30
Jan
Study Finds Pesticide Exposure to Bees During Dormancy or Overwintering Disruptive of Reproductive Health
(Beyond Pesticides, January 30, 2025) A research article in Biology Letters, published by The Royal Society, finds that the neonicotinoid insecticide imidacloprid disrupts survival and reproductive patterns in Bombus impatiens bumblebees. The study adds to the wide body of science highlighting how exposure to pesticides “can result in immediate mortality or cause long-term detrimental effects on pollinators‘ health, lifespan and reproductive success,†the authors state.
The researchers performed two experiments to assess the effects of various concentrations of imidacloprid, one of which focuses on bees during diapause, a period of dormancy. “Wild bees, which provide the majority of pollination services worldwide, undergo an annual life cycle that includes a winter diapause, that can span over 75% of their life cycle and during which their metabolism, growth and development are halted,†the authors note. They continue: “The time spent in diapause can have lasting effects on pollinator fitness and their ability to establish nests or colonies in the following spring. This period is especially critical for social bee colonies, which are founded by a single queen after diapause and play a vital role in large-scale pollination.â€
Exposure to pesticides during diapause can occur while bee species overwinter in contaminated soils. In the U.S., many pesticides are highly water-soluble and are found in many nontarget plants, such as in the pollen, nectar, and soil that bee species encounter. (See additional coverage on soil pesticide residues here, here, and here.) “Diapause is a critical period in the life cycle of most bees with profound effects on their health. Exposure to sublethal doses of pesticides may increase bees’ resistance to stress/cold during diapause but may also trade off with reduced reproductive performance later in life,†the researchers state.
They continue: “Bumblebees are annual and social pollinators of great importance to pollination of agricultural and wild crops. A single queen starts a nest in the spring and can produce up to several hundreds of workers at the peak of the season. Towards the end of the summer, the colony produces sexuals [drones and worker bees], and only the new queens (gynes) survive the winter by entering a diapause of six to nine months following mating.â€
In this study, the authors assess hormesis in bumblebees with imidacloprid exposure. Hormesis is “a general adaptive response characterized by low-dose stimulation and high-dose inhibition, where overcompensation or stimulation occurs after homeostasis is disrupted,†they denote. By feeding bees different concentrations of imidacloprid and examining “(i) the effect of imidacloprid on longevity and reproduction of gynes, males and workers and (ii) how imidacloprid exposure affects gynes’ diapause survival length,†the researchers find hormetic stress responses in Bombus impatiens.
Twelve colonies of bees were used in the first experiment while six colonies were used in the second experiment. Within the first experiment, the control groups were not subjected to any pesticides, while the treatment groups were fed concentrations of either 6, 60, 150, 300, or 600 ppb of imidacloprid. “These concentrations were chosen to cover both field-realistic and LD50 [lethal] doses of imidacloprid shown to cause mortality within days in other species of bees,†the researchers report. Mortality and the total number of offspring produced were recorded until all bees died.
In the second experiment, gynes (new queens) were collected and “provided pesticide-free sugar solution for the first 3 days, then divided into treatment groups and received either 0, 6 or 60 ppb imidacloprid during days 3-6 after emergence [and] [g]ynes were then weighed and placed in cold storage†and checked weekly for survival until all died. This experiment represents the process of diapause.
As a result, the authors note that imidacloprid treatments higher than 150 ppb reduce survival in gynes, workers, and males. The number of offspring produced was also highest in the control group, showing that even low concentrations of imidacloprid, such as the field-realistic doses of 6 and 60 ppb, can impact reproduction.
