It's still a mystery, but Harvard researchers have made progress!
For some years now, bees are decimated because of neonicotinoids. These substances are used as poison to defend the crops from the attack of other species of insects, so that, even if not voluntarily, also bees are hit. It would seem that, once these substances are taken through the pollen, they hit the nervous system and, even if it does not lead directly to their death, they cause loss of memory, making them unable to get food.
It is very difficult to study this type of phenomenon because it is not easy to closely monitor the behaviour of bee colonies. However, researchers have discovered a method for following swarms of bumblebees. In fact, they were equipped with a sort of tiny backpack with a tracking system to detect their movements. A group of researchers at Harvard University discovered in detail how these animals suffer from the effects of various neonicotinoids.
An example? By becoming inactive and less attentive to puppies.