3.62-fold), and high level of tolerance to malathion (16.30-fold). While all adults from irradiated pupae elucidated different levels of tolerance to malathion, formothion and s. fenvalerate, the effectiveness of Duradin on those adults was significantly enhanced. This means that the application of Duradin in combination with gamma radiation is more effective than their separate applications. The dissimilarity of adult responses to the insecticides may be attributed to the enzyme alterations, either quantitative or qualitative and are the key to these irradiation effects on toxicity of insecticides to insects.