VIII SIMPOSIO INTERNAZIONALE ICPBR

HAZARDS OF PESTICIDES TO BEES

Bologna, 4-6 Settembre 2002

Effetti dell’imidacloprid sulle api

Effects of imidacloprid administered in sub-lethal doses on honey bees’ (Apis mellifera L.) behaviour. Laboratory tests

Piotr Medrzycki (1) (2), Rebecca Montanari (1), Laura Bortolotti (1), Anna Gloria Sabatini (1), Stefano Maini (2) and Claudio Porrini (2).

(1)Istituto Nazionale di Apicoltura, via di Saliceto 80, 40128 Bologna, Italy. E-mail: piotr@entom.agrsci.unibo.it
(2) Dip. di Scienze e Tecnologie Agroambientali, University of Bologna, via Filippo Re 6, 40126 Bologna, Italy

Imidacloprid is a molecule of which the effect on honey bees may not be, in certain cases, of immediate perception. In fact, at very low doses, although it does not cause the death of individuals, it may cause behavioural changes that can lead to temporary damage in all the family. The aim of the present research was to verify the impact of the active ingredient, administered at sub-lethal doses, on the behaviour of the specified insect and to analyse how it changes over time. For this reason experimental laboratory tests were outlined.

The bees, isolated in experimental cages (in groups of 10 individuals) were placed in a climatic chamber at 25°C and in 24 hours darkness. Their activity was monitored with infrared camera and video-recorded. The dietary supply was given in the form of a sugar solution at 50%. The substance in analysis was added to the sugar solution at two different levels of concentration (100 ppb and 500 ppb) and tested by ingestion. Two administration procedures (unique dose - 20 µl per bee and ad libitum) were carried out. Four theses were this way obtained, always accompanied by a control thesis. For each thesis three repetitions were conducted. Insect behaviour was subsequently analysed with the help of "The Observer®" computer program, from Noldus Information Technology B.V. Thus, the average percentage of time spent by the bees on each of the three behaviours related to mobility: "stationary"; "walking" and "running" was measured. To follow the behaviour evolution of the bees over time, the first two hours after administration were analysed constantly; then a subsequent 30 minutes after 5.30 hours and then another 30 minutes after 22 hours were evaluated. The bee mortality in each cage was measured as well.

During the two first hours after administration (from ad libitum procedure) the "stationary" behaviour in the bees treated with imidacloprid at 500 ppb and at 100 ppb was, respectively, 81% and 29% more frequent in comparison to the control thesis (U-test, p<0.05). On the other hand, when analysing only the second half-hour and the second hour, this difference was respectively 92% and 181% for 500 ppb and 46% and 69% for 100 ppb (U-test, p<0.05). In the first half-hour after administration, no significant difference was found. Similar results were obtained from the unique dose administration procedure. It was obvious that the substance in question had a negative effect on the insect mobility, which started to be revealed only sometime after ingestion. It was also observed that each bee subsequently ran for a determined period of time and than remained stationary.

The apparent difference between the control and the treated bees was the fact that the former had a regular tendency to reveal this behaviour in a co-ordinated way, that is: almost all of them running vs. almost all of them being stationary. This phenomenon was not observed in the treated bees, where each individual did not seem to be influenced by other individuals’ behaviour. This observation is supported by the Brown-Forsythe HOV test (p<0.05), applied to the variance in the number of "running" bees in the treated group, confronted with those in the control. This result could indicate a decreasing communicative capacity in the treated bees with a subsequent decline in the social behaviour. However, six hours after the dose administration, no impact of imidacloprid was observed. The mortality rate did not show any significant differences between the treated and the control theses (U-test, p<0.05), regarding the fact that the doses used where sub-lethal.

Taking into account the results obtained from the present research, we can forecast that bees, accidentally intoxicated in a field with imidacloprid, could find difficulties in returning to the hive, and in spite of the fact that the molecule activity period may be low, it could also be lethal to individual bees.