Behavioral and neurophysiological evaluation of pyrethroid effects on honey bee olfactory perception and learning

Honey bees are recognized as keystones of terrestrial ecosystems, being responsible for about 80% of human crop pollination. Unfortunately, bee populations have significantly declined over the past years due to various factors including pesticide use. Although pyrethroid insecticides have been intensively used since the 70 s and recognized for their high toxicity to bees, their possible deleterious effects on honey bee cognition have received little attention. Here, we investigated the impact of sublethal doses of two pyrethroids (permethrin and cypermethrin) on honey bee perception, learning and memory. We used a battery of cognitive tests of increasing complexity, all based on the conditioning of the proboscis extension response (PER). This protocol replicates in the Lab the final phase of bees' foraging behavior, in which they learn to associate odorants with sucrose rewards. Topical exposure to pyrethroids impaired both non-associative learning (habituation) and associative learning (olfactory learning and discrimination performances). Using in vivo calcium imaging, we then assessed odor-induced activity in the antennal lobe, the primary olfactory center of the insect brain. Surprisingly, cypermethrin treatment showed no effect on odor representation within the antennal lobes, neither in terms of signal intensity nor in the separability of glomerular activity patterns. This suggests that pyrethroid effects may be located in higher-order centers of the honey bee brain. These results show although they are widely used, sublethal doses of pyrethroids gravely affect bees' cognitive abilities. More generally, our test battery can be used for a more thorough systematical assessment of cognitive effects of insecticides in pollinators.