Honey Bee Poisoning: Beyond Sublethal Effects
Previous articles analysed the sublethal effects of pesticides on bees and the consequences of accumulated, prolonged impairments. This article elaborates on how these sublethal effects kill bees. Three case studies of pesticide toxicity are examined: Neonicotinoids, the pyrethrins (commonly used inside hives for Varroa mite control) and the inactive, “inert” ingredients, those adjuvants chemicals used in pesticide formulations.
Clickbait title aside, Pesticides can directly kill bees at high exposures. It’s the sub-lethal effects like impaired navigation, learning, foraging and immune function that combine with other stressors to reduce overall survival and “quality of life”. In most, if not all instance, it’s the chronic low-level exposure that matters more than acute poisoning; how often is someone going to walk up to a hive and spray it directly?
There’s a shirt that goes with this article, designed & printed by yours truly:
Neonicotinoid Pesticides & Honey Bees: Case One
Chemically, neonicotinoid pesticides have two structural analogues: the N-cyanoamidine (eg acetamiprid, thiacloprid) and N-nitroguanidine (eg imidacloprid, thiamethoxam), as covered in the article neonicotinoids on bees. Of the two groups, the N-nitroguanidine compounds are more toxic to bees, since their chemical structure contains the N-nitro-groups, which are more reactive due to the presence of oxygen atoms. In N-cyanoamidine neonicotinoids, the cyanoamidine group replaces the nitro group, and this absence of oxygen atoms makes them less reactive and subsequently less toxic to insects.
Imidacloprid & Digestive Alterations – not in Honey Bees!
Imidacloprid has been implicated in many cases of honey bee mortality, as this is one of the mostly commonly used pesticides in the U.S. Like other neonicotinoid pesticides, its primary mechanism of action is central nervous system toxicity. In laboratory experiments with the fruit fly larvae Drosophila melanogaster, it shows digestive toxicity (Daisley et al., 2017). One may assume that imidacloprid is harming bees through alterations in its gut microbiome, however the work of Raymann and colleagues found no significant differences in the core gut microbes inside honey bees when compared to control. The authors suggests that this because the resident gut bacteria do not have the ability to metabolise imidacloprid, and its breakdown is mediated by the bee’s detoxifying enzymes (CYP450 subfamilies).
Decreases in Honey Bee Survival – Increased Infections
The decrease in honey bee survival is attributed to a prolonging imidicaloprid-induced sublethal effects, such as behavioural defects or immune system dysfunction. When imidacloprid-treated bees were exposed to the opportunistic (i.e. takes advantage of impaired immunity) bacterial pathogen Serratia marcescens, they had an increased mortality compared to control bees, suggesting that imidacloprid increased the bees’ susceptibility to infections as a result of impaired immunity (Raymann et al., 2018).
Imidacloprid’s effects could be attributed to the reductions in the hemocyte density, encapsulation response, and overall antimicrobial activity1, which have been observed in other neonicotinoid pesticides like clothianidin (Di Prisco et al., 2013). Research by Balbuena & colleagues found that chronic imidicaloprid exposure in the presence of Nosema cerana resulted in an additive effect between the two stressors, impacting the gut biota. Such results suggest that honey bees are resilient to imidacloprid alone, but not in combination with other stressors. Even at doses previously reported as sublethal, this combination was found to be more toxic to honey bees and increasing in a dose-dependent manner.
How Pyrethrins Pesticides Harm Bees: Case Two
Pyrethrins have been used for over a hundred years as pesticides derived from pyrethrum, a chrysanthemum flower extract. They are approved for use in organic farming, with examples including allethrin, resmethrin, and tetramethrin. Despite such certifications, these organic pesticides can still affect bees.
Pyrethrins bind to voltage-gated sodium channels within the nerve axons and prevent them from closing. As a result, the insect experiences rapid tremors and convulsions, leading to death. This class is indiscriminate; pyrethrin pesticides have been shown to harm pollinators and other beneficial insects (Oliver et al., 2015; National Pesticide Information Center Factsheet).
