Lethal and sublethal effects of two bacterial-based pesticides on Bombus terrestris (Hymenoptera: Apidae)

Abstract

In this study, the lethal and sublethal effects of Bacillus thuringiensis var. kurstaki (Btk) and Bacillus velezensis (formerly B. amyloliquefaciens) QST 713 on Bombus terrestris were determined. The worker bees and colonies were fed sugar syrup prepared with four doses of both bacteria, including a 1/1 dose as the recommended field application dose, and 1/10, 1/100, and 1/1000 doses. In the lethal effect experiments, 90 caged bee boxes (a total of 900 worker bees) were used. In sublethal effect experiments, 90 B. terrestris colonies with 15-20 worker bees were used, and characteristics of colony development were determined. In the lethal effect experiment, the corrected mortality rates at 1/1 and 1/10 doses of both bacterial agents were statistically significantly higher than from the other doses on day 28 (p < 0.001). When B. terrestris workers were exposed to both bacterial agents for the third time, the corrected mortality rates were 100% and 57% for B. thuringiensis var. kurstaki (1/1 dose) and B. velezensis QST 713 (1/1 dose), respectively. In the sublethal effect experiment, the first male emergence time at 1/1, 1/10, and 1/100 doses of Btk was significantly different from that at 1/1000 of Btk, all doses of B. velezensis QST 713, and the control. The time of first queen emergence at Btk doses of 1/10 and 1/100 differed from the control. The competition point at the 1/10 dose of B. velezensis QST 713 was significantly different when compared to the 1/1, 1/10, and 1/100 doses of Btk. According to the results, Btk was more harmful than B. velezensis QST 713 for B. terrestris workers.