Synergy and Indirect Effects From Pesticide Use
When discussing chemical use in residential and agricultural settings, insecticides are quickly blamed due to their harmful impacts on bees. However, Insecticides are just one element within the “pesticide” category, with other components being fungicides and herbicides.
Pesticides are blamed for killing beneficial insects and contributing to the colony collapse phenomenon. The issues with herbicides and fungicides is that they are often applied without restriction during major flowering periods for crop protection, exposing honey bees and other insects to their effects. They may not kill bees “directly”, they’ll cause sublethal effects through gut microbiota disruption, reduced food resources, weakened immunity, and interactions with other chemicals.
This next article in the pesticides on honey bees series covers the two other pesticides classes used in agricultural settings: herbicides and fungicides. We describe their effects on honey bees Apis mellifera, and Bumble bees Bombus terrestris. It identifies the limitations of studies conducted under laboratory conditions, and provides beekeepers with management tips around pesticide exposure (In short, the best advice is to assume they’re ALL toxic).
There’s a shirt that goes with this article, designed & printed by yours truly:
What Effects do Herbicides and Fungicides Have on Honey Bees?
The answer is “it depends”. Some formulations may be safe for use in solo applications, yet result in severe health consequences when used as a combination within a mixing tank. The chemicals may impact bees directly, or affect their gut microflora. The potential combinations of insecticides, herbicides and fungicides are large in number, with new chemical classes being released more frequently than the safety studies (not to mention the number of organic pesticides currently available on the market).
As stated in the introduction, even if they may not kill bees outright, they can still weaken colonies, reduce immune function, and interact with other chemicals to make bees more susceptible to stress and pathogens.
The Impacts of Sublethal Effects on Honey Bee Health
Measuring sublethal effects is no easy task; the causes & symptoms are multi-factorial and readily change when one component is altered. Sublethal effects can be defined as the “biological, physiological, demographic or behavioural effects on individuals or populations that survive exposure to a chemical at its lethal or sublethal doses/concentrations” (de França et al., 2017).
Ultimately, sublethal effects contribute to honey bee mortality as a result of continuous, chronic exposure; prolonged pesticide use can cause population decline both directly and indirectly. Examples of sublethal effects include memory loss, impaired movement and erratic foraging behaviour. Bees from social species like Apis mellifera and the buff-tailed bumblebee, B. terrestris, will spread the contaminated forage to other bees in their hive, rapidly increasing the pesticide concentrations within the colony (Banks et al., 2020).
The sublethal effects are great and vary by chemical class. Snippets have been covered amongst other articles on this site, and a summary of their effects is as follows:
- Reduced feeding and/or foraging activity; navigational skills
- Impaired physiology and/or metabolism
- Impaired larval/pupal development
- Generalised negative impacts on colony performance over time
The Impacts of Fungicides on Honey Bee Health
Active ingredients may be deemed “safe” for wild bees and insects in agricultural systems. However, contact exposure to their adjuvants can impact the bee gut bacteria. Fungicides have been shown to disrupt the microfloral balance in the honey bee midgut, and make bees more susceptible to insecticides by impairing the detoxifying activities of their P450 enzymes.
DeGrandi-Hoffman et al., (2016) showed that when using field realistic concentrations, the fungicide Pristine (a proprietary combination of oxathiin and a strobilurin fungicide) did not have a clinically significant impact on honey bee health, despite strobilurin fungicides showing antibacterial properties. Several suggestions for this observation include a reduction in palatability of fungicide-coated pollen, or the dilution of experimental doses with nectar (as in when pollen is used to make bee bread).
Their experiment suggests that the health impacts are not driven by the microflora, rather the organism as a whole. By contrast, honey bees fed a combination of insecticides and fungicides showed a disrupted gut microflora that led to a reduced lifespan and increased susceptibility to infections (Al Nagger et al., 2022). This study was also conducted under laboratory conditions, and was one that did not observe synergism between their two tested pesticides (insecticides flupyradifurone and sulfoxaflor and the fungicide azoxystrobin).
Fungicides on Bumble Bee Species
A 2021 study by Tamburini and colleagues showed that the systemic fungicide Azoxystrobin had a negative effect on the foraging behaviour of individual B. terrestris bumble bees, reducing the rate of pollen collection in affected colonies. This was studied in conjunction with a new class of systemic insecticides, the sulfoximine sulfoxaflor (Closer), novel compounds heralded as the replacement to neonicotinoid insecticides1.
Alcohol ethoxylate, the adjuvant in the azoxystrobin blend Amistar, appeared to negatively influence the health of bumble bees. Examined under field conditions, alcohol ethoxylate (a type of surfactant – products used to ensure that the pesticide penetrates its target plants or insects) led to midgut damage, which then cascaded into a reduced appetite, weightloss and increased bumble bee mortality. The authors conclude that while the fungicide applications did not have a large impact on flower visitation, flight activity, or colony growth, further investigation are required to assess the sublethal effects following chronic and higher-dosed exposure to Amistar.
