By: Jay Evans
Honey bees are interconnected within colonies, with a crowded three-dimensional space that favors constant contact. Add shared feeding and a mite that is mobile enough to connect viruses from one bee host to another, and it is safe to say that no colony member is socially distant from the whole. Fortunately, bees have a range of defenses, from molecules to mandibles, that help them reduce the spread and impacts of disease. When this fails, beekeepers can add another layer, most importantly by reducing mite levels and being overly cautious with signs of brood disease. Thanks to these defenses, honey bees are bruised but still with us and hardworking beekeepers are still providing a great service to agriculture.
One research area that has received much attention is the ability of environmental stressors, including pesticides, to reduce the defensive posture of bees and colonies against disease. Early work by Gennaro di Prisco and colleagues suggested that chemical stress directly impacts the immune responses of bees toward viral infection, a fact that was championed to explain an apparent upswing in viral impacts on bee health (“Neonicotinoid clothianidin adversely affects insect immunity and promotes replication of a viral pathogen in honey bees”. 2013. Proceedings of the National Academy of Sciences of the United States of America, 110(46), 18466-18471, https://www.pnas.org/content/110/46/18466). While mite vectors are arguably far more important for viral disease than chemical stress, determining a role for chemical stress on viral immunity has led to several important research studies. These studies are summarized in a timely review by Gyan Harwood and Adam Dolezal in the open-access journal Insects (“Pesticide–virus interactions in honey bees: Challenges and opportunities for understanding drivers of bee declines”. 2020. Viruses 12, 566. https://doi.org/10.3390/v12050566). Alongside a plea for additional research, the authors of this review make a good case for connecting the dots between chemical insults and vulnerability to viruses. Interestingly, while they describe many laboratory studies that show an increase in viral levels when bees are exposed to chemicals, such viral outbreaks have not been confirmed in colonies in the field. Why is that?
First, field experiments are far more costly and it is hard to generate the numbers of independent control and exposed colonies needed to see subtle changes in viral infections. Second, in even the most ambitious field experiments, colonies are able to collect food sources beyond those prepped with chemicals, and it is possible they are diluting pesticide intake. Weakening these two hypotheses, bees in field colonies HAVE been shown to express behavioral and longevity effects from exposure to chemicals. An alternative explanation for survival of honey bee colonies after chemical stress is resilience (the topic of my July Bee Culture article). Several recent papers suggest that this resilience starts in the bee gut.
Having done numerous lab studies with live bees, it is really hard to give them adequate ‘lab-based’ nutrition. It is quite likely that most laboratory stress assays for bees involve suboptimal nutrition. Further, as documented abundantly by May Berenbaum and colleagues at the University of Illinois, natural chemicals found in many pollen and nectar sources can themselves trigger defenses in bees that help reduce the impacts of dangerous pesticides (for example, ‘Increase in longevity and amelioration of pesticide toxicity by natural levels of dietary phytochemicals in the honey bee, Apis mellifera. 2020. PLoS ONE 15, e0243364; doi:10.1371/journalpone.0243364). The impacts of plant-based chemicals on bee health merits it’s own discussion given strong evidence that these chemicals can also reduce bee disease, but the link to pesticide tolerance is especially interesting. Certain chemicals found in pollen and nectar trigger the same detoxification enzymes bees use to dampen the impacts of pesticides. If this triggering offers protection (a phenomenon called hormesis or, informally, ‘hair of the dog’) this might lead to practical avenues of reducing chemical stress on bees. Of course, it is also possible that plant chemicals over-tax the same enzymes and other processes that are needed to smother pesticides, in which case synergies might arise to the harm of bees. Synergies between agrochemicals that lead to unexpectedly dire results for bees are known, but so far there are no identified synergies of this sort between chemicals found in bee forage and synthetic chemicals used in farming or beekeeping.
So what is the evidence that nutrition, or these plant chemicals alone, can provide real-world protection to honey bees against pesticide stress? A recent paper by Lena Barascou and colleagues shows that supplemental pollen gives some measure of protection against the pesticide sulfoxaflor (Pollen nutrition fosters honey bee tolerance to pesticides. 2020. R. Soc. Open Sci. 8: 210818. https://doi.org/10.1098/rsos.210818). Bees given a pollen boost had lowered mortality after both acute and chronic exposure to sulfoxaflor. After chronic exposure, bees had 2.5- and 2-fold greater survival when exposed to sulfoxaflor IF they were fed a pollen mix heavy with mustard and oak sources, or one with pollen from willow trees and other sources, respectively. Only the former pollen offered protection from an acute pesticide dose in this study. Interestingly, oak pollen is heavy with one of the plant chemicals highlighted by Berenbaum’s group as being especially important for bee detox responses. Given the different chemistries of these pollens, it is also possible that pollen as a whole can provide some protection from chemicals, i.e., a ‘better nutrition equals better resilience’ hypothesis is not ruled out.
Bees in colonies also carry abundant microbial associates in their guts and there is evidence that these associates themselves enable greater tolerance toward pesticide stress. This is important for beekeepers both because antibiotic use in hives is known to reduce microbe numbers and change their constituencies, and because several researchers and companies are developing microbial supplements (prebiotics and probiotics) purported to improve bee health. In one recent study, Brendan Daisley and colleagues tested the abilities of a common environmental bacterium to reduce the impacts of imidacloprid on fruit flies (“Neonicotinoid-induced pathogen susceptibility is mitigated by Lactobacillus plantarum immune stimulation in a Drosophila melanogaster model”. 2017. Scientific Reports, 7:2703, https://www.nature.com/articles/s41598-017-02806-w). As with bees, low-level chemical exposure increased disease susceptibility, but that susceptibility was offset by supplementing the insects with a single bacterial species. The authors argue that this dynamic could help bees in the field survive chemical insults and they are actively pursuing this. In support of this, researchers in the laboratory of Nancy Moran (a pioneer who has made huge discoveries in the roles and makeup of bee gut bacteria) have shown that bees given a probiotic ‘cocktail’ after antibiotic cleansing of their guts show greater resistance to bacterial disease (“Field-realistic tylosin exposure impacts honey bee microbiota and pathogen susceptibility, which Is ameliorated by native gut probiotics” 2021. Microbiology Spectrum. 9(1):e0010321. doi: 10.1128/Spectrum.00103-21. They and others have also shown interactions between these native gut bacteria and pesticides used in agriculture.
In summary, different experimental outcomes in field versus laboratory experiments require us to push for more realistic lab setups (modeled perhaps by the Barascu paper and with a ‘common’ pollen source). It is also important to invest, where possible, in more costly but more realistic experiments in the world of beehives. Finally, we are learning more and more about both the subtle harms of a range of environmental stressors, including pesticides, and potential means for reducing these impacts on bees and other key pollinators.