How the Active Ingredient Glyphosate Affects Bees
Glyphosate-based herbicides like the Roundup products have been investigated for a variety of health and ecological reasons. Despite the differences in glyphosate pesticide formulations, studies show that this active ingredient has lethal and sublethal effects on exposed bees. Readers familiar with the effects of herbicides and fungicides on bees know that extensive exposure to agrichemical products can affect bees at the whole organism level or within their gut microbiome.
In laboratory and semi-field studies on Apis mellifera, glyphosate exposure has been shown to harm honey bees by impairing their learning capabilities, memory formation and ability to distinguish between complex odours (Herbert et al., 2014).
This article examines the effects of Glyphosate-based herbicides in formulations like Roundup on bees, focusing on A. mellifera and the bumble bee Bombus terrestris. It investigates the sublethal effects and the toxicity of inert ingredients used in whole formulations that were previously reported as safe. It ends with suggestions on the best practices to minimise off-target effects during pesticide use on flowering plants.
There’s a shirt that goes with this article, designed & printed by yours truly:
Current Research on How Roundup Kills Bees
The effects of herbicides on bees tend to be chronic, sublethal effects arising from the active ingredients like glyphosate, or its adjuvant and inert ingredients such as surfactants (chemicals that assist the product in penetrating its target). The accumulation of chronic effects leads to a decrease in productivity within the individual bees, peaks into eventual colony collapse as a result of a dysfunctional population. Roundup is one of the many pesticide products that contain the herbicide glyphosate, and is frequently applied in residential and agricultural settings.
Bees are mainly to glyphosate when foraging on contaminated flowers or water sources (that have glyphosate runoff), and at the field level, the exposures are (usually) below lethal concentrations, producing subtle effects on bee health and behaviour.
Mortality Rates
A 2021 meta review by Battisti & colleagues stated that glyphosate-based herbicides are “now recognised as moderately toxic to bees”. Multiple studies have shown that bees in glyphosate treatment groups had higher mortality rates compared to the controls, both within laboratory and field conditions at field-realistic or label-recommended doses.
The 2014 study by Herbert and colleagues found no significant increases in honey bee worker mortality when exposed to glyphosate over 14 days. However, when honey and bumble bees were exposed to glyphosate products like Roundup, there was a significant increase in mortality. Such results strongly suggest that bee mortality is driven by the toxic, inert ingredients, rather than glyphosate use alone (Tang et al., 2023).
Further, the hygienic grooming behaviour of honey bees increases the risk of oral exposure to glyphosate from contaminated surfaces. Topical contact on the bee body hair triggers the autogrooming (self-grooming) response, which leads to oral ingestion of the compound. This direct exposure fuels the cycle of sublethal effects and morbidity on honey bees, turning a successful evolutionary adaption against them.
Sublethal Effects
As previously stated, sublethal exposure to glyphosate impacts honey bee behaviour, navigation, learning, and immune responses, impacting the overall health and productivity of bee colonies. Glyphosate triggers the antioxidant and detoxification mechanisms of honey bees, as denoted by the significant increases in enzymes like superoxide dismutase (Tang et al., 2023).
Increases in superoxide dismutatse and the innate immune protein prophenoloxidase (PPO) was also observed in bumble bees following consistent sublethal glyphosate exposure. The results suggest that this herbicide increases oxidative stress, which requires the upregulation of antioxidant enzymes to mitigate the damage. This relationship is complicated by the upregulation of PPO, which indicates that the bumble bee humoural immune defence is engaged in response to herbicide exposure (Wang et al., 2020).
The gut enzyme amylase assists with the uptake and utilisation of nutrients in insects, modulating energy production. Glyphosate-exposed bumble bees had significantly reduced enzyme activity, which resulted in reduced bumble bee digestion and body weight. While this suggests a reduction in colony productivity, the authors highlight that further studies are required to properly determine the consequences on colony health (Wang et al., 2020).
Synergistic Effects
Glyphosate can synergize with other stressors such as pathogens and pesticides, or beneficial compounds like antibiotics, resulting in a more pronounced negative impact on bee and colony health compared to exposure to the stressors or glyphosate alone.
Author’s note: As of writing, many of the current studies on bees investigate glyphosate alone or in comparison to other pesticides, rather than in a tank-mix combination. I have yet to find a publication that shows the impacts of synergy on bees between glyphosate and another pesticide formulation. If you know of any studies, please reach out!
