Appetitive and Aversive Learning in Spodoptera littoralis Larvae

Page 725 Adult butterflies are capable of associative learning that serves foraging flowers or finding oviposition sites. In experimental paradigms, they learn to extend their proboscis in response to floral orders associated with a sugar reward or to avoid them if paired with bitter substances. Such settings have been used to elucidate the neuronal circuitry and mechanisms underlying the aforementioned behaviors. Butterfly larvae have been less well studied for their learning capabilities, possibly because for them to feed on a plant the mechanisms mediating adaptation, habituation, and specialization appear more relevant. However, larvae would profit from associative learning in numerous situations, including navigating back to their host plant, avoiding photosensitive toxins, finding less toxic plant tissues, or sampling different host plants. Salloum et al. now adapted a paradigm recently developed for Drosophila larvae to study if Egyptian cotton worm larvae can associate a taste with an odor. To this end, they exposed groups of larvae to a sugar or a bitter compound as unconditioned stimuli (US) paired with an odorant as a conditioned stimulus (CS) and subsequently tested their reaction to the CS at different time intervals. Depending on the US used larvae learned to avoid or prefer the CS. This learning was specific because trained larvae discriminate CS from another odorant present during the training that remained unrewarded. Moreover, the aversive bitter stimulus was much more efficient than the sugar reward as US. Thus, the butterfly larvae were able to quickly store and memorize information crucial for their survival that offers them a substantial ecological advantage over using habitual mechanisms solely.