Taking account of environment of bees to better evaluate insecticide-related risks

Faced with the worrying phenomenon of "bee decline," researchers, engineers, farmers and beekeepers have been working together to try and explain its causes. In 2012, a study carried out by INRA in collaboration with ACTA, CNRS and ITSAP-Institut de l'Abeille had already shown that low doses of an insecticide could disturb the orientation of bees and increase losses during their foraging activities. As a continuation of this study, the scientists have since analysed this disorientation phenomenon as a function of both environmental parameters, weather conditions and landscape complexity.

Meteorological conditions and landscape complexity as factors for variation

The scientists fixed RFID microchips onto the thorax of nearly a thousand bees. These microchips were able to check on the return to the hive of individuals, thanks to a series of electronic sensors. The foraging bees had or had not been exposed previously in the laboratory to non-lethal doses of Thiamethoxam, the active substance in a pesticide used by farmers. The bees were then released 1 km from their hive into landscapes with different structures (a bocage landscape (with enclosures) or a plain given over to intensive agriculture) and under more or less favourable weather conditions (clear skies and temperatures higher than 28°C or cloudy skies and temperatures between 15°C and 20°C).

The results revealed a significant influence of weather conditions and landscape complexity on bee sensitivity to the insecticide. The scientists were able to establish that the pesticide induced an average risk of loss that rose from 3% to 26% (or one bee in four) when the weather conditions became unfavourable. This insecticide-related loss rate was also modulated by the landscape environment, reaching 35% (one bee in three) in bocage landscapes versus 18% in open landscapes with a less complex structure.

The sensitivity of bees to the insecticide was therefore not identical everywhere and in all types of weather, but varied as a function of environmental conditions. The scientists were thus able to show that depending on the landscape or weather context, the effects of a pesticide could be under- or over-estimated by a factor of six.

Bocage which becomes a maze

To return to the hive, bees navigate according to the position of the sun and visual landmarks (trees, hedges, forest boundaries) that they have memorised during their past foraging experiences. In unfavourable weather conditions, bees tend to use visual landmarks more to navigate, but this study showed that they did not appear to achieve this if they had been exposed to the insecticide. Exposure to low doses of pesticide thus appeared to alter their ability to call upon their spatial memory. A dense network of trees and hedges (bocage) thus became a veritable maze for these bees who had become less able to recognise their landmarks. It is possible that the increased loss rates during poor weather may also be linked to the additional physiological and energy constraints involved when flying at low temperatures.

A new source of variability concerning the effects of pesticides

The scientists now need to explore the complexity of these environmental and toxicological interactions. Several years ago, they had already evidenced the interactive effects of different insecticides (the "cocktail" effect) or between insecticides and pathogenic agents -- the effects of the insecticides being exacerbated in bees already weakened by viruses or parasites. This new study reveals an interaction of a different type: between pesticides and the environmental context. By characterising the environmental conditions that constitute the most risks for bees, scientists will be better able to evaluate toxicological risks in the field and improve the design of epidemiological watch networks.