Neonicotinoid Insecticides Contribute to Honey Bee Vulnerability to Parasitic Varroa Mites
(Beyond Pesticides, July 10, 2024) An article last month in Entomology Today, a publication of the Entomological Society of America, highlights the important findings of a study published earlier this year in the Journal of Insect Science. While there has been debate on whether neonicotinoid (neonic) insecticides or Varroa mites (Varroa destructor) are more detrimental to the survival of bees, evidence suggests that neonicotinoids are not only harmful individually but can increase vulnerability to parasitism from mites in western honey bees (Apis mellifera).
The Entomology Today article reads: “Some researchers and organizations have pointed to neonics as directly harming bees. Others have pointed to other issues, like Varroa mite infestation, as more hazardous to honey bee populations.” There is scientific evidence supporting each claim, as both cause stress to bee species that can lead to population decline. The study in the Journal of Science, however, is “the first experimental field demonstration of how neonicotinoid exposure can increase V. destructor populations in honey bees and also demonstrates that colony genetic diversity cannot mitigate the effects of neonicotinoid pesticides.”
As the article states, “The researchers were not looking for impacts on Varroa mites at first. Instead, they were looking to understand how genetic diversity of a honey bee colony could buffer the bees from environmental toxins, says Lewis Bartlett, Ph.D., assistant professor of entomology at the University of Georgia and lead author of the study.” The study uses controlled honey bee colony exposure to field-relevant concentrations of clothianidin and thiamethoxam in pollen while also simultaneously manipulating intracolony genetic heterogeneity.
Genetic diversity, as a result of queen polyandry (a queen bee mating with multiple drones), can improve colony function, health, and fitness. As the study shares, “A wealth of studies have linked increased queen polyandry to reduced parasite (including Varroa) pressure in colonies,” which led the researchers to study the effects on colony vitality with increased polyandry.
“We used brood exchange (transplantation) treatments to simulate the effects of a higher degree of polyandry in a colony. We tested whether this induced period of genetic heterogeneity not only improved colony health (for example, better brood survivorship, greater pollen foraging, reduced parasitic Varroa pressure, and increased queen survival) but also whether it improved colony tolerance to exposure of a mix of 2 common agricultural neonicotinoids (clothianidin and thiamethoxam) fed to colonies in contaminated pollen,” the authors write.
Data was collected across three sites at the University of Delaware, University of Georgia, and Auburn University in Alabama. The honey bee colonies were force-fed clean pollen patties or pollen patties dosed with the two neonicotinoid insecticides and were denied the importation of foraged pollen by pollen traps being placed at all colony entrances. “Neonicotinoid-dosed patties were supplemented with 4.5 ng g−1 thiamethoxam (Sigma-Aldrich) and 1.5 ng g−1 clothianidin (Sigma-Aldrich) at the mixing stage,” the researchers note. “These concentrations reflect those found in pollen from flowering crops and crop-adjacent wildflowers in agricultural systems using these neonicotinoids.”
The colony responses assessed at all sites include “size (capped brood area), queen survival (whether marked queens were confirmed as still present), pollen (mass of corbicular pollen gathered in 24 h), aggression (recruitment of guard bees to colony entrance following alarm pheromone exposure), per-capita Varroa parasitism (phoretic mites washed using ethanol from 300 adult bees), Varroa mite drop (mite drop rate onto sticky screen colony bottom boards), comb construction (area of comb constructed onto undrawn frame inserted for 24 h), and brood survival (proportion of identified L1–L2 larvae that survived to capping),” the authors report.
As a result, the researchers, “find evidence of a detrimental effect of neonicotinoid exposure on colonies, where both mite drop rates and mite wash counts were higher in colonies exposed to neonicotinoids; colonies exposed to neonicotinoids had on average 5.3 more mites drop in 24 h and 1.11 more phoretic mites per 100 bees.” They continue in saying, “We found that field-relevant neonicotinoid exposure increased the severity of Varroa parasitism rates in exposed colonies. We did not find evidence supporting the hypothesis that this negative effect could be mitigated by increased genetic diversity in the colony, as in no instance did we find a significant interaction between pesticide exposure and brood mixing. Additionally, we found no evidence that increased genetic diversity compensated for the negative effects of the neonicotinoids, as the 2 treatments did not overlap in which colony phenotypes they significantly affected. We did, however, confirm the beneficial effects of increased genetic diversity in the form of improved brood survival.”
The Entomology Today article reiterates that the authors did not initially seek to find effects on Varroa mites with a quote from Dr. Bartlett: “‘We didn’t expect to find our main result at all…The only negative effect we observed was the increased severity of parasitism, presumably due to some immunosuppression,’ Bartlett says.”
As the original study emphasizes, “Agrochemical exposure is a major contributor to ecological declines worldwide, including the loss of crucial pollinator species. In addition to direct toxicity, field-relevant doses of pesticides can increase species’ vulnerabilities to other stressors, including parasites… Both wild and managed bees are threatened by interacting combinations of stressors, including forage loss, parasite pressures, and exposure to agrochemicals, including pesticides.”
Most notably, pollinators are severely impacted by neonicotinoids, a class of insecticides that share a common mode of action that affects the central nervous system of insects. Studies show that neonicotinoid residues accumulate in the pollen and nectar of treated plants and represent a risk to pollinators by causing paralysis and death upon exposure. While the U.S. Environmental Protection Agency (EPA) has a system in place for risk assessment pertaining to pesticides that impact bees that includes a tiered process, there are many flaws in place regarding the assessments and regulations of neonicotinoids. (See more on Beyond Pesticides’ coverage of EPA’s risk assessment shortcomings here.)
As the study authors maintain, “Agricultural pesticides are known to interact in their toxicity, leading to suspicions that their dangers to bee pollinators are underestimated.” Not only are there synergistic effects between pesticides, but they also interact with other environmental pollutants and cause greater harm to organisms and the environment. As seen in the body of science that this study adds to, neonicotinoids and Varroa mites are interacting stressors for bees that also have synergy. Previous studies have shown how the interacting stressors of neonicotinoids and mites can cause 70% reductions in overwintering honey bee survival.
Additional studies have found that bee-toxic neonics do not provide any benefits to agriculture, and only sow seeds of doubt. The impact of pesticides on bees is an unreasonable amount of harm, even with any potential benefits, according to environmental advocates. Populations of bees continue to decline, which threatens pollinator’s critical contribution to plant health, crop productivity, and the preservation of natural resources.
The researchers remark, “There is no shortage of observational and experimental studies emphasizing the synergistic impacts of pesticide and parasite exposure on honey bee health, including neonicotinoid and Varroa pressure. Moreover, field observations have explicitly linked neonicotinoid exposure to vulnerability to Varroa, with crucial laboratory studies… demonstrating how the neonicotinoid clothianidin increases Varroa reproduction rates due to reducing honey bee hemolytic immune response. Our work completes this literature body by showing that the individual-level findings… are mirrored at the colony level with contaminated pollen and is the first (to our knowledge) demonstration of this link between neonicotinoid and Varroa parasitism using a manipulated field experiment.”
These results, specific to honey bees, may not represent all bee species. As previously covered by Beyond Pesticides, pesticide sensitivity in different species can vary dramatically. The authors of this study confirm that notion in saying, “While our neonicotinoid treatments were at field-realistic doses, honey bees may be more resilient in the face of these pesticides compared to other bees.” This could leave other bee species even more vulnerable than the honey bees that experienced increased parasitism in this experiment.
The authors conclude that, “The combination of pesticide exposure and parasitism has antagonistic, additive, or synergistic impacts on honeybee health compared to either effect in isolation… The impact of the neonicotinoid in increasing Varroa populations both in absolute terms and in per-capita parasitism is in agreement with laboratory and observational studies and with this present work has now been demonstrated in the field, contributing to a large body of literature on the multiple and interacting stressors on pollinators. It is a notable finding that we confirm in-field that neonicotinoids can exacerbate the abundance of the parasite Varroa, which is arguably the single most severe contributor to managed honey bee losses in the United States.”
Given these interacting stressors, organic agriculture is understood to be the solution. Without the use of harmful pesticides like neonics, the threat to bees and other pollinator species can be diminished. Alternatives are available for sustainable agricultural practices and for reducing parasites in hives. Research suggests that growing sunflowers near honey bee colonies help in reducing mite problems. Organic beekeeping practices are proven successful, as is organic crop production. Worldwide adoption of organic land management practices continues to increase, with entire towns going pesticide free.
Protecting honey bees and pollinators from pesticides is crucial to agricultural and economic productivity, as well as food security. Take action to advance organic, sustainable, and regenerative practices and policies. Be part of the organic solution by becoming a member of Beyond Pesticides today and subscribe to the Daily News to see the latest information on the health and environmental hazards of pesticides, pesticide regulation and policy, pesticide alternatives, and cutting-edge science.
