The insecticidal and antifeedant activity of extracts derived from different plants of Aesculus hippocastanum L. (Hippocastanaceae), Viscum album L. (Loranthaceae), Sambucus nigra L. (Caprifoliaceae), Buxus sempervirens L. (Buxaceae), Diospyros kaki L. (Ebenaceae). Artemisia absinthum L. (Compositae), Alnus glutiosa Goertn. (Betulaceae), Origanum vulgare L., (Labiatae), Hypericum androsaemum L. (Hypericaceae) and Ocimum basilicum L. (Labiatae) are reported. The 95% ethanol extracts of plants were tested for toxicity against the third-fourth instar larvae of the Thaumetopoae solitaria Frey. (Lep.: Thaumetopoeidae). Antifeedant activity of the extracts was assessed through tests conducted on the larvae of T. solittaria by the feeding protection bioassay. In the tests carried out on the larvae of T. solittaria, A. hippocastanum, A. glutiosa, H. androsaemum, and O. basilicum extracts showed high toxicity with 48 h values of 90%, 80%, 70%, and 70%, respectively. The toxicity levels of the V. album, S. nigra, B. sempervirens, D. kaki, A. absinthum, and O. vulgare extracts were determined as LC50 values of 60%, 60%, 50%, 50%, 40%, and 30% within the same period, respectively. Ten per cent mortality was determined in control group I. However, no mortality was determined in control group II. Alcohol extract from A. hippocastanum, A. glutiosa, and H. androsaemum showed high antifeedant activity against larvae of T. solitaria with the absolute deterrence coefficients of 52.05, 41.18, and 40.06, respectively. In addition, A. hippocastanum and B. sempervirens extracts caused a decrease in the consumption of food per 1 mg of larva body weight accompanied by the higher body weight loss values of 76.3 and 58.6 mg/larva, respectively. The highest food consumption (59.6 mg/larva) was observed with the extract from D. kaki and the lowest food consumption (17.6 mg/larva) was with the extract from A. hippocastanum. The other tested extracts showed similar activities. © TÜBİTAK.