In summarizing these results, the authors state: “As anticipated, bee longevity and reproductive performance declined with increasing concentrations of imidacloprid. However, when gynes were exposed to sublethal concentrations and subsequently entered diapause, their survival was greater compared with the control, demonstrating a classic hormetic response. Notably, a recent study showed that bumblebee gynes actively seek out pesticide-contaminated soil, potentially indicating an adaptive strategy to enhance their fitness.â€
Similar to previous study results, within the second experiment there was a higher proportion of gynes that survived diapause in the treatment groups versus the control group. This reflects hormetic responses that allow organisms to survive unfavorable conditions. The researchers postulate that a “mechanism linking pesticide exposure to greater diapause survival involves its effects on thermal tolerance and metabolism, possibly enhancing cold resistance at the cost of reduced reproductive performance later in life… Pesticides are known to affect the expression of heat shock proteins [HSPs], and several studies have shown that pesticide exposure alters heat or cold tolerance… Sublethal pesticide exposure often impairs reproduction even without increasing mortality, suggesting a trade-off between detoxification and reproduction.â€
Studies show that HSPs “act to protect organisms from various environmental stressors such as heat, cold, desiccation, toxins, pathogens, and others. In this regard, levels of transcripts and translated proteins encoded by HSP genes can be used as biomarkers to monitor cellular and physiological responses to various environmental stimuli.†Another study finds hormetic responses in insects along with an increased expression of detoxification genes, allowing insects to withstand higher levels of pesticides.
Hormetic responses to stressors such as pesticides have been documented to enhance performance in some areas but may reduce the fitness of the next generation. While additional research is needed to determine the full consequences of their enhanced detoxification abilities, studies do show that there is a cost for the hormetic stress responses including decreased reproduction in future generations. (See more on multigenerational effects here.)
One study finds that “Osmia bees exposed to anoxia hormesis exhibit improved flight, mating and longevity compared with controls, yet their offspring experience developmental delays and incomplete adult emergence.†Another study finds that: “exposure to neonicotinoids affected Osmia females and reduced the reproduction of their offspring, even when the offspring were raised in a pesticide-free environment. This highlights the complex, diverse and long-lasting effects of pesticides on pollinators and the potential stimulatory effect they may have in addition to their harmful effects.â€
Pollinators provide ecosystem services and are essential for food security but face threats from habitat loss, pesticide exposure, climate change, and many other factors. One of the largest chemical threats is from neonicotinoids. “Although their use in Europe has been banned or restricted, they are still very popular in the United States, with 24% of the global insecticide market. On top of that, they are highly soluble in water, persist in the soil and of a particular danger to diapausing bees,†the authors note.
Exposure to pesticides, even at low concentrations, can compromise pollinator health. A multitude of studies find exposure affects key traits such as survival, reproduction, learning and memory, flight, and foraging, among others. The risk assessments conducted on pesticides by the U.S. Environmental Protection Agency (EPA) lack comprehensive data regarding their effects on bees. The limited studies that are performed do not adequately assess the varying impacts throughout all bee species, which display differing levels of sensitivity, nor do they account for the cumulative effects through various routes of exposure to pesticide mixtures. (See more on EPA failures here, here, here, here, and here.)
The researchers conclude that: “[T]he accumulation of pesticide residues in soil is frequently overlooked in standard assessments, leading to incomplete evaluations of pesticide safety. To fully understand the impact of pesticides on pollinators, it is essential to assess their effects across all life stages and account for the varying vulnerabilities throughout the insect life cycle… Not accounting for these hormetic effects can skew survival tests, causing underestimations of the risks neonicotinoids pose to beneficial pollinators. Additionally, without a thorough understanding of the latent effects of pesticides and the trade-offs between diapause survival and post-diapause performance, the full extent of the problem remains unclear, potentially undermining conservation efforts and agricultural policies. This is particularly critical for social bees, such as bumblebees, whose populations depend heavily on the successful emergence of individuals from diapause.â€
As an alternative to harmful petrochemical pesticides, crop production practices can consider pollinator preservation, which in turn protects food security and biodiversity, by employing organic land management strategies. Organic agriculture is the solution that not only eliminates exposure to toxic synthetic pesticides for pollinators but also supports soil health and the health of all organisms, including humans.
In previous coverage, Beyond Pesticides reports how researchers find that organic farming provides the highest benefit to bees, as organic practices lead directly to lower parasite load and higher colony growth. See more on the benefits of organic land management here and make The Safer Choice to avoid hazardous home, garden, community, and food use pesticides.
Image: Art Page submission from Marion Yaglinski, “Bumblebee on Oriental Chives.”
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Amsalem, E., Derstine, N. and Murray, C. (2025) Hormetic response to pesticides in diapausing bees, Biology Letters. Available at: https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2024.0612.
