What Effects do the Neonicotinoid Insecticides Have on Bees?
Neonicotinoid insecticides are are synthetic compounds modelled after the natural plant-based pesticide nicotine and are systemically distributed throughout the plant tissue (including its nectar and/or pollen) when applied as a seed coating. These agricultural chemicals work by binding to the nicotinic acetylcholine receptors (AChR for short) in the insect nervous system, leading to death by overstimulation.
This widespread distribution makes neonicotinoids effective in controlling sucking and chewing insects. However, the pesticides are also present in the plant’s pollen and nectar, which leads to off-target effects in bees and other pollinators as they forage on contaminated crops. In commercial pollination settings, neonicotinoid pesticides are applied on plants like sunflowers, soybeans and various fruit trees.
This article will continue on the concepts covered in the effects of pesticides on honey bees by examining the neonicotinoid class of insecticides. It will outline the mechanism of action of different neonicotinoid variants and the role of these pesticides on the health of Apis mellifera. A description on the potential synergy between neonicotinoids and other agrichemicals (ergosterol biosynthesis inhibiting fungicides) will be made, as well as an analysis into the risk assessment and current regulations placed by environmental protection agencies on their use. It concludes by listing some of the controversies surrounding neonicotinoid use.
There’s a shirt that goes with this article, designed & printed by yours truly:
Neonicotinoid Exposure and Honey Bee Health
Neonicotinoids exposure scenarios have been linked in several detrimental effects on honey bee health. At high doses, it is an unceremonious death to the individual bee. At lower, sublethal doses, the impacts range from cognitive impairments on learning and foraging behaviour, to complete colony collapse when enough bees return contaminated pollen and nectar to their hives. The severity of these effects depend on the concentrations and length of exposure, resulting in reduced colony health over time even in the absence of “immediate” death.
Honey bees are widely studied for this topic, however bumble bees and solitary bees are probably even more sensitive to neonicotinoid exposure owing to their smaller colony size (and subsequently reduced detoxification potential).
How do Neonicotinoid Pesticides Cause Paralysis and Death of Honey Bees?
A clickbait-ey headline, I’ll admit. At high doses, neonicotinoids leads to death as a result of nerve overstimulation. Broadly, they target the acetylcholine (ACh) system, prolonging the presence of ACh in the post-synaptic nerve terminal by binding to the ACh molecules themselves as follows:
The process begins with oral exposure to a neonicotinoid insecticide, where it travels to the bee nervous system.
Here, neonicotinoids disrupt the transmission of nerve signals as the molecules binds to the AChR, leading to overstimulation and prolonged receptor activation.
This overstimulation interferes with normal muscle function, resulting in paralysis. In honey bees, this manifests as tremors and uncoordinated movement, eventually rendering the bee immobile; unable to fly.
As the contamination spreads, it stresses the normal functional of honey bee colonies as nonfunctional and dead bees being accumulate within the hive.
Effects of Acute Neonicotinoid Exposure
Acute exposure effects are not easy to detect in the field. Instead, experiments under laboratory conditions are used to show the first signs of how pesticides alter honey bee physiology. In a 2014 study by Williamson & Colleagues, bees placed under field-realistic exposure scenarios of imidacloprid started exhibiting subtle alteration to motor function. Predominantly, these were shown as increases in grooming time, and impairments to the bees’ righting reflex, in which affected affected bees were found to spend more time upside down.
Under normal conditions, bees rarely spend time idle or standing still – they are busy maintaining hive functions. Indeed, the control bees spent more time walking (presumably between hive duties) and rarely spent time idling or grooming compared to the neonicotinoid-exposed bees; they readily righted themselves after falling over. At higher doses of imidacloprid, the affected bees spent most of their time stationary, suggesting severe impairment or inhibition of their motor function (Williamson et al., 2014)
Based on the results of Williamson & Wright in their 2013 study, the authors suggest that this motor impairment is reversible: Imidacloprid-associated learning defects were reversed after bees were given an untreated sucrose solution the three days following insecticide exposure. The authors extrapolate that feeding bees with a standard sugar syrup within certain timeframe could reverse the motor dysfunctions resulting from pesticide exposure.
Neonicotinoids Agonize Nicotine Receptor Activity to Increase Grooming Behavior
Non-technical readers are welcome to skip this section.
