Associative learning of plant odorants activating the same or different receptor neurones in the moth Heliothis virescens

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Abstract
SUMMARY The importance of olfactory learning in host plant selection is well demonstrated in insects, including the heliothine moths. In the present study olfactory conditioning of the proboscis extension response was performed to determine the moths9 ability to learn and discriminate three plant odorants:β -ocimene and β-myrcene (activating the same receptor neurone type), and racemic linalool (activating two different types). The conditioned stimulus (CS) was an air puff with each odorant blown into a constant air stream and over the antennae, and the unconditioned stimulus (US) was sucrose solution applied first to the antennal taste sensilla, then to the proboscis. Conditioning with increasing odorant concentrations induced increased learning performance. The concentration threshold for learning was 100 times lower for racemic linalool than for the two other odorants, a fact that can be correlated with a higher sensitivity of the moths9 antennae to racemic linalool as shown in electroantennogram recordings. After correcting for the different odour sensitivities, the moths9 ability to discriminate the odorants was studied. Differential conditioning experiments were carried out, in which moths had to distinguish between a rewarded (CS+) odorant and an explicitly unrewarded odorant (CS-), choosing odour concentrations giving the same learning rate in previous experiments. The best discrimination was found with β-myrcene as the rewarded odorant and racemic linalool as the unrewarded. The opposite combination gave lower discrimination, indicating a higher salience for β-myrcene than for racemic linalool. The moths could also discriminate betweenβ -ocimene and β-myrcene, which was surprising, since they activate the same receptor neurone type. No difference in salience was found between these two odorants.