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Selected aromatic oils and terpene fractions of plant origin were tested in a bioassay to evaluate their toxicity to and growth regulatory activity on three insect pests of economic importance, namely the desert locust (Schistocerca gregaria), castor semilooper (Achaea janata), and Bihar hairy caterpillar (Spilosoma obliqua). Terpene fraction of the mint oil (Mentha arvensis) has the highest biological activity on the locust, causing deformities at the nymphal adult moult. Such effects were also noticed from Artemesia oil and Himalayan cedarwood oil, though to a lesser extent. Terpene fraction of mint oil was also active on A. janata, causing deformities at larval-pupal moult, but was toxic to S. obliqua. Mint oil was very toxic to A. junata larvae, causing desiccation. Oil of sweet basil was moderately toxic to locust hoppers and non-toxic to A. janata larvae.