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

Abstract

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 moths' 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 moths' antennae to racemic linalool as shown in electroantennogram recordings. After correcting for the different odour sensitivities, the moths' 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 between β-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.

Cognition in Invertebrates

“…While there is evidence in vertebrates of how this reduction in the error signal may be implemented biologically, such evidence is still lacking in invertebrates. Conditioning using the PER has been so successful that it has been established in other insects such as moths and flies (Medioni and Vaysse, 1975;DeJianne et al, 1985;Holliday and Hirsch, 1986;Brigui et al, 1990;Fan et al, 1997;Fresquet, 1999;Hartlieb et al, 1999;Daly and Smith, 2000;Fan and Hansson, 2001;Skiri et al, 2005) and for use with mechanosensory stimuli (Giurfa and Malun, 2004).…”

Effects of two bitter substances on olfactory conditioning in the moth Heliothis virescens

“…The sucrose US (5·s) was applied with a toothpick 2.5·s after the onset of the odour puff, first to both antennae, and then to the extended proboscis. Because moths tend to be unresponsive at the beginning of