Over a hundred years ago, Ivan Pavlov trained dogs to salivate at the sound of a bell that rang before every meal. Since then, scientists have trained all sorts of animals to link scents and sounds with food. For instance, honeybees regularly participate in learning experiments, which train them to reflexively extend their mouthparts when they smell a specific odor they've come to associate with sugary rewards. Although bees are usually good students, they often struggle to learn when they've been exposed to pesticides such as Pristine, which is sprayed on crops to kill mold and mildew. These toxic chemicals can interfere with a bee's ability to remember which flowery smells indicate nutritious nectar sources. Pristine doesn't degrade a bee's sense of smell but there are other ways to disrupt the insect's ability to make lasting memories. For the memory to stick, bees must not only eat sugar in the presence of a particular odor, but also digest and absorb the nutrient. Nicole DesJardins, Brian Smith and Jon Harrison of Arizona State University, USA decided to find out if digestive issues could explain why Italian honeybees (Apis mellifera linguistica) can't remember scents after eating foods laced with Pristine.
The researchers started six colonies of honeybees in their lab and allowed three of the colonies to feed on pollen laced with levels of Pristine that bees would realistically encounter when collecting pollen and nectar from flowers. After letting the colonies grow for 4 months, DesJardins and colleagues began collecting individual bees to see how the pesticide had affected them. They fed the bees sugar water before checking to see if the bees that had eaten the fungicide-laden pollen had trouble absorbing the sugary energy source glucose from their meals or storing it afterwards. To do this, the team collected samples of each bee's hemolymph – the fluid that circulates nutrients throughout an insect's body – to see if the sugar levels in the bees’ hemolymph changed after the insects ate their meals. The researchers tested each sample of hemolymph for trehalose, a sugar molecule that insects use when storing glucose for later. They found that the bees’ trehalose levels didn't change, regardless of whether the bees had been given the pesticide or not. The researchers concluded that Pristine did not interfere with the bees’ capacity to convert glucose to trehalose or to regulate trehalose levels.
DesJardins and colleagues then turned their attention to glucose itself. The team found differences in glucose levels between the bees given normal food and the bees given the food laced with pesticide. For pesticide-free bees, glucose levels consistently started at a low value before the bees ate. After feeding, the bees began absorbing the sugar from their guts, and the glucose levels in their hemolymph rose to a peak about 3 minutes later. However, bees that had been eating Pristine-laced pollen had higher glucose levels both before and after they ate. Even though the bees appeared to absorb the sugar from their meals, the researchers couldn't see glucose levels rise in the bees’ hemolymph. DesJardins and colleagues concluded that Pristine interferes with the bees’ ability to regulate how much glucose is in their hemolymph at any given time. Because they always have lots of glucose in their hemolymph, the bees treated with the pesticide don't experience the same spikes in glucose levels after meals that untreated bees do. Without that final, important step, bees can't seem to make the lasting memories they need to navigate flowering farms and meadows and find their food again.