15
Nov
Synthetic Fertilizers and Pesticides Make Plants Less Attractive to Bumblebees, Research Shows
(Beyond Pesticides, November 15, 2022) Spraying a flowering plant with synthetic fertilizers makes it less attractive to bumblebees, according to research published this month in PNAS Nexus. “A big issue is thus—agrochemical application can distort floral cues and modify behaviour in pollinators like bees,” said study author Ellard Hunting, PhD, of the University of Bristol, UK. The findings underscore the limited understanding that proponents of chemical agriculture have for the complex processes that food production relies upon and reinforce calls for a broad scale transition to regenerative, organic farming practices.
Scientists began with the knowledge that spray applications of various agrichemicals affect the visitation patterns of bumblebees and other pollinators through a range of different processes. Past research finds that notorious bee-killing neonicotinoid insecticides not only kill bees outright, but also result in a range of complex damage, including their ability to impede bees’ olfactory senses and adversely affect their vision and flying ability. Other chemicals like glyphosate weaken bees’ ability to distinguish between colors.
A growing area of research is investigating the ways in which pollinators use static electric fields surrounding flowers to find food sources. A 2013 study found that bumblebees use floral electrical fields to discriminate between potential food sources. Subsequent reporting shows that bees use “mechanosensory” hairs on their body to detect these fields, that find that other pollinators like the hoverfly also use these cues. Flowers produce these fields “from the negative bio-electric potential within the flower and positive charges in the atmosphere, including the electrosphere and positively charged insects such as bees,” the study explains. Pollinators interacting with flowering plants can change a flower’s electric field, as the plant responds by producing more sap, for instance.
The authors confirm that applications of synthetic fertilizers and the neonicotinoid insecticide imidacloprid to flowering plants both resulted in significantly reduced foraging by bumblebees. To make this determination, a series of experiments were conducted to rule out other factors. To test whether the fertilizer was adversely affecting visual cues, researchers observed the reflectance spectra of spray applications with fertilizer, finding that they were no different than simple demineralized water. To rule out odor as a factor in their findings, bees were provided sugar solution with and without fertilizer, and found that the bees showed no preference for one or the other.
Then researchers began exploring the effect of agrichemicals on the electric fields flowers produce. Cut flowers (Geranium pratense) were sprayed with simple water, or water with fertilizer, both with the addition of positively charged colored particles in order to observe the electrostatic deposition of the colored particles. The colored particles show significant differences between the two applications. The experiment was then repeated with a rooted, still growing flowering plant (Jacobaea vulgaris), and this time researchers measured the electrical field around the flower. Scientists found that fertilizers increase the flower’s electric field, which then slowly returned to its previous state.
Digging deeper into the issue, researchers focused in on the bio-electric potential energy within a plant’s stem. Plants are known to respond to environmental stressors like cutting/herbivory and chemicals by changing the water flow and ion transport within their stems, which can subsequently be measured and manipulated. Moreover, changes observed within the bio-electric potential of a plant’s stem are directly proportional to a plant’s floral electric field. To test this process, cut Lavandula angustifolia flowers were sprayed with either water or a water-synthetic fertilizer solution, and their stem potential changes were measured. While water resulted in a change in stem potential that lasted up to a minute, synthetic fertilizers changed stem potential for 16 minutes, and the neonicotinoid imidacloprid showed alterations that lasted for up to 25 minutes. These measurements aligned directly with observed declines in bumblebee foraging interest in flowers recently sprayed with the agrichemcials. The authors note, “Since many chemicals used in agriculture and horticulture carry an electric charge, the observed mechanism could potentially be relevant for a wide array of chemicals.”
To add additional weight to their findings, scientists conducted another experiment in which they artificially maintained flower stems with altered electrical signals mimicking the changes seen with a fertilizer or insecticide application. Manipulated flowers experienced 62 bumblebee approaches, while unaltered control flowers saw 47. However, only out of this 62 only 35 bumblebees landed on the manipulated flowers, while control flowers received 43 landings. “This suggests altered floral E-fields affect bee foraging when approaching the flower, and that bumblebees can detect and discriminate small and dynamic alterations in the electric landscape induced by agrochemical deposition,” the study explains. To provide further context, study co-author Sam England, PhD adds, “It’s much like motorboat noise that hinders the ability of fish to detect their predators, or artificial light at night that confuses moths; the fertilisers are a source of noise to bees trying to detect floral electrical cues.”
Not only do these chemicals interrupt the daily foraging of pollinators, they may also represent a longer-term threat for pollinators. In a final experiment, researchers mimicked a rain event after an initial fertilizer treatment. Plants responded with a similarly prolonged alteration of their electrical signal. The authors note that fertilizer applications may thus chronically reduce pollinator foraging either by recurring electric alterations after a rain event or from learned negative associations with the altered plant.
“The fact that fertilisers affect pollinator behavior by interfering with the way an organism perceives its physical environment offers a new perspective on how human-made chemicals disturb the natural environment,” Dr. Hunting notes.
These results fly in the face of outdated toxicological approaches that agrichemical companies hide behind when confronted with the on the ground impacts of their dangerous products, such as 15th century Paracelsian concept that “dose makes the poison.” As modern science delves deeper into the inner working of plants and insects and the interactions between these critically important groups, it finds the world to be incredibly more complex than an inaccurate truism.
Solutions to the problems of chemical-intensive agriculture exist and provide proof of concept that farming can occur without a range of negative impacts on the surrounding environment. Organic agriculture has never permitted the use of synthetic fertilizers, nor do organic farmers ever use synthetic insecticides like the bee-killing neonicotinoid imidacloprid. Instead, regenerative organic farming embraces a natural systems approach, taking efforts to work with and enhance the existing ecological services in their region. Organic farming yields multiple bottom line benefits for wildlife and the wider environment, human health and the economy.
For more information on the dangers of synthetic fertilizers and alternative, organic companies you can support, see Beyond Pesticides page on Fertilizers Compatible with Organic Landscape Management. Get active in your own community to eliminate synthetic fertilizers and toxic pesticides by sending a letter to your local officials today.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: PNAS Nexus, University of Bristol press release
