Fertilizers change how bumblebees ‘see’ flowers

A new paper in PNAS Nexus, published by Oxford University Press, indicates that fertilizers and pesticides can change the way bees ‘see’ a flower, and this reduces the number of bees that visit a flower.

Flowers produce a diverse range of cues and attractants to insects that promote feeding and pollination. Bees use colour, sun and magnetic fields to navigate the landscape. On a smaller scale, they use cues like flower odour and colour, but also humidity and electric fields to identify plants. 

Farmers routinely apply mixtures of chemicals, in particular fertilizers, on plants using large-scale spray applications. The widespread use of chemicals in agriculture and horticulture has been linked to reductions in bee population size and diversity.

While researchers have long recognized that many of these chemicals are toxic, they know little about how agrochemicals affect the immediate interaction between plants and pollinators. Spray applications can change the properties of flowers in several ways. Many agricultural chemicals carry an electric charge designed to adhere to plants. Thus, spray applications can potentially change the electric fields surrounding a flower.

A big question is whether chemical application can distort floral cues and modify behaviour in pollinators like bees.

Researchers tested the effect of fertilizer sprays on various floral cues used by bees. They observed that the chemical did not affect vision and smell, but there was a response in the electric field surrounding the flower. To visualize this, the researchers sprayed cut flowers with positively charged, coloured particles released as an aerosol.

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To get a better idea of what changed in the flower, the researchers measured an electric property of the flower, the bio-electric potential in the stem. This potential is an important source of the electric field surrounding flowers. 

They observed that sprays with chemicals changed this bio-electric potential for up to 25 minutes. This change is substantially longer than natural fluctuations, such as those caused by wind, and aligns with observed declines (about 20 minutes) in bee-feeding efforts observed in nature. Interestingly, they observed that the plant had the same lasting response when there was a simulated rain event after the use of chemicals, suggesting the effect persists beyond the single use of chemicals. 

To test whether the observed changes in the plant’s electrical signature are indeed perceived by bees, the researchers mimicked the electrical changes caused by fertilizers in the field by electrically manipulating flowers. While bumblebees were approaching the flowers, they observed that bees were less keen to land on a flower that was electrically manipulated compared to a control flower that was not manipulated. This showed that bees were able to detect and discriminate the small and dynamic electric field alterations that are caused by agricultural chemicals.

The fact that fertilizers affect pollinator behaviour by interfering with the way an organism perceives its physical environment offers a new perspective on how human-made chemicals disturb the natural environment and emphasizes the importance of considering the seemingly hidden senses that are used by insects to understand and learn about their environment.

“That fertilizers affect bee behaviour by changing the way it experiences its physical environment gives a new perspective on how humans disturb the natural environment,” said the paper’s lead author, Ellard Hunting. 

“Imagine yourself not being able to distinguish apples from tomatoes because someone sprayed some chemicals in the vegetable department. This may be relevant for all organisms that use the electric fields that are virtually everywhere in the environment.’’