The motor neuron assay first described by Maze & colleagues in 2006 observes bees using a petri dish that serves as the arena, monitored with specialised computer software to determine the frequency and duration certain behaviour. The motor circuits that govern grooming are related to cholinergic neurotransmission in the ventral nerve chord. Studies using radiolabelled imidacloprid and thiamethoxam demonstrated binding to the same area as α-bungarotoxin (an irreversible competitive antagonist to AChR,commonly, used as a comparative in neuroscience research in its fluorescent form) to produce a a simulatory effect.
The authors suggest that a sublethal dose of this nerve stimulation is what drives the grooming behaviour in honey bees. More research is required to determine the distribution of these neonicotinoid-sensitive receptors in the ventral nerve cord ganglia and neuromuscular junctions (Williamson et al., 2014).
Effects of Chronic Neonicotinoid Exposure
Chronic exposure to neonicotinoids can affect learning and recognition in honey bees. In one study, the long-term effects of neonicotinoids saw changes in the antenna responses to volatile organic compounds. Honey bee antennas are responsible for detecting odours (eg on flowering plants) and pheromones within the hive (such as the Queen Mandibular pheromone or alarm pheromone). Forager bees exposed to thiacloprid had a reduced associative learning ability, in which a variety of floral scents could not be equated with food quality1. Of particular note were the increases in response to the compounds benzyl alcohol and 1-hexanol, suggesting that olfactory disruption causes bees to inaccurately perceive the richness of floral sources, impairing their foraging behaviour. (Favaro et al 2022).
Exposing honey bees to pollen samples containing field-realistic thiamethoxam and clothianidin doses resulted in decreased honey production and pollen collection. These neonicotinoids increased the frequency of memory loss episodes in forager bees and compromised their navigational memory when by reducing the rate of homing success (Sandrock et al., 2014). The reductions in pollen collection is attributed to the premature forager recruitment of younger bees as a consequence of increased forager bee loss. This triggers downstream effects of having fewer nurse bees available for brood rearing as the colony reallocates worker resources to ensure survival (superorganism resilience at work!).
The work of Sandrock & colleagues showed that chronic neonicotinoid insecticide exposures did not increase overwintering losses when appropriate Varroa mite controls were in place. This contrast previous work from Lu & colleagues, who found that honey bee colonies experience greater overwintering losses in the presence of neonicotinoids, and attributed it as a driver of colony collapse disorder (Lu et al., 2014).
Two Queens, One Hive: How Neonicotinoids Increase Queen Supersedures
Interestingly, Sandrock & colleagues showed that in the long term, honey bee colonies affected by neonicotinoids were able to survive the winters despite a significant difference in colony performance to the control bees. They attributed this to an increase in queen supersedures: at the final autumn inspections, colonies had possessed marked queen bees with clipped wings. When the treated hives were inspected in spring, the researchers found newer, non-tagged egg laying queens alongside the original queens from the prior autumn in 60% of the colonies, indicating that the honey bees recognised the failing queen, and underwent supersedure (Sandrock et al., 2014).
Colonies raising queens in the autumn are playing it dangerously: it puts the virgin queen at risk of insufficient mating due to a reduced drone availability. This (presumably) newly poorly-mated queen, along with the presence of the original neonicotinoid-treated queen is thought to contribute to the reductions in hive growth the following season, but not complete honey bee colony collapse.
Synergism Between Neonicotinoids and Other Pesticides
The widespread use of neonicotinoids on agricultural crops2, in combination with other pesticides used inside honey bee hives provides the potential for synergistic effects. One such study showed that the commonly used organophosphate coumaphos for Varroa mite control can synergise with neonicotinoid pesticides as both compounds target ACh signalling (Gregorc et al., 2018). This additive toxicity is due to the involvement of the same detoxification pathways, namely the p-glycoprotein xenobiotic efflux transporters and the P450 superfamily.
A 2016 study by Sgolastra et al. showed that co-application of an ergosterol biosynthesis inhibitor fungicide (propiconazole) with clothianidin led to increased lethality in three bee species. In commercial settings, these pesticides are often applied together on sunflowers, canola and various fruit trees. As bees were exposed to contaminated pollen and nectar containing the fungicide, they were found to be more susceptible to clothianidin. This effect is due to the inhibition of P450 by propiconazole, which increases neonicotinoid concentrations, and subsequently toxicity, inside the bees.
Ergosterol biosynthesis inhibiting fungicides are known to inhibit the bee P450 enzymes. With neonicotinoids, this synergy between P450 enzyme inhibition and pesticide toxicity was class-dependent:
- N-cyanoamidine neonicotinoids like acetamiprid or thiacloprid have a relatively low toxicity to honey bees, but when applied with fungicides like triflumizole or propiconazole, their toxicity increased by several hundred times (Uwasa et al., 2004).
