Bulletin of Insectology 56 (1): 63-67, 2003 ISSN 1721-8861 Effects of sub-lethal imidacloprid doses on the homing rate and foraging activity of honey bees Laura BORTOLOTTI 1 , Rebecca MONTANARI 1 , José MARCELINO 2 , Piotr MEDRZYCKI 1,3 , Stefano MAINI 3 , Claudio PORRINI 3 1 Istituto Nazionale di Apicoltura, Bologna, Italy 2 Departamento de Protecção Vegetal, Escola Superior Agrária de Castelo Branco, Portugal 3 Dipartimento di Scienze e Tecnologie Agroambientali, Università di Bologna, Italy Abstract 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, indi- vidually 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 cap- tured, 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. Key words: imidacloprid, Apis mellifera, sub-lethal doses, behavioural effects, foraging, orientation. Introduction In recent years, many beekeepers from European and American countries have been complaining about un- usual bee mortality rates and high honey bee losses. In France, one molecule was indicated as the main respon- sible for these damages, imidacloprid, particularly in its formulation Gaucho®. French and Italian beekeepers report that hives placed near sunflower (in France) and maize (in Italy), originated from seeds dressed with Gaucho®, show high levels of damage due to a progres- sive decline in the hive populations (the so called “disappearing disease”), until the complete loss of the colonies. Trying to connect the beekeepers’ complaints with the observed effects of imidacloprid, one question arises: can sub-lethal doses of imidacloprid be responsible for the “disappearing disease”? Is there a link between the use of imidacloprid and the effects reported by beekeep- ers? In a first experiment, during summer 2001, our group investigated the effects of sub-lethal doses of imidaclo- prid on the foraging behaviour of honey bees. Bees were trained to forage on an artificial feeder, then orally treated with two concentrations of imidacloprid (100 ppb and 1000 ppb) and immediately released. We re- corded the number of honey bees returning to the hive and their foraging activity on the feeder for the follow- ing two days. We found a decrease in the foraging ac- tivity of bees treated at the two doses, lasting until the second hour after the treatment, but we did not find any effect on the honey bees’ homing ability. All the treated bees were able to return to the hive within two hours from the treatment (Bortolotti et al., unpublished data). Our results were coherent with those of a comprehen- sive field research on the effects of imidacloprid on honey bees, presented by Curé et al. (2001). In this study the authors found that imidacloprid at a concen- tration of 100 ppb, disrupts the communication of the location of the food source between foraging bees and house bees, causing a temporary reduction or the end of the foraging activity. However, foraging activity is re- stored the day after the treatment, and no other adverse effect was noticed. One hypothesis to explain these results is that treated honey bees, returning immediately to the hive, regurgi- tate the content of their stomach and therefore the active ingredient does not have enough time to enter the bees’ body fluids. In fact, from the results of a laboratory study, we know that the active ingredient starts to affect the behaviour of treated bees only 30-60 minutes after ingestion (Medrzycki et al., 2003).