The term synergy means that a combined effect is greater than the sum of the effects. Basically, if two stressors act synergistically, it means that the effect of the two stressors together is more than what you would see if you just added together the effects from when the stresses occur alone. We see synergistic effects when one stressor increases the risk or magnitude of the effect of another stressor.
Pollinators can come into contact with many stressors simultaneously. While foraging, they can become exposed to chemicals like herbicides, insecticides, fungicides, antibiotics, repellants, systemics, growth regulators, heavy metals, and diesel exhaust. Some exposures may not be that harmful if experienced alone, but become very harmful in the presence of another environmental hazard.
The types of chemicals that bees are commonly exposed in their environment.
Even chemicals that are deemed “safe for bees” can become very hazardous when they are combined with other chemicals. For example, many fungicide products that are used on plants can make the effects of certain pesticides much worse. Insects have very simple detoxification systems. It can be very dangerous for a pollinator to experience multiple hazards at once, because the presence of one hazard may prevent it from detoxifying a second one. If the first hazard wasn’t there, the pollinator may be able to clear the second hazard, and no effect would be seen. Because the pollinator’s detoxification system is tied up with the first hazard, the second one can cause increased harm.
Synergistic effects can make it very difficult to characterize pesticide risks. We have to be very careful with what we use, because we don’t know what other exposures a pollinator is experiencing.