Background The pine processionary moth, Thaumetopoea wilkinsoni Tams, 1926 (Lepidoptera/Notodontidae) is one of the most harmful insects that destroys pine ecosystems by feeding on pine leaves at its larval stage. Because of its urticating setae, the insect also causes severe skin reactions to animals and humans. Instead of chemical control, eco-friendly biological control methods are preferred to combat this species. Results The purpose of this study was to evaluate the efficacy of five different Beauveria bassiana Vuill, 1912 (Hypocreales/Cordycipitaceae) isolates (TR-SM-10, TR-SM-11, TR-SM-2, TR-SK-1 and TR-D-1) and one B. pseudobassiana Rehner & Humber (Hypocreales/Cordycipitaceae) isolate (TR-SM-1) against the fourth instar larvae of T. wilkinsoni under laboratory conditions. T. wilkinsoni larvae were collected from the Ondokuz Mayis University Kurupelit Campus in Samsun, Turkey, in 2021, and the fourth instar larvae were used in the experiment. Two ml of spray of the six fungal isolates were applied to every ten larvae at each concentration (1 x 10(7) and 1 x 10(8) conidia ml(-1)). The experiment was carried out in five replicates per group, and the larvae were observed for 10 days. As a result, all isolates of B. bassiana caused 100% mortality at 1 x 10(8) conidia ml(-1) concentration. B. pseudobassiana isolate also caused 100% mortality at both concentrations. At 1 x 10(7) conidia ml(-1) concentration, the larvae treated with the B. pseudobassiana isolate (TR-SM-1) had the lowest LT50 (2.89 days) and LT90 values (4.79 days), while the larvae treated with TR-SM-10 isolate had the highest LT50 (5.65 days) and LT90 values (9.39 days). At 1 x 10(8) conidia ml(-1) concentration, the larvae treated with TR-SK-1 isolate had the lowest LT50 (2.89 days) and LT90 values (4.79 days), while those treated with TR-SM-10 isolate had the highest LT50 (3.95 days) and LT90 values (8.15 days). Conclusion It has been recommended that the five different isolates of B. bassiana and B. pseudobassiana isolates were virulent to T. wilkinsoni larvae and can be used for biological control of T. wilkinsoni.