For example, in the common South American toad Rhinella arenarum (toads eat bees so they’re considered relevant now), glyphosate synergised with the pyrethroid insecticide cypermethrin. When applied as a combination of Atanor & Xiper respectively, the synergy was found to have significant mortality on the larvae of this amphibian species (Peluso et al., 2021). The pesticides were combined and applied in equal amounts, and these subsequent formulations were more toxic than either of the individual compounds at the same dose. To confirm that these outcomes were not due to the adjuvants, other co-formulations were examined (glyphosate: Glifoglex & cypermethrin: Glextrin), and the authors found similar synergistic effects.
In the field, the use of pyrethroid pesticides on honey bees is commonplace. Whether as a tank mixture with herbicides and fungicides or other agrichemicals. From the example above, it is easy to see the large potential for synergy between glyphosate herbicides and other compounds; the outcomes of which may be additional sublethal effects or complete colony collapse.
Impact of Glyphosate on Honey Bee Gut Microbiome
Main article: Bee Gut Bacteria.
It is worth listing the eight main bacterial taxa contributing to the biological diversity within the honey bees’ gut bacteria:
- Phylum Firmicutes: Lactobacillus spp. firm-4 & firm-5.
- Phylum Actinobacteria: Bifidobacterium spp..
- Phylum Pseudomonadota: Prasaccharibacter apium, Snodgrassella alvi, Bartonella apis, Gilliamella apicola & Frischella perrara
Honey bees treated with Glyphosate had alterations to their gut bacteria populations, and were more likely to be affected by the opportunistic pathogen Serratia marcescens1 compared to the control group (Motta et al., 2018). Glyphosate showed a large reduction in S. avli, and smaller reduction in G. apicola populations, whereas it increased the populations of Lactobacillus species (Blot et al., 2019).
In contrast, Blot and colleagues found no significant infections in overwintering honey bees when exposed to Nosema ceranae; their study showed that modifications to gut bacteria do not always produce negative impacts. (Indeed, artificially increasing the populations of gut bacteria [specifically with Bifidobacteria and Lactobacilli spp.] through supplementation has been successful in reducing the susceptibility of honey bees to N. ceranae infections [Baffoni et al., 2016].)
The authors suggest that this discrepancy is a result of the multi-factorial parameters involved in honey bee exposure to stressors, and that as laboratory experiments themselves are stressful conditions, they may have inadvertently contributed to the negative effects of glyphosate-based herbicides on honey bee health.
Glyphosate on the Bumble Bee Gut Microbiome
In one study, the gut bacteria of bumble bees was unaffected by glyphosate exposure (Tang et al., 2023). Instead, the authors noted a disturbance within the fungal microbiome: sublethal glyphosate exposure was found to impact the Ascomycota Zygosaccharomyces populations. The role of this fungi is not completely understood in B. terrestris; in honey bees, this fungi is associated with food consumption and nutrient metabolism. When honey bees were supplemented with a number of yeast strains including Z. mellis, they were able to maintain a stable body fat composition and immune gene expression under stressful (laboratory) conditions (Canché-Collí et al., 2021).
Glyphosate’s Mechanism of Action
Glyphosate is a non-selective herbicide that inhibits the shikimate pathway enzyme EPSPS (5-enolpyruvylshikimate-3-phosphate synthase). EPSPS is involved in the synthesis of the aromatic amino acids: phenylalanine, tryptophan and tyrosine. This enzyme is present in plants, bacteria and fungi, but not animals, which contributed to the success of glyphosate herbicides.
In plants, glyphosate is absorbed by the leaves and stems, where it travels throughout the vascular tissue via translocation. Over 1-3 weeks, plants begin displaying stunted growth, loss of green colouration, leaf malformations and ultimately, death. The shikimate pathway generates several other phenol compounds that are involved in plant immunity (Yokoyama et al., 2021), and inhibition of EPSPS by glyphosate-based herbicides in off-target plants found them to be more vulnerable to soil-borne pathogens.
EPSPS inhibition disrupts the balance of honey bee gut bacteria. The aforementioned Lactobacillus spp. & B. apis taxa were unaffected by glyphosate due to the resistant nature of their molecular EPSPS structure2 (Blot et al., 2019).
Effects of Glyphosate in Mammals
A note on mammalian and human health: glyphosate use in herbicide products is generally deemed safe to mammals as they do not posses the EPSPS enzyme. However, as in honey bees, the mammalian gut microbiome can be impacted. Ongoing studies are compiling safety data on glyphosate and its commercial formulations to determine the extent of harm from this active ingredient and its adjuvant formulations on mammalian health (Gill et al., 2017).
Are “Inert” Ingredients in Pesticides Toxic?
Inert ingredients such adjuvants, surfactants, and carrier solvents, are specifically added to pesticides in order to increase the effectiveness of their active ingredients. These inert ingredients may lead to hazardous effects in products that contain active substances previously reported as safe (Boily et al., 2013).
