24
Sep
Neonicotinoid Insecticide Linked to Honey Bee Decline, Threatening Reproductive Function of Hive
(Beyond Pesticides, September 24, 2025) A novel study of chronic toxicity of the neonicotinoid insecticide thiamethoxam to honey bees (Apis mellifera), published in Insects, finds sublethal effects that threaten the survival of bee larvae and the health of bee colonies. “We evaluated the effects of thiamethoxam on the entire larval development cycle of reproductive bees and conducted a comparative analysis, demonstrating that thiamethoxam significantly alters ecdysone [a hormone that controls molting in insects] and juvenile hormone titers [hormones for insect growth] in both queen and drone larvae, impairing metamorphosis and reproductive development,†the authors state.
Risk assessments and scientific literature focus primarily on honey bee workers, rarely including how pesticides can impact queen and drone bees that are necessary for stable colonies. In focusing on this data gap, the researchers reveal that the survival rate of bee larvae decreases gradually with increasing doses of thiamethoxam, adding to the wide body of science on pesticide-related threats to honey bee health. (See Beyond Pesticides’ What the Science Shows on Biodiversity page for more information, as well as Daily News coverage on risk assessments here, here, and here.)
Honey bees provide crucial ecosystem services as pollinating insects and play a vital role in maintaining biodiversity. “In healthy colonies, the reproductive success of the queen and the genetic contribution of drones are essential for long-term survival, directly affecting brood production, genetic diversity, and environmental adaptability,†the researchers note.
This study assesses effects on reproductive bees (queens and drones) with exposure to thiamethoxam for key developmental and physiological parameters, including survival, pupation rate (ratio of immature insects that successfully develop into pupae, the stage between larva and adult), eclosion rate (the speed or proportion of insects that emerge from their pupal cases to become adults), hormone titers, and detoxification enzyme activities. “Our finding reveals that thiamethoxam exerts sublethal effects on larvae, significantly impairing the fitness of reproductive bees,†the authors explain. “Specifically, exposure altered juvenile hormone III, ecdysone titer, and acetylcholinesterase activity [enzyme activity necessary for nervous system and cognitive functioning] in reproductive larvae, with these effects showing a negative correlation with pesticide concentration.â€
Pollinators and Pesticides
The delicate balance seen in nature, with proper ecosystem functioning, relies on rich biodiversity. Insects, such as honey bees, are particularly crucial for pollination, which many plants depend on for survival and reproduction. The role of pollinators in global crop yields and biodiversity in natural ecosystems is threatened by environmental contaminants.
Research in the early 2000s documented the phenomenon of Colony Collapse Disorder (CCD), which is “characterized by the abrupt and widespread disappearance of worker bees from a colony, resulting in hives inhabited solely by the queen and immature brood, while honey and pollen stores remain intact and unplundered.†As the researchers describe: “This syndrome typically results in the eventual collapse of the colony. Declining honey bee populations and CCD are spreading globally, raising concerns about food supply shortages and diminishing biodiversity in ecosystems.†(See study here.)
The role of hazardous chemicals in CCD is widely documented in scientific literature, with research highlighting the multifactorial stressors that impact bees, including climate change, parasite infestations, pathogenic infections, inadequate nutrition, and pesticide exposure. (Learn about how pesticides exacerbate effects of parasitic Varroa mites in Daily News articles here and here.)
Study Methodology and Results
With research on thiamethoxam and bees mainly focused on worker bees, gatherers in particular, this study offers valuable insight into impacts on drones and queens that contribute to the stable reproduction of bee colonies. The quality of bee colonies is determined by the reproductive bees (drones and queens) as the mated drones transfer semen to queens. “As a result, elements that affect drone fecundity will also have an immediate impact on the colony’s offspring,†the authors point out. They continue, “Although a drone’s lifespan is short and its function is simple, the drone holds significant value in bee breeding because of its unique genetic structure.â€
To investigate the sublethal effects of thiamethoxam on the reproductive bees, the researchers added three concentrations of the compound alongside fresh royal jelly (a substance produced by worker bees as a food source for queen bees and larvae) and supplied it to larvae obtained from honey bee colonies. To determine the dosages for the study, additional research was utilized, where levels of thiamethoxam in nectar, pollen, and plant secretions range from 1 to 100 mg/kg (equivalent to 1,000 to 100,000 μg/L), to provide environmentally relevant concentrations.
Within the study, the low, medium, and high concentrations of 3 μg/L, 25 μg/L, and 2300 μg/L all fall well below the highest values referenced above. After exposure, statistical analyses of survival rate data, larval enzyme activity, hormone titer, larval mortality, pupation rate, eclosion rate, and pupal weight and length were performed and analyzed.
