Examination of mass honey bee death at the entrance to hives in a paddy rice production district in Japan: the influence of insecticides sprayed on nearby rice fields

Abstract

SummaryBeekeepers in Japan have been finding as many as 1,000 dead honey bees piled up in front of hive entrances. They have observed that these mass bee deaths tend to occur at about the same time as measures are taken to control rice bugs, which appear every year during the summer in rice-producing districts. We set up and conducted controlled field studies from July to August in 2012 in a rice-producing district of northern Japan where frequent mass bee deaths are observed each summer. As had occurred in the area in previous years, and as we directly observed during our field investigations, masses of honey bees were found dead at the entrance to their hives. Samples of pollen and honey bee corpses were found to contain measurable concentrations of five active ingredients of insecticides: clothianidin, dinotefuran, ethiprole, etofenprox, and phenthoate. Insecticides containing these active ingredients are known to have been used for rice bug control in this area. Furthermore, we did not observe other known honey bee death agents in the area, such as mites and hornets. Thus, we suspected that the phenomenon of “piled honey bee death” (PD) at hive entrances is related to the influence of insecticides used to control rice bugs in paddies. The insecticide concentrations in dead honey bees were, however, extremely low compared with the median lethal dose (LD50) for honey bees. Moreover, PD was observed after rice anthesis, when bees would be collecting pollen from flowering plants other than rice. This calls into question any direct link between mass bee death and insecticides. Further study is necessary to understand insecticide exposure routes and the actual mechanism of the PD phenomenon.