- N-nitroguanidine neonicotinoids like imidacloprid and thiamethoxam are more toxic to honey bees alone, yet showed weak toxicity when paired with ergosterol biosynthesis inhibitors in comparison to N-cyanoamidine compounds (Yao et al., 2018).
- This suggests that N-nitroguanidine neonicotinoids are weakley metabolised by P450 enzymes, or not metabolised by them at all, which could explain why they are more toxic to bees. More on that later.
- In the case of wild bees, experiments on the horned-face Mason bee Osima cornifrons revealed that the combination of acetamiprid (N-cyanoamidine) with the ergosterol biosynthesis inhibitor fenbuconazole, showed significant synergy in acute toxicity, whereas the N-nitroguanidine imidacloprid, with these fungicide classes showed very little synergistic effects (Biddinger et al., 2013).
- Within the scientific literature, a few studies have shown that this synergy can lead to sublethal effects like impaired ovary maturation and decreased female lifespan. The impacts of this synergy are moreso detrimental for solitary bees as each affected female means the loss of a generation; newly emerged bees will see a gradual reduction in populations as a consequence of pesticide exposure.
- In other wild bees like B. terrestris, the results are mixed: some studies exposing bumble bees to imidacloprid and the fungicide imazalil observed a synergy that reduced bee survival, however it did not impact their sucrose responsiveness to the contaminated food sources. This is one such combination used in agriculture, and the authors emphasise that future studies are required to understand these effects in the field (Raimets et al., 2018)
As mentioned in the article on the effects of herbicide and fungicide on bees, the vast majority of studies acknowledge that there are multiple factors at play when the same dose of a neonicotinoid pesticide is applied in the presence of fungicide pesticide. In the face of the overwhelming number of synergistic potentials, we recommend the precautionary principle: assume that all combinations are harmful and can lead to lethal or sublethal effects on bee health.
How P450 Enzymes Detoxify Neonicotinoid Pesticides
Non-technical readers are welcome to skip this section
In bees, removal of neonicotinoids is largely mediated by the P450 superfamily. These enzymes catalyse reactions that aim to increase the rate of pesticide excretion, with the vast majority proceeding as follows:
- When certain neonicotinoid pesticides enter the bee’s system, the P450 enzymes begin Phase I metabolism to modify their molecular structure by oxidation, reduction, or hydrolysis, depending on the compound.
- In the case of thiacloprid, it undergoes oxidation and hydroxylation to form hydroxylated thiacloprid metabolites (Manjon et al., 2018).
- Phase II metabolism begins with conjugation of the metabolite. Certain molecules (eg glucuronic acid or sulfate) are attached to the hydroxylated metabolite to increase its water solubility and prepare it for excretion.
- The now-metabolised molecule undergoes excretion and is moved out of the cells using membrane transporters to the excretory organs in the Malphighian tubule system, and ultimately eliminated in the bee’s waste, such as its feces.
What are the Differences Between the Two Types of Neonicotinoids?
Neonicotinoids can be structurally distinguished into two types: N-nitroguanidine and N-cyanoamidine neonicotinoids. The structure of the first group contains N-nitro-groups, which are more polar and reactive due to the presence of oxygen atoms. Imidacloprid, clothianidin and thiamethoxam belong to this group, and are more toxic to bees than N-cyanoamidine neonicotinoids like acetamiprid and thiacloprid. Within the molecule the cyanoamidine group replaces the nitro group, and this absence of oxygen atoms in the cyanoamidine group makes them less reactive and less toxic to insects (further reading is found in this 2022 paper by Schumann et al.).
N-cyanoamidine vs N-nitroguanidine Neonicotinoids Detoxification
In general, the detoxification process aims to make a molecule more water-soluble. In bumble & honey bees, the members of the P450 superfamily responsible for detoxifying neonicotinoids are found in the CYP9Q subfamily, who catalyze N-cyanoamidine neonicotinoids but not N-nitroguanidine compounds (Haas et al., 2022). The actions of CYP9Q enzymes (specifically CYP9Q4, CYP9Q5 & CYP9Q6) are so effective in detoxifying N-cyanoamidine compounds that thiacloprid was classified as ‘practically non-toxic’ to B. terrestris in acute contact bioassays (Troczka et al., 2019).
