Sublethal effects of thiamethoxam on immune system cells in the workers of Bombus terrestris (Hymenoptera: Apidae)

Neonicotinoids harm especially bumblebees and other species in agricultural areas all over the world. The toxic effect of thiamethoxam from the neonicotinoid group has been little studied especially on bees. This research aimed to evaluate the effects of thiamethoxam on the immune system cells of Bombus terrestris workers. Different ratios of 1/1000, 1/100 and 1/10 of the recommended maximum application dose of thiamethoxam formed the experimental groups. Ten foraging workers were used for each dose and control groups. Contamination was ensured by spraying the prepared suspensions at different ratios to the bees for 20 s at a pressure of 1 atm. The effects of thiamethoxam on the structures of immune system cells of bumblebees and the amount of these cells were investigated after 48 h of exposure. In general, anomalies such as vacuolization, cell membrane irregularities and cell shape changes were detected in prohemocyte, plasmatocyte, granulocyte, spherulocyte and oenocytoid in each dose group. Hemocyte area measurements in all groups were examined comparatively between groups. In general, granulocyte and plasmatocyte sizes were decreased, while spherulocyte and oenocytoid were increased. It was also determined that there was a significant decrease in the amount of hemocytes in the 1 mm(3) hemolymph as dose increased. The results of the study revealed that sublethal doses of thiamethoxam negatively affected hemocytes and their amounts of B. terrestris workers.