Multi-stress exposures are considered the most putative cause of honey bee decline.
Three experimental sites were chosen: a control one far from direct anthropogenic stress sources, a pesticide-stress site and multi-stress one, adding to the exposure to pesticides the presence of an electric and magnetic field from a high-voltage power line. The distance between the two sites was about 300 m (near enough to be exposed by foraging activities to the same exposure to pesticides, but sufficiently distant to be differently exposed to the electric and magnetic fields). In each experimental site (control, pesticide- and multi-stress), four colonies of honey bees were used. Apiaries were monitored weekly for one year.
Exposure duration: continuously
The data showed that bee health conditions were the worst in the multi-stress site (pesticide exposure and exposure to electric and magnetic fields) with only one colony alive out of the four colonies present at the beginning. In this site, a complex picture of adverse effects was observed, such as disease appearance (American foulbrood), higher mortality, behavioral alterations (queen changes, excess of honey storage) and biochemical anomalies (higher alkaline phosphatase enzyme activity at the end of the season).
Even if this study did not allow for testing of the synergistic vs. additive effect of the multi-stress conditions, as single stress was only tested for pesticide-stress, the authors suggest that at least additive effects can be defined for the multi-stress condition investigated.
In conclusion, the results indicate that the multi-stress condition was able to induce biochemical, physiological and behavioral alterations which severely threatened bee colony survival.