The delicate buzz of the honey bee, a sound synonymous with productive orchards and flourishing fields, may be growing quieter, and the very tools used to protect our crops could be contributing to this silence.
Honey bees (Apis mellifera) are far more than just producers of golden honey; they are indispensable pollinators in global ecosystems and agriculture. It is estimated that a staggering 90% of the world's flowering plants rely on animal pollinators to reproduce, and bees play a leading role in this vital process. They are responsible for pollinating over 70 of the 100 crop species that provide the majority of the world's food, making them a cornerstone of both natural biodiversity and our food security.
of flowering plants depend on animal pollinators
crop species pollinated by honey bees
However, in recent decades, bee populations have faced alarming declines, a complex phenomenon driven by a web of factors including habitat loss, pathogens, and climate change. Among these stressors, exposure to pesticides has emerged as a significant concern. The scientific and regulatory scrutiny that led to restrictions on certain neonicotinoid insecticides created a demand for alternatives. One such class of chemicals that has entered the agricultural landscape is the sulfoximines, with sulfoxaflor being the first and most prominent member. While developed to be more target-specific and to combat insecticide resistance, a growing body of scientific evidence is revealing that this insecticide may not be as benign to pollinators as once hoped, posing subtle yet serious threats to the health of honey bee colonies.
To understand the threat sulfoxaflor poses to bees, we must first look at its mechanism of action. Sulfoxaflor is a systemic insecticide, meaning it is absorbed by the plant and distributed throughout its tissues, including the pollen and nectar that bees forage on. Like its controversial neonicotinoid cousins, sulfoxaflor targets the nicotinic acetylcholine receptors (nAChRs) in the insect nervous system. These receptors are critical for facilitating nerve impulses.
Sulfoxaflor acts as a potent agonist at these receptors, essentially mimicking the natural neurotransmitter acetylcholine and binding to the receptor sites. However, unlike the natural compound, sulfoxaflor overstimulates the nervous system.
This leads to uncontrolled neurotransmission, which can cause muscle tremors, paralysis, and eventually death in target pests. What makes sulfoxaflor particularly effective against resistant insects is its unique chemical structure, centered on a sulfoximine moiety, which makes it a poor substrate for the detoxification enzymes that render other insecticides ineffective.
The problem, however, is selectivity. While designed to target sap-feeding pests like aphids, the fundamental neurobiology of the honey bee is similar enough that they are vulnerable to the same effects. When foraging bees consume nectar or pollen containing sulfoxaflor residues, the insecticide enters their system and binds to their own nicotinic acetylcholine receptors, triggering a cascade of sublethal physiological effects that can be just as damaging as immediate death to the individual bee and the overall health of the colony.
While acute toxicity studies are important, the real-world threat to bees often comes from chronic exposure to low, sublethal concentrations of pesticides. A pivotal 2024 study published in Pest Management Science delved into precisely these effects, providing a comprehensive view of how field-realistic and sublethal doses of sulfoxaflor impact honey bees at different levels, from their development to their molecular machinery.
The researchers designed a multi-pronged investigation on honey bee colonies with low pathogen loads. The study was conducted in two main parts:
Bee larvae were reared in the laboratory and divided into control and treatment groups. The treatment groups were fed a diet containing sulfoxaflor at a field concentration (5.0 mg/L). The researchers then meticulously tracked key developmental indicators, including larval survival, pupation rates, and emergence rates (eclosion). They also measured changes in physiological markers such as weight, and the levels of energy reserves—glycogen, fat, and protein—at different developmental stages.
Adult bees were exposed to a lower, sublethal concentration (2.0 mg/L) of sulfoxaflor. The researchers then used a powerful technique called RNA sequencing (RNA-Seq) on the heads of these bees to analyze changes in their gene expression profiles. This allowed them to see which biological pathways were being disrupted by the insecticide at a molecular level.
The findings from this experiment painted a concerning picture of sulfoxaflor's sublethal impact.