The Impacts of Herbicides on Honey Bee Health
Existing studies have shown that oral exposure to herbicides for 20 days (clethodim & haloxyfop-p-methyl) resulted in reduced honey bee survival and body weight measurements compared to the control. Chronic sublethal exposure to either clethodim alone or as a mixture with haloxyfop-p-methyl resulted in physical deformities, hypopharyngeal gland deficiencies and reduced haemocyte count (equivalent of bee blood) (Mohamed et al., 2022).
This experiment compared the application of herbicides in sugar syrup under laboratory conditions with a control. Exposure to the combination of the two herbicides was found to be less toxic than the individual application of the two (generally, combinations tend to be more toxic). Meanwhile, the reduction in hypopharyngeal gland size is a common side effect of pesticides application. This subsequent reduction in the quality and quantity of royal jelly production had a negative effect on the overall colony survival, development and maintenance.
Additional data from from field & semi-field studies will be required in order to place the impact of sublethal herbicide exposure on the health of other bee species & pollinators. In this study, the authors mentioned that these effects were also observed in other Apis species, such as the giant honey bees A. dorsata.
Honey Bee Pesticide Exposure in Australian Almond Pollination
Commercial pollination is common in the Australian almond industry. Each almond flower has a five-day window in which it is viable for pollination, requiring temperatures between 18 – 27°C for pollen release. This typically occurs in early spring, and that is when large amounts of honey bees are transported to almond orchards to provide pollination services.
The almond board is aware of the synergy between insecticides and fungicides, and discourages tank-mixing of the two compounds. Their guidelines state that insecticides should not be used in the ten days prior to hives arriving in the orchard, and throughout the pollination period. Fungicides on the other hand, should only be applied if necessary and in the late afternoon or evenings when foraging honey bees return to their hives.
I suggest that beekeepers and farmers to adopt the precautionary principle and assume that all chemical combinations are toxic and should not be used in situations that can affect honey bees. This heuristic is useful in the absence of sufficient evidence on the vast possibilities of agrichemical combinations. Further, the almond board of Australia recommends that flowering weeds be maintained and even encouraged, as they present honey bees with better nutrition and prolongs the foraging period.
It is well known that the honey bees used in migratory beekeeping receive an unvaried diet that impacts their health. A proposed solution to reduce pesticide use and provide additional nutrition to bees during commercial pollination is to sow flowering cover crops (eg clovers). Research conducted by the Californian almond industry showed that these alternative floral resources did not compete with the almond blooms, but rather enhanced the fruit set by providing bees with natural nutrition during the pollen shortages between blooms.
Pesticides Commonly Used on Almond Crops
Common fungicides applied in Australian almond orchards include chlorothalonil, azoxystrobin and copper oxychloride. The types of insecticides applied vary depending on location and need, such as tetraniliprole or the pyrethroid insecticides bifenthrin and permethrin. The issues with pyrethroid insecticides arise from their direct impacts on honey bees and the natural predators of the target pest. Growers are advised to adopt integrated pest management methods are as a more ecologically conservative solution.
How Plant Growth Regulators Enhance Almond Crop Yields
As a bonus topic, plant growth regulators such as ReTain are emerging chemical compounds used to extend the pollination periods by targeting the plant hormone ethylene, resulting in longer flower viability and subsequent increases in the pollination window for almond yields.
Other plant growth regulators include a mixture of micro-nutrients, antioxidants and growth enhancing co-factors (Flower Power), which enhances the success of pollination by increasing plant auxin hormone levels. Whether they have any impact on bee (or human) health, the research is ongoing.
What are the Effects of Glyphosate Herbicides (RoundUp) on Honey Bees?
Glyphosate, the active ingredient of formulations like RoundUp, is the most commonly used herbicide in the world. A 2014 study from Herbert & colleagues found that glyphosate herbicide use reducing the learning capabilities of bees, impaired new memory formation (even when memories were formed, they were rapidly forgotten) and hindered honey bees in differentiating between complex odours.
Paradoxically, the sublethal effects from glyphosate exposure did not alter honey bee foraging behaviour. As a consequence, these same forager bees become carriers of the herbicide, collecting more contaminated nectar and pollen to store in the hive. This combination of consistently visiting the same flowering plants, high stability of glyphosate in stored honey, and the capacity of A. mellifera to rapidly distribute food to its hive mates leads to a rapid accumulation of glyphosate at the colony level. A more detailed examination is provided in article “Does Roundup Kill Bees?”
Why Would Adult Bees Prefer Low Concentrations of Unnatural Compounds?
Honey bees can discern between food sources that contain additives such as pesticides (Liao et al., 2017). This behaviour is concentration-dependent, as the foraging bees will disregard a food source at higher adulterant concentrations, rather than continue collecting from it if it was at lower concentrations. Under semi-field conditions and low concentrations, these honey bees were found to prefer sugar syrups containing glyphosate and chlorothalonil, a broad-spectrum fungicide.