For example, fenpropathrin is used as a broad-spectrum insecticide in cotton and cereal crops for mites and similar pests. When it was applied on newly-emerged bees (<12 hours old), researchers found that their homing ability and overall survival was decreased (Liao et al., 2017). This was a dose-dependent relationship, with bees exposed to higher doses having a shorter lifespan – 15 days at highest dose.
The Effects of Tau-Fluvalinate Inside Honey Bee Hives
Beekeepers commonly use tau-fluvalinate (commonly called fluvalinate) as a Varroa mite control in times of low brood numbers and after honey harvests. However, fluvalinate has been shown to affect bee larvae, impairing the olfactory abilities of the emergent adult bees. This indirectly affects honey bee survival by impacting memory formation (Ko et al., 2022), and was previously confirmed by Frost & colleagues. As with other pesticides, these effects are dose-dependent, impacting colony survival at the brood and adult level (Zhu et al., 2013; Berry et al., 2013).
Fluvalinate accumulates within the wax comb, eventually reaching highly toxic levels as it chronically exposes bees to the pesticide (as well as contaminating honey). Indeed, Benito-Murcia & colleagues found that while accumulated pesticide levels in bee bread are lower than the lethal values, larvae fed contaminated food had a greater likelihood of developing negative effects compared to a control brood population. (There is also the risk of mites developing resistance to fluvalinate, a topic for later).
Honey bees given oral fluvalinate develop sublethal effects. Inside the hive however, it is topical contact with the fluvalinate strips that’s more likely. Lim and colleagues (2020) found that when bees contacted fluvalinate, they developed olfactory defects. The authors suggest that a better understanding of the mechanisms behind these non-target chemicals inside hives can help in determining the underlying causes behind phenomena like the colony collapse disorder.
How Antibiotics Make Bees More Susceptible to Fluvalinate
The 2020 research of Wu & colleagues found that some pesticide detoxification is mediated by the honey bee gut microbiome, which regulates P450 gene expression. When bees were administered tetracycline, an antibiotic used in the treatment of European Foulbrood (EFB), they found these bees more susceptible to the neonicotinoid pesticide thiacloprid and fluvalinate. This susceptibility led to a higher bee mortality compared to control and bees treated with pesticides only (100% of tetracycline-thiacloprid treated workers died after ten days & 40% died in tetracycline-fluvalinate within the same timeframe).
P450 genes are also expressed in the legs and antenna of the bees; structures that commonly come into contact with toxic pesticides during foraging (Mao et al., 2015). While antibiotics impacted gut P450 expression, the authors found no alterations in P450 expression within the legs and antennas of treated bees, suggesting a decentralization of the honey bee immune system.
Effects of Adjuvants in Pesticide Formulations: Case Three
Inert ingredients such as adjuvants, surfactants, and carrier solvents, are specifically added to pesticides to increase the effectiveness of their active ingredients. An example covered in the article on herbicides and fungicides on bees, is alcohol ethoxylate. Research found that when bumble bees were 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). Different formulations of a pesticide contain different inert ingredients, which may lead to hazardous effects, even if the active substances were previously reported safe (Boily et al., 2013).
Adjuvants in Glyphosate Pesticide Formulations
Extract from “Does Roundup Kill Bees?“
The most common organophosate used in residential and commercial settings is glyphosate. This active ingredient has found itself into many formulations of systemic pesticides, and is commonly used for weed control. The safety data on its inert ingredients is constantly being re-assessed for off-target effects- there are many formulations available, from wettable powders, emulsifiable concentrates to liquid sprays, and each have a different impact on bee populations and the environment.
Glyphosate salts have different molecular weights, leading to variations in pesticide formulation, stability, and translocation within the plants and soil. In the field, these salts dissociate at the plant tissue, and exhibit no further herbicidal effect, whereas in bees, the other chemicals within the compound can give rise to sublethal effects.
