18
Jul
Variability in Effect of Pesticides on Bumblebee Survival Tied to Gut Microbiome Health
(Beyond Pesticides, July 18, 2025) A study in Royal Society Open Science shows intraspecific differences (between individuals of a species) in wild bumblebees (Bombus vosnesenskii) exposed to an herbicide (glyphosate), a fungicide (tebuconazole), and an insecticide (imidacloprid), with gut microbiome health as a factor. “Wild pollinator declines are increasingly linked to pesticide exposure, yet it is unclear how intraspecific differences contribute to observed variation in sensitivity, and the role gut microbes play in the sensitivity of wild bees is largely unexplored,†the authors explain. “Here, we investigate site-level differences in survival and microbiome structure of a wild bumble bee exposed to multiple pesticides, both individually and in combination.â€
In collecting 175 individuals of this wild, foraging species from an alpine meadow, a valley lake shoreline, and a suburban park and exposing them to a diet with individual pesticides and mixtures, the researchers assess the varying lethal and sublethal effects that can occur with pesticide exposure. Between the three sites, the survival differences “emphasize the importance of considering population of origin when studying pesticide toxicity of wild bees†and highlight how pesticide sensitivity not only varies between species but within individuals of the same species with site-specific impacts. (See previous Daily News Literature Review Analyzes Pesticide Sensitivity in Bee Species on a Molecular Level for further analysis.)
“Bumble bees and other wild bees are vital pollinators of crop plants and wildflowers in North America, often outperforming managed species like honey bees,†the authors note. (See studies here and here.) They continue: “Due to their agricultural and ecological importance, bumble bee population declines have gained global attention, which have been linked to a combination of human-induced stressors. Key among these is the widespread use of insecticides, fungicides, and herbicides.†(See studies here, here, and here.)
Pesticide residues are ubiquitous in natural settings, including urban, suburban, and conservation areas. “Moreover, pesticides are rarely observed in isolation and are often detected as combinations of different chemical classes (i.e., insecticides, fungicides, and herbicides) in a complex chemical cocktail,†the researchers say. “Understanding synergistic effects of pesticides on bee health is therefore an active area of study.â€
To perform the study, bees were collected from the three sites in northern Nevada and assigned to one of five groups. These groups include experimental sucrose solution diets with the different treatments: herbicide (glyphosate), insecticide (imidacloprid), fungicide (tebuconazole), combination, and control. “These chemicals were selected because they had the highest estimated use for each chemical class (insecticide, herbicide, fungicide) in our region (Washoe County, Nevada),†the authors note. The concentrations also reflect field-realistic exposure for the area.
Bees were monitored for survival throughout the experiment until day 20 or until they died, at which point they were weighed, measured for body size, and dissected to extract DNA from their guts for bacterial abundance evaluations. An evaluation of the gut microbiome provides insights into the indirect effects that chemicals can have on bee health in addition to the direct physiological harm.
As the study states: “The bumble bee gut harbours a simple community of microbial symbionts that aid in digestion, growth, protection against parasites and pathogens, and detoxification. Gut microbes also aid in resilience to pesticides by facilitating expression of cytochrome p450 enzymes involved in detoxification, so disruptions to the microbial community can have consequences for bee survival.†(See studies here, here, here, and here.)
The study results reveal variations in body size at the different sites. “Bees from the alpine site were an average of 8.2% larger than bees from the shoreline site, and an average of 11.2% larger than bees from the suburban site,†the researchers say. “Shoreline bees were only an average of 3.0% larger than our suburban bees.â€
This sizing correlates with trends in survival. The average alpine bee survived to day 12.1, the shoreline bee to day 8.1, and the suburban bee to day 7.0. The authors also note: “Here, we also saw evidence of treatment effects. Bees assigned a diet containing the combination of all three chemicals showed marginally shorter survival time than control bees, but this was the only treatment that had an overall impact on survival. However, we did observe a site-by-treatment interaction with the fungicide, herbicide and all chemical treatments.â€
The larger bees, mostly noted at the alpine site, also show the presence of more gut bacteria. “We observed site-level differences in community composition and we also observed difference in gut community composition depending on if bees survived longer over the course of the experiment,†the researchers write. They continue: “We found a change in the gut community in bees that survived longer, which is consistent with other studies demonstrating a change in gut bacterial communities over time. We also observed site-level differences in gut community structure wherein our suburban site differed from the other two sites, and post hoc analysis showed this site had fewer gut bacteria. Bees from this site also had the poorest overall survival.â€
Previous Beyond Pesticides coverage (see here, here, here, and here) highlights studies that show how pesticide exposure disturbs and shifts the abundance of certain microbes in the bee gut microbiome. These disturbances primarily occur in one of two ways—either by directly harming microbes or indirectly harming the host (bee) and subsequently shifting the microbiome.
In summary, the study authors write: “We found that, regardless of the site, bees showed a marginal reduction in survival when given our chemical combination treatment. Yet, site of origin played a strong role in determining a bee’s survival. We found evidence for a site-by-treatment interaction influencing the day a bee died across all pesticide treatments, except the insecticide group. The microbiome did not differ when bees were exposed to the pesticide treatment, within or across sites. Instead, gut microbiome differences were associated with the presence of conopid parasitoids and body size, both of which varied by site. These findings suggest that site-specific factors influence pesticide sensitivity and should be considered in ecotoxicological studies of wild bees.â€
This study demonstrates that bee responses, even within the same species, to pesticides and their mixtures can vary greatly and are dependent on various factors. In testing field-realistic concentrations of common pesticides, the researchers show how intraspecific and site-specific mortality can occur, while also highlighting that “it is essential to note that these chemicals can have serious sub-lethal effects such as impacts to immune response, metabolism, reproduction, cognition, and more.†(See research here and here.)
The authors also comment that “pesticide sensitivity assessments must account for these variables to fully capture population-level patterns. Expanding research to include wild bees will allow for a deeper understanding of ecological complexities and help inform more effective conservation strategies.†Current pesticide risk assessments that analyze effects on bees primarily focus on a limited subset of species and do not provide comprehensive protection of all wild bees. (See Daily News Study of Pesticide Risk in Wild Bee Species Highlights EPA Risk Assessment Inadequacies and Field Study of Bumble Bees Finds Exposure to Chemical Mixtures, High Hazard, Flawed Regulation for more about these regulatory deficiencies.)
“Although there have been efforts to examine pesticide sensitivity in wild species, the degree to which population-level differences influence sensitivity to pesticides is not often considered,†the researchers postulate. They continue: “This is crucial because stressors that influence bee abundance vary across landscapes, and pesticide tolerance interacts with these stressors like parasite load, nutritional status and even temperature. In addition, wild bumblebees can exhibit variation in abundance of gut microbes across landscapes, likely due to variation in these stressors and floral resources. If we want to identify generalizable patterns regarding which populations of wild bees (or their microbiomes) are most vulnerable to disruption by agricultural chemicals, a deeper understanding of factors driving sensitivity across landscapes is thus essential.â€
Beyond Pesticides has long documented the failure of risk assessments and regulatory processes that do not properly protect health and the environment. With these deficiencies, the only solution is a systems-based, holistic transition to organic land management that removes the threats of petrochemical pesticides and synthetic fertilizers. This protects pollinators, such as bumblebees, from harmful chemicals, as well as enhances biodiversity and mitigates the crises of climate change and associated public health implications.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Tatarko, A. et al. (2025) A wild bumble bee shows intraspecific differences in sensitivity to multiple pesticides, Royal Society Open Science. Available at: https://royalsocietypublishing.org/doi/10.1098/rsos.250281.
