Fertilizers Alter Electric Field of Flowers, Changing Bee Behavior

Researchers find that bees can detect electric field (Efield) changes surrounding flowers after they have been sprayed with fertilizers. The study is published in the journal PNAS Nexus.

Fertilizers impacting bee populations

Fertilizers, pesticides and other chemical mixtures are often applied in large-scale sprays to crop fields by farmers. These substances might be used to enhance nutrient supply to the crops or protect against pests, for example.

The high volume of chemicals used in agriculture is adversely affecting pollution rates, and has been recognized as a contributing factor to the declining population of bees and their reduced diversity. Agricultural chemicals have also been shown to alter the behavior of pollinators – like bees – in how they interact with plants. Why exactly these changes occur has remained elusive until now.

A new study by researchers at the University of Bristol, led by Dr. Ellard Hunting, research associate in sensory biophysics, has tested the impact fertilizer sprays can have on floral cues used by bees, such as vision, smell and Efields.

“We are biologists, and we study how different organisms use the static Efields that are virtually everywhere in the environment,” says Hunting. “For instance, flowers have an Efield that bees can sense. These Efields of flowers can change when the flower has been visited by a bee, and other bees can use that information to see whether a flower has been visited. The positive electric charge of bees also helps in the transfer of pollen from plants, as pollen are typically negatively charged.”

Testing how agricultural chemicals change plant Efields

The fertilizers used in the study were formulas that can be bought commercially. “Virtually all the fertilizers you buy have the same compound formula (nitrogen, phosphorus and potassium),” says Hunting. A pesticide – imidacloprid – was also used in the chemical mix. This pesticide is banned for outdoor use in the UK and the EU, though the UK does permit emergency applications. In the US and other countries, imidacloprid is still utilized widely.

The researchers found that spraying plants with the agricultural chemicals did not affect cues such as vision and smell. However, there was a response to the Efield surrounding the flower.

Hunting and colleagues measured the bio-electric potential in the stem and found that applying chemicals altered this field for up to 25 minutes. In nature, stimuli such as the blowing of the wind can also change the bio-electric potential of the stem, but this lasts a maximum of 30 seconds, typically. Even when mimicking the effect of rain fall post-application of the chemical mixture, the Efield change persisted.

Can bees detect these changes? How does it impact their behavior?

To investigate whether bees could detect the Efield changes, the researchers manipulated flowers such that their electrical signature matched that of the change caused by fertilizer application in the field. Bees were less likely to land on these flowers compared to un-sprayed flowers.

“This is the first study to show that chemical-induced changes in plant electrophysiology translate to a change in floral Efields,” explains Hunting. “So, chemistry translates to a change in plant physiology, changing the physics around a flower, which in turn translates to a change in bee behavior. What happens is that the plant has a stress response to the chemical. This results in metabolite (ions) transportation to the flower, and this ion flux is essentially an electric current. This translates to changing Efields.”

The impact of human-made chemicals

The study emphasizes the potential impact that human-made chemicals can have on the natural environment and pollinator behavior. 

Hunting and colleagues hope that their work can spread awareness – not all interferences caused by humans are obvious and easily detectable, but they are certainly there. It is also possible that other insects known to be electroreceptive, such as hoverflies or nematodes in the soil, could be affected by perturbations to plant/ soil Efields.

“The results have been obtained across both the field and laboratory environments, but it remains a reductionist approach,” Hunting reflects on the study. Whether the mechanisms observed indeed operate in complex agricultural settings remains to be explored. This will be the focus for future research by the Bristol team – to test the data’s relevance in more complex field settings, to explore other biological interactions, assess other chemicals such as nanoplastics and – eventually – develop chemicals that can trigger less of a physiological response in flowers.

Reference: Hunting E, England S, Koh K et al. Synthetic fertilizers alter floral biophysical cues and bumblebee foraging behavior. PNAS Nexus. 2022. doi: 10.1093/pnasnexus/pgac230.

Dr. Ellard Hunting was speaking to Molly Campbell, Senior Science Writer for Technology Networks.