Fumigant toxicity of eight essential oils obtained by hydrodistillation from eight plant species viz. Artemisia maritima L., Cinnamomum zeylanicum Blume, Citrus hystrix DC., Colebrookea oppositifolia Sm., Pelargonium hortorum Bailey, Rabdosia rugosa Wall. ex Benth, Thuja occidentalis L. and Zanthoxylum armatum DC. were tested against different immature stages of Tribolium castaneum (Herbst) and Plodia interpunctella (Hubner) both are serious pests of stored products worldwide. 8-10 day and 18-20 day old larvae of P. interpunctella and T. castaneum were used for the bioassay. R. rugosa oil gave 80.03±5.6% mortality at 100 µl/ml followed by 72.28±4.9% at 50 µl/ml against 8-10 day old larvae. Whereas 65.47±2.9% mortality at 100 µl/ml followed by 62.09±1.8% mortality at a dose of 50 µl/ml was obtained against 18-20 day old larvae of T. castaneum after 120 hrs. At a lowest concentration of 10 µl/ml R. rugosa oil produced 53.8.33±2.2 % mortality for 8-10 day old larvae and a mortality of 40.55±3.9% respectively was obtained for 18-20 day old larvae of P. interpunctella after 120 hrs. Among eight essential oils tested R. rugosa had the highest toxicity followed by A. maritima, C. zeylanicum, Z. armatum, T. occidentalis, P. hortorum, C. oppositifolia and C. hystrix against both the insect pests. The results obtained also showed that tolerance increases as the immature stages grow older and 18-20 day old larvae were least susceptible to all the treatments as compared to 8-10 days old larvae. Larvae of T. castaneum were found to be most susceptible for all treatments than P. interpunctella.