Short- and long-term effects of neonicotinoid application in rice fields, on the mortality and colony collapse of honeybees (Apis mellifera)

Declines in honeybee (Apis mellifera ) colonies have elicited great concern worldwide. Recently, many Japanese beekeepers have implied that midsummer use of a new insecticide, neonicotinoid, in rice fields, is causing widespread mortality of neighboring honeybees and frequently resulting in colony collapse. Since few field experiments have directly tested the effects of neonicotinoids, I addressed four research questions in the field. The questions are: 1) Does clothianidin application in rice fields cause the collapse of neighboring honeybee colonies? 2) Is colony collapse related to hive distance from the rice field? 3) Is the number of dead honeybee workers after spraying, related to hive distance from the field? 4) What are the long-term effects of neonicotinoid use on honeybee colony growth, especially brood production? In the late summer of 2010, honeybee hives were placed adjacent to two separate rice fields for 1 week. The hives were placed at the distance of 0, 30, 60, and 90 m. After spraying clothianidin, a daily count of dead worker honeybees was done for a week. Hives were weighed, and capped-brood areas were estimated weekly, for 2 months following insecticide application. Although the average number of dead workers ranged from 40 to over 100 within 24 hours after spraying, only a few dead workers were observed in the subsequent days. Distance from the rice field had no significant effect on the number of dead workers. There were no collapsed colonies during the 2-month, post-spray observation period. Hive weight and capped-brood area did not significantly differ among those hives placed at varying distances from the rice field. These results indicated that clothianidin spraying of the rice field increased the mortality of the honeybees, but did not always cause colony collapse.