The Effects of Organosilicone Surfactants in Foraging Bee Species
Organosilicone surfactants are silicone-based polymers whose surface tension and solubility can be adjusted for a variety of applications, from tank additives to improving drug delivery (Reviewed in Chen et al., 2018). They greatly enhance pesticide effectiveness and are are considered the “gold standard” of adjuvant ingredients, however they are also the most toxic to bees (as well as other animals). These adjuvants contribute to the overall toxic load of applied insecticides, and foraging bees exposed to these products also show signs of pesticide poisoning (Boily et al., 2013).
Several literature reviews have investigated the science and mechanisms behind organosilicone surfactants, declaring that they are unsafe for bees and humans alike (Krogh et al., 2003; Mullin 2015; Mullin et al., 2016). The basis of these findings stem from:
The ability of the surfactants to enhance pesticide toxicity by increasing cellular membrane permeability and subsequently, increased pesticide uptake,
Sublethal effects, even in the absence of pesticides – from impaired immune functions and altered bee behaviours (bee-haviors?),
Synergism with other chemicals, whether in a tank mix or in the environment. By interacting with other stressors, they can lead to a sudden loss of productive colonies even when all individual stressors were thought to be at safe levels.
Not Just Organosilicates – Other Inert Ingredients Causing Harm to Bees
Organic pesticides are praised as being safer for insects. While this is true of the active ingredient(s), it is not always the case in complete formulations. Bacillus thuringiensis is not toxic to bees alone, and experiments applying pure formulations of the Bt aizawai strain found the median lethal dose (LD50) to be greater than 100 μg/bee, considering it practically nontoxic. Any harm to pollinators was likely mediated by the synthetic chemicals used as adjuvants in the pesticide formulation, or due to the route of exposure.
Under laboratory conditions, oral pesticide exposure showed greater toxicity compared to other methods, such as spraying or contact exposure with strips (Mommaerts et al., 2010; Libardoni et al., 2018). This is likely due to systemic exposure occurring following pesticide ingestion, as opposed to the localised exposure from topical contact.
Difficulties in Conducting Research on Inert Toxic Materials
Within the U.S. and European Union, the Environmental Protection Agencies are attempting to improve the transparency around the use of these adjuvant ingredients, such as renaming them to “other ingredients” since the term “inert” can be misleading.
The difficulty in conducting such studies stems from the fact that laboratory conditions are not an accurate depiction of real-world harms. In a given formulation, countless complex interactions take place between its component chemicals, and when these compounds are used in the environment, these interactions compound (pun intended).
Furthermore, due to proprietary or regulatory reasons, pesticide companies are not required to disclose the specific inert ingredients in their products, which further complicates the research process. There are many varieties of pesticide formulations, and the same active ingredient can be paired with different adjuvants in different products (see: glyphosate formulations, or another herbicide example below), making it hard to draw generalized conclusions on the effects of a certain adjuvant on bee health.
Agrichemicals Vs. Beneficial Insects
The three examples listed above are but one of the many ways pesticides impact insects in the field. Research continues to show that even when an agrichemical is approved for use, it will show sublethal or lethal effects over prolonged field use, simply due to the complexities of the environment in comparison to laboratories. Worth noting that pesticides do not act in isolation: poor nutrition, parasites (e.g., Varroa), and pathogens can amplify their impacts and increase the morbidity:mortality risks beyond what single exposures would predict… many such cases.
Further research on pesticide classes like herbicides and fungicides, or on the effects of neonicotinoids on bees will only repeat the message that agrichemicals are a net loss for insects and other pollinators, yet are necessary to sustain humanity’s food supplies. So my dear reader, adopt the precautionary principle: assume these chemicals are harmful without waiting for the evidence to say so.
Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.
Footnotes
- In insect immune systems, pathogens that are too large for breakdown by phagocytosis are encapsulated, a process by which a layer of cells (such as granulocytes) surround the pathogen. This layer is then surrounded by plasmatocytes (the insect equivalent of macrophage white blood cells) and then surrounded by another layer of granulocyte cells. In some cases, the capsule may become melanized – darkened to cause pathogen death by starvation or oxidative damage as a result of isolation from the nutrient-rich haemolymph. Further reading on insect immunology is available in Hillyer, 2016. ↩︎