The widely used neonicotinoid (“neonic”) insecticide imidacloprid was the focus of the study done by a team at Imperial College London, whose key finding is summarized in a press release issued by the college:
“Baby bees can also feel the effects of the contaminated food brought back to the colony, making them poorer at performing tasks later in life.” (Dunning, 2020)
Imidacloprid has previously been shown to have negative behavioral impacts on adult bumblebees. In this new study, bumblebee colonies were exposed to levels of imidacloprid equivalent to what bees experience when foraging on, or near newly-treated fields. Impacts of exposure to brooding young and young adult bees on neurodevelopment and behavior were tracked and compared to colonies in a control group that was not exposed to neonic insecticides.
Twenty-two bumblebee (Bombus terrestris audax) colonies were procured for this study. The bees in the five control colonies were not exposed to imidacloprid. The other colonies were split between three treatment groups:
- Pre-Eclosion: This group was treated before the young bees molted into their adult form, starting at about Day 2 and ending around Day 21. This group was “used to investigate the effects of exposure during brood development” (Smith et al., 2020)
- Post-Eclosion: This group was treated after molting into their adult form starting at Day 22 and extending until the end of the experiment. This group was to “investigate the effects of exposure during early-adult development” (Smith et al., 2020).
- Continual: This group was exposed throughout their development from in their egg case through to adulthood.
Once these bees morphed into their adult form, they were randomly divided into 3-day and 12-day post-molting cohorts for behavioral testing. This entailed using standard protocols that paired sucrose feeding with a lemon odor to see if pesticide exposure had an impact on learning behavior.
The results? According to the Imperial College team: “we detected for each exposure treatment a significantly lower proportion of learners relative to the control” (Smith et al., 2020). Plus, learning impairments were visible at both the 3-day and 12-day time points, suggesting that bees were not able to overcome this deficit, at least not in 9 days.
The research caught the eyes of a science writer at CNN, who points out that “this impaired learning ability could make it harder for the worker bees to navigate and forage…potentially imperiling the bee colony” (Hunt, 2020). This in turn, suggests an explanation for a long-standing bee colony collapse disorder mystery:
“Our findings of early exposure affecting later adult behaviour can provide an explanation for why reduced colony growth has been detected two to three weeks after the onset of neonicotinoid exposure in previous studies” (Smith et al., 2020).
After their behavior tests, bees underwent a “swift decapitation” so that their brain tissue could be scanned to determine if there were any measurable neurodevelopmental impacts from imidacloprid exposure. This entailed using a micro-CT machine to investigate brain tissue morphology.
This novel element of the research found that “exposure at either stage of development was associated with workers possessing smaller mushroom body calyces” (Smith et al., 2020). These mushroom bodies are an important functional component of the bumblebee brain. The volume of the calyces is a predictor of a worker bee’s learning ability and capabilities as an adult. This reduced mushroom body was consistent in both 3-day and 12-day tested bees, suggesting that the impact came from pre-eclosion imidacloprid exposure.
In other words, neonics caused early-life bee brain defects that were not fatal, but impaired hive viability when adult bees could not adequately perform the complex, highly social behaviors essential to hive integrity.
Taken together, these results indicate that exposure in-hive, rather than direct exposure in the field, to imidacloprid caused “impeded developmental plasticity” in worker bumblebees. These findings show “the importance of considering different life-stages when assessing pesticide risk” (Smith et al., 2020).
In 1993, the scientific community made essentially the same point in the seminal National Academy of Sciences report Pesticides in the Diets of Infants and Children. Prenatal and early life exposures to pesticides in humans also can disrupt neural development in subtle ways that impact intelligence, behavior, and the life-long health of humans.
Is the huge increase in herbicide use in the Midwest doing the same thing? See how one group of physicians and scientists are trying to find out.
Hayley Dunning, “Pesticides impair baby bee brain development,” Imperial College London Press Release, Date published:March 4, 2020, Date Accessed: March 4, 2020.
Katie Hunt, “Pesticides damage the brains of baby bees, new research finds,” CNN, Date Published: March 3, 2020, Date Accessed: Marc 4, 2020.
Smith, Dylan B., Arce, Andres N., Ramos Rodrigues, Ana, Bischoff, Philipp H., Burris, Daisy, Ahmed, Farah, & Gill, Richard J.; “Insecticide exposure during brood or early-adult development reduces brain growth and impairs adult learning in bumblebees;” Proceedings of the Royal Society B: Biological Sciences, 2020, 287(1922); DOI: 10.1098/rspb.2019.2442.