The data on bee development revealed clear detrimental effects. The table below summarizes the key developmental parameters that were inhibited by exposure to the field concentration of sulfoxaflor 3 .
| Developmental Parameter | Effect of Sulfoxaflor |
|---|---|
| Larval Survival Rate | Decreased |
| Pupation Rate | Inhibited |
| Emergence Rate (Eclosion) | Inhibited |
| Energy Reserves (Pupal Stage) | |
| ↳ Fat Content | Significantly Increased |
| ↳ Glycogen Content | Decreased |
The transcriptomic analysis provided a molecular explanation for these observations. Exposure to the sublethal concentration led to the down-regulation (silencing) of many critical genes. The affected genes were involved in a wide range of vital biological systems, including 3 immune function, detoxification, nervous system function, and olfaction.
In essence, the study demonstrated that sulfoxaflor doesn't have to kill bees outright to cause harm. It can inhibit their normal development, disrupt their energy metabolism, and compromise their immune and nervous systems, making them more vulnerable to other environmental stressors.
The findings from the featured experiment are not an isolated case. They are part of a growing consensus among toxicologists and entomologists regarding the risks of sulfoxaflor. Research from other institutions has consistently shown that chronic exposure to field-realistic concentrations can have profound consequences.
A 2022 semi-field study found that feeding colonies a minuscule concentration of a sulfoxaflor formulation (Closer® at 0.3 ppb) for 21 days led to a significant decline in overall colony health and productivity. The forager bees themselves were smaller and less effective in their duties 7 .
Concentration causing colony decline
Furthermore, a 2023 study investigated the physiological changes in forager bees exposed to the same low concentration. It found that sulfoxaflor significantly altered key immunological and physiological markers after just 15-20 days of exposure, suppressing the activity of enzymes vital for digestion, detoxification, and immune defense . Another study confirmed these findings, showing that sulfoxaflor exposure increased the activity of the detoxification enzyme glutathione-S-transferase (GST) and caused clear histological damage to the brain and midgut of honey bees 5 .
To conduct the sophisticated research outlined in this article, scientists rely on a suite of specialized reagents and methods. The following table lists some of the key tools used in studying sulfoxaflor's effects on bees, illustrating the multi-faceted approach required in modern toxicology.
| Reagent / Method | Function in Research | Example of Use |
|---|---|---|
| Technical Grade Sulfoxaflor | The pure active ingredient; used to study intrinsic toxicity without formulation additives. | Assessing precise mechanisms of action and receptor binding 1 . |
| Formulated Products (e.g., Closer® SC) | Commercial products containing active ingredient + adjuvants; used for realistic risk assessment. | Semi-field in-hive studies to mimic real-world exposure scenarios 7 . |
| RNA Sequencing (RNA-Seq) | A technique to profile the expression of thousands of genes simultaneously. | Identifying down-regulated genes in immune, digestive, and nervous systems 3 . |
| Enzyme Activity Assays | Biochemical tests to measure the activity of specific enzymes (e.g., GST, GOX, ALP). | Quantifying detoxification, metabolic, and immune response stress 5 . |
| Histological Staining | Using dyes to visualize tissue and cellular structures under a microscope. | Revealing brain cell degeneration and midgut damage 5 . |
RNA sequencing reveals gene expression changes at the molecular level.
Enzyme activity tests quantify physiological stress responses.
Tissue staining visualizes cellular damage in organs.
The evidence is clear and accumulating: sulfoxaflor, an insecticide designed to solve one agricultural problem, presents a significant, subtle threat to honey bees. The danger lies not only in immediate mortality but, more insidiously, in the chronic, sublethal effects that undermine the very foundation of a colony. From stunted development and altered energy reserves to suppressed immune function and neurological damage, the physiological toll is heavy.
"This research presents society with a difficult challenge: balancing the need for effective pest control in agriculture with the urgent imperative to protect our pollinators."
The findings call for a precautionary approach and more sophisticated risk assessments that fully account for sublethal and colony-level effects, not just acute mortality. As we move forward, the health of the humble honey bee must remain a key indicator of our ability to cultivate our crops in a sustainable and ecologically harmonious way. The silent buzz in our fields depends on it.
How do we balance agricultural productivity with pollinator protection? Further research is needed to develop insecticides that target pests without harming beneficial insects like honey bees.