On N-nitroguanidines like imidacloprid, only CYP9Q1–3 showed a capacity to metabolise this pesticide, and at a lower efficiency when compared to thiacloprid (Manjon et al., 2018). Such non-selectivity to N-nitroguanidine neonicotinoids could explain why they are more toxic to bees, as the main metabolite remains active within the insects’ body for a longer period than N-cyanoamidine compounds.
Risk Assessment on Pesticide Exposure
Some studies have shown that bees willingly feed on sugar solutions that contain low doses of a pesticide, suggesting that bees either cannot detect them or do not object to their taste (Kessler et al., 2015). Whether this is consistent at higher, lethal doses, the data is unclear, but one can assume that bees will be less likely to consume from these sources due to a more pronounced (and presumable foreign) taste.
One aspect missing from tests under laboratory conditions is that foraging bees typically consume more sugar (nectar) than caged bees due to the metabolic demands of flight. Laboratory studies that measure the consumption of confined bees may produce underestimated values compared to those in the field, and as a result, measuring the degree of pesticide exposure can be much lower than reality.
The extent of pesticide toxicity depends on the lifestyle of the bee species. Individual bees in social species like Apis mellifera and Bombus terrestris are not as vulnerable to chemical exposure as they buffer their effects at the colony level, termed “superorganism resilience” (whereby the colony as a whole functions as a single unit to withstand and recover from stressors or disturbances[Straub et al., 2015]). An analysis of bees foraging on clothianidin-coated canola showed negative effects on wild bee nesting and B. terrestris colony growth, but no significant effects on the development of A. mellifera colonies; as attributed to the size differences between bee colonies (Rundlöf et al., 2015).
Are Neonicotinoid Insecticides Banned?
The European Union is continually monitoring the use of neonicotinoids, starting with a partial ban on the N-nitroguanidine pesticides (clothianidin, imidacloprid and thiamethoxam), and later expanded by restricting use only within permanent greenhouses to minimise bee exposure.
Within the United States, neonicotinoid are approved for use, with some interim decisions put forward to minimise the harm to insects and other pollinators. These proposals include:
- Improved management methods to reduce off-target effects,
- Restrictions on application during blooming crops,
- Cancelling spray uses of certain neonicotinoid pesticides like imidacloprid due to health concerns,
- Increasing the use of personal protective equipment to address occupational hazard risks,
- Mandatory labelling of neonicotinoid products to notify residential users not to use these products.
In Canada, neonicotinoids are approved for use, with some restrictions to foliar applications on important pollinator plants and other crops. Seed treatments are deemed acceptable, with a focus on minimising bee exposure to neonicotinoid-contaminated dust during planting. In France, a 2023 court ruling declared that the country will no longer use neonicotinoids, whilst the Australia government is currently conducting a chemical review to assess the benefits of neonicotinoids field use.
Studies like the 2018 research from Manjon & colleagues showed that bee toxicity is driven by the differences in metabolism and excretion, rather than the binding affinity between the two types of neonicotinoids: radioligand binding assays found no significant differences between the binding of imidacloprid and thiacloprid to the AChR. Such work continues supporting the legislation around the restrictions for the use of neonicotinoids on plants that require pollination.
Why are Neonicotinoids Controversial?
The neonicotinoid controversy arises from their adverse effects on honey bees and other pollinators. Multiple studies beyond the ones referenced here have highlighted their detrimental impact on the long-term health and survival of bees. While it’s easy to jump to conclusions and say “these pesticides kill bees”, we live in a world where unless the evidence is obvious then the answer is always “it’s complicated, but…”. The cumulative sublethal effects often take many months to manifest at a significant level, making it difficult to draw direct causalities between a pesticide and its effects.
The potential for environmental contamination in soil and waterways increases the off-target effects on bird and mammal species, impacting entire ecosystems in complex ways that are not always studied. All this is enveloped by the regulatory responses put in place to protect pollinators and the environment around the world.
Again, the precautionary principle: assume that all combinations are harmful with respect to honey bee health, and this extends to any pesticides in honey sold for consumption .
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
- To the foraging bee, floral scents are a source of information that’s based around the concentrations of volatile organic compounds. This differential sensitivity aids bees in determining the quality of a food source. ↩︎
- That’s not to say there are no policy responses – regional regulators have provided guidelines on when and when not to use neonicotinoids due to their harm on bees & pollinators. For example: the European Food Safety Authority concluded that most neonicotinoid uses pose a risk to wild bees and honeybees, leading to restrictions. ↩︎