An example covered in the article examining herbicides and fungicides on bees, is the adjuvant alcohol ethoxylate. Bumble bees exposed to alcohol ethoxylate under field conditions developed midgut damage, which then cascaded into a reduced appetite, weightloss and increased bumble bee mortality rates (Tamburini et al., 2021).
Inert Ingredients in Glyphosate Pesticide Formulations
The safety data on inert ingredients within glyphosate based herbicides are being reassessed for their off-target and sublethal effects. Within the U.S. and European Union, the Environmental Protection Agency have been taking steps to improve the transparency around their use, such as renaming them to “other ingredients” since the term “inert” can be misleading.
The difficulty in such studies is due to the fact that laboratory testing is not always an accurate depiction of the real-world harms. Furthermore, due to proprietary or regulatory reasons, pesticide companies are not required to disclose the specific inert ingredients in their products, which complicates the research process by introducing the intellectual property laws.
There are several commercial pesticide formulations that have glyphosate as their active ingredient, including:
- Roundup (ready-to-use formulations) – active ingredient is glyphosate as a isopropylamine salt.
- Roundup ProActive – active ingredient is glyphosate as a potassium salt.
- Touchdown – active ingredient is glyphosate as an ammonium salt.
- Durango DMA – active ingredient is glyphosate as a dimethylamine salt.
As different glyphosate salts have differing molecular weights, the variations between each pesticide formulation would vary in stability, absorption and translocation within plants. In the field, the salts dissociate at the plant tissue, and exhibit no further herbicidal effect. In honey bees, each of these products may give rise to sublethal effects as a result of other ingredients within the whole formulations, such as the undisclosed inert ingredients added to enhance the handling, safety and solubility of these products.
It’s worth noting that studies differentiate between pure glyphosate, the active ingredient, and Glyphosate formulations (e.g., Roundup products above) which contain other ingredients. It could be that the surfactants and other coformulants in formulations are more harmful than glyphosate alone, and these ingredients may be responsible for observed toxicity in exposure tests
Best Practices for Bee-Safe Weed Killers on Gardens and Farms
Bee-safe weed control best practices depend on type of method and the timing of its application, ranging from physical to long-term controls that include:
- Physical controls. Remove weeds by hand or with garden instruments, like a hoe.
- Planting cover crops. Crowds out weeds, and has the nice bonus of improving soil health and providing forage to bees (if they’re the flowering type).
- Mulching. An effective way to suppress weed growth, conserve soil moisture and improve soil health.
- Biological Controls. Introducing or promoting environments for beneficial insects, animals or microbes to control weeds and pests. Be sure they won’t harm bees! (see: Cane Toads in Australia).
- Bee-safe herbicides. Some “natural” herbicide formulations are less toxic than synthetic varieties, such as a mixture of vinegar and dish soap.
- However, direct exposure with such “bee-safe herbicides” can still harm bees by blocking the breathing tubes within their tracheal system, or contaminate the nectar and pollen.
- Application timing. As in commercial settings, herbicides should be applied when bees are less active in the early morning or late evening.
- Spot treatments. One way to limit herbicide use is to spot treat target plants rather than spray down the entire ara.
Readers can appreciate why these methods are difficult to scale on larger farms and properties, and why pesticides are the answer to this scaling issue; for the home beekeeper or grower, these practices can be easily put in place.
How Do You Make Bee-Safe Herbicides?
Perhaps the best herbicide is no herbicide. Any agrichemical is bound to have off-target effects, whether on the whole organism or its gut microbiome (in the case of honey bees, it’s their hive products).
Honey bees used in commercial pollination operations are at a high risk of exposure to such agrichemicals, simply due to the economic demands placed on agriculture. At this stage, there are insufficient alternative avenues to meet demands without offloading the aftereffects to the consumer or corporations, and so beekeepers and farmers must adapt to the procedures at hand.
As stated in the effects of herbicides and fungicides on bees, in the face such uncertainties beekeepers and farmers are encouraged to adopt the precautionary approach: assume that any glyphosate-based herbicide will impact bees, and work hives accordingly.
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Footnotes
- No distinct symptoms are attributed to infections S. marcescens. It is generally thought to compete with the bee’s gut bacteria for nutrients, leading to increased honey bee mortality or brood diseases (Steele et al., 2021). ↩︎
- The glyphosate-sensitive structure of other taxa is in the Class Iα configuration (s. alvi, G.apicola), and the glyphosate-resistant enzyme is in the Class II configuration (Lactobacillus spp. & B. apis) ↩︎