Effects of sub-lethal imidacloprid doses on the homing rate and foraging activity of honey bees

Laura Bortolotti, Rebecca Montanari, José Marcelino2, Piotr Medrzycki, Stefano Maini,
Claudio Porrini

For several years, reports by French and Italian beekeepers have been suggesting a lethal effect of imidacloprid on honey bees; in particular, the molecule has been related to honey bee mortality and decrease of hive populations, affecting the orientation and ability of honey bees to return to the hive.
In this paper we investigate the effects of sub-lethal doses of imidacloprid on foraging activity and homing ability of honey bees. Honey bees from one hive were trained to forage on an artificial feeder filled with a 50% sucrose solution. The feeder was gradually moved up to a distance of 500 meters from the hive. Thirty bees, foraging on the sucrose solution, were captured, individually marked with coloured number tags and transferred into a flying cage, acting as control. The feeder was then replaced with a new one, filled with an imidacloprid supplemented sucrose solution. Again, thirty bees foraging on this feeder were captured, individually marked with different coloured number tags and transferred into an other flying cage. Three concentrations of imidacloprid were tested: 100 ppb, 500 ppb and 1000 ppb. The solutions at 500 ppb and 1000 ppb of imidacloprid had a repellent effect and the bees stopped visiting the feeder, hence only 10 and 20 honey bees, respectively, were captured for the two doses.
Since the effects of imidacloprid start half an hour to one hour after ingestion, bees were released from the flying cage 1 hour after confinement. After the release, the behaviour of the bees was followed for 2 hours: two observers at the hive and one observer at the feeding site recorded the arrival and the departure of the marked bees. The presence of the bees at the hive and at the feeder was also recorded for one hour, 5 and 24 hours after the release.
The results show that almost all the control honey bees returned to the hive, and started again visiting the feeder between 2 to 5 hours after the release. Honey bees fed with the concentration of 100 ppb also returned to the hive, but they returned to visit the feeder only 24 hours after the release. Honey bees fed with 500 ppb and 1000 ppb completely disappeared after the release, and they were not seen during the following 24 hours, neither at the hive nor at the feeding site.

Bron: Bulletin of Insectology 56 (1): 63-67, 2003