The results reveal sublethal effects of thiamethoxam on the overall survival of queen and drone honey bee larvae that lead to chronic impacts impacting development into adulthood. All concentrations significantly reduce the total survival rate of queen bee larvae. However, in drones, only the highest concentration results in a significant reduction in the overall survival rate of larvae. At this dosage, pupation rates of drones also significantly decrease.
Exposure to thiamethoxam at the medium and high concentrations leads to pronounced detrimental effects on pupal development. In the queen bees, they significantly reduce pupal weight. Drone pupae at these levels also exhibit a significant decrease in pupal length, indicating impacts of thiamethoxam on growth and morphogenesis during pupal development. In drones, all three concentrations significantly lower pupal weight, “suggesting that even low thiamethoxam concentrations could negatively affect nutrient accumulation or metabolic processes during development,†the researchers explain.
At the high level, both emergence and eclosion rates significantly decline when compared to the control group. Enzyme activity, particularly in those related to oxidative stress and detoxification, is impacted, with both drone and queen larvae experiencing dose-dependent decreases. The hormones related to insect development and growth also exhibit dose-dependent effects in all treatments.
“We showed that the exposure of the reproductive bee larvae to thiamethoxam may influence the survival rate, pupation rate, eclosion rate, hormone titer, and enzyme activity when present in the field,†the authors summarize. They continue: “Pesticide exposure resulted in the spreading of poor-quality semen and affected the offspring, although the physical integrity of the drone was maintained. If the physiology and detoxification enzyme system of queens is strongly impacted and jeopardizes their survival, the mating success rate might be compromised and generate a shortage of healthy workers in congregation areas, which would be highly detrimental to the species.â€
The findings of this study demonstrate how thiamethoxam adversely affects the growth and development of reproductive bees, thereby threatening colony stability and success. “Collectively, these interactions form a vicious cycle: pesticide-induced stress perturbs detoxification and neuroendocrine functions, which in turn exacerbate metabolic deficits and ultimately lead to behavioral abnormalities, reduced longevity, and colony decline,†the researchers conclude.
Previous Research
There are a large number of studies highlighting the effects of neonicotinoids on honey bees, as well as other species of bees. Many studies highlight the impacts of various chemicals within the class of neonicotinoids on the learning and foraging of honey bees that impair colony fitness, while additional studies specifically involving thiamethoxam report effects on visual learning, altered decision times, and increased abnormal behaviors. (See examples of scientific literature here and here.)
One experiment in a colony feeding study of honey bees demonstrates how thiamethoxam exerts significant sublethal effects on larval development. As the current study authors note: “Their experimental findings indicate that chronic exposure to environmentally relevant concentrations of thiamethoxam severely impairs larval viability and leads to a marked reduction in adult emergence rates, particularly in queen-rearing colonies. Our current findings on chronic toxicity in reproductive bees corroborate and extend these observations, revealing that elevated thiamethoxam concentrations induce statistically significant reductions in emergence success for both queen and drone.â€
Previous Daily News coverage further highlights risks from neonicotinoid exposure that extend to other species, including humans. See Study Adds to Findings of Neonic Insecticides’ Threat to Soil Communities; Ground Beetles at Risk for an analysis of a study conducted in Pennsylvania and published in Environmental Entomology that reveals threats to nontarget organisms from neonicotinoid insecticide exposure. Neonicotinoid Exposure Threatens Fish Health, Highlighting Cascading Effects to Humans as Consumers reports on a study in Environmental Pollutants and Bioavailability that finds genotoxicity, oxidative stress, and changes in tissue structure, among other threats to organ function and overall fish health, from thiamethoxam exposure that can lead to impacts throughout the entire food web.
Organic Solution
Sublethal effects from thiamethoxam to drones and queens, as documented in the current study, threaten their essential role in the life cycle of a honey bee colony. To mitigate these effects, as well as numerous others to pollinators and other insects, Beyond Pesticides urges the widespread adoption of organic agriculture and land management. Not only does this holistic solution remove the use of petrochemical pesticides and synthetic fertilizers, including but not limited to neonicotinoid insecticides, that threaten honey bees, but it also protects and enhances biodiversity and mitigates both the climate change and public health crises we are currently experiencing.
For more information on the direct impacts of organic practices on pollinators, see Study Adds to Wide Body of Science Highlighting Benefits of Organic for Insect Biodiversity. Additional health and environmental benefits are available here and here.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Li, M.-J. et al. (2025) Systemic Assessment of Chronic Toxicity of Thiamethoxam on Honeybees (Apis mellifera), Insects. Available at: https://www.mdpi.com/2075-4450/16/9/936.