This preference for chlorothalonil can explain its presence as a frequent contaminator of beeswax and stored pollen. The fungicide has been linked to an increased risk of Nosema infections, lower colony biomass, and increased honey bee toxicity due to synergy with the Varroa mite acaricide fluvalinate.
Why bees may prefer such compounds is yet to be understood, suggesting that the taste perception of honey bees may be more complex than initially thought. Indeed, when the taste receptors of honey bees were examined in a 2011 review by de Brito Sanchez, the author challenged the notion that honey bees had a limited tasting capability. This observation is based on the ten gustatory repertories identified in honey bees, which suggested that they were lacking in sensory input with respect to taste.
The biology of honey bee taste is more complex than just sampling nectar and pollen sources. Honey bees are capable of “tasting” different compounds throughout their lives, from salts and organic acids, to lipids and growth regulator factors (de Brito Sanchez, 2011).
How Chlorothalonil produces “Entombed” Pollen
The ubiquity of chlorothalonil fungicides in honey bee colonies has been associated with “entombed pollen”, a type of pollen that is sealed-off inside the hive (vanEngelsdorp et al., 2009). The difference being that unlike the normal & colourful uncapped cells, the entombed pollen cappings appear sunken, (pointing arrows in figures A & B below) and are made entirely of propolis rather than the standard beeswax.
Analysis of the entombed pollen showed that it contained a plethora of pesticide residues, with chlorothalnoil being the most abundant. Compared to normal pollen, the entombed variety chlorothalnoil’s levels were 40 times higher, and high concentrations of other pesticides like coumaphos and fluvalinate were detected.
Interestingly, entombed pollen did not appear to impair larval development when compared to the control. The authors highlight that overall colony mortality increases with entombed pollen levels, and require future studies to determine the role and potential threats of this condition.
Effects of Herbicides and Fungicides on Solitary Bee Species
Most research is conducted on the honey bees Apis mellifera. A. mellifera is the most abundant and easiest bee species to study in agricultural settings. Different bee species such as native and solitary bees, have a much lower population compared to the superorganism that is the honey bee colony. As a result, the impact of insecticides, herbicides and fungicides often go undocumented.
Exceptions exist, with the Red Mason Bee Osmia bicornis being the example organism for studying solitary bees. In general, solitary species lack the superorganism resilience exhibited in honey bee colonies. If adult bees succumb to fatalities as a result of pesticide exposure, the colony survives and can adapt to meet this challenge. However, in the populations of (generally-solitary) native bees, the affected adult bees die out, which impacts the ecosystem and subsequent generations.
There are Issues with Studies on Agrichemical Use
Honey bees and other pollinators are exposed to a variety of acaricides, fungicides, herbicides and insecticides. The combined effects from the enormous number of possible agrichemical combinations remain largely unknown, and future research is in a constant competition to match the frequent releases of novel pest control measures.
Furthermore, the impacts of these pesticides are exacerbated by other stressors such as pathogens, climate variability and resource limitations. Very few studies have taken these factors into account and as such, they are usually overlooked during the pesticide risk assessment process by regulatory authorities.
Herbicide and fungicide effects are less studied than insecticides, and studies are difficult to replicate, with mixed findings depending on species, formulation, dose, and environmental context – what works for the almond orchards of California may not be the same for those orchards in Australia.
Benefits of Studying Bees Under Laboratory Conditions
Laboratory conditions are a starting point to investigate the toxicity of different pesticide types. However, even when a product is applied in accordance with the pesticide label, the true impact of field realistic doses may not become apparent. Regardless, lab-based studies allow researchers to detect subtle impacts and infer potential risks that may otherwise be masked in the more complex context of real-world conditions.
Which Herbicides and Fungicides Have Been Studied?
An analogy for our current understanding of the long-term outcomes of insecticides, herbicides and fungicides is like the Herculean Hydra: For every one compound studied, two new ones appear. Taking the metaphor further, these two new compounds “revive” their predecessors (as they are often used in combination), which further widens the knowledge gaps. Studying the active ingredients may not always be sufficient, as sublethal effects can arise not only from the pesticide, but also its adjuvant compounds.
In the face of these uncertainties, beekeepers and farmers are encouraged to adopt the precautionary principle: assume that all agrichemicals will impact bees and other pollinators, whether by directly affecting the organism’s health, or indirect effects on its external and internal environment.
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Footnotes
- Sulfoximines have a similar mechanism of action to neonicotinoid insecticides, in that they target the acetylcholine receptors as agonists, leading to insect death from nerve overstimulation. Countermeasures to preserve honey bee health include the banning of field and non-permanent greenhouse applications of these compounds by the European Food Safety Authority in 2019. Further reading on pesticide mechanisms of action is available on the effects of Pesticides on Honey Bees & the effects of Neonicotinoids on Bees. ↩︎