Insecticide exposure during brood or early-adult development reduces brain growth and impairs adult learning in bumble bees

Social bees such as bumble bees are crucial pollinators whose populations are in decline. One of the reasons for the decline is thought to be exposure to certain pesticides. Commonly used pesticides, such as neonicotinoids, have been shown to negatively affect colony health when found in pollen or nectar that is distributed throughout the hive.

The research team led by Dr. Dylan B. Smith wanted to explore any potential effects that imidacloprid, a common active to manage insect pests, would have on a colony of bumble bees in terms of brain development and learning capacity. The UK researchers tested imidacloprid-spiked sugar water at 5ppm, which was administered to several treatment groups (one group imidacloprid was administered throughout the study’s duration, another just during larval development, and the last after adult bees emerged). The data from the experimental groups were then compared to a control group where bees were fed only sugar water. The researchers used impressive imaging technology called Micro-computed Tomography Scanning to measure parts of the bumble bee brain where an increased volume of neuropil (a grouped network of nerve fibers) is positively correlated with increased learning capacity. This area is called mushroom body lobes and calyces. Additionally, the researchers also set up a learning performance test which measured a worker bee’s ability to learn once adulthood was reached by attempting to teach the bees to associate a specific odor with a sugar-water reward. This test was given to each worker adult 9 times, and the data recorded.

Bees fed spiked sugar water had fewer learners compared to bees fed only sugar water, with the larval exposure group and the continual exposure group reporting the lowest proportion of learners. When examining the bumble bee brain images, the volumes of the neuropil were lower in the treatment groups than the control group, but these differences in volume were not always statistically significant. Regardless, this study (and others) suggest that imidacloprid can have potentially negative effects on the neurological development of worker bees exposed to it and that further research into short and long-term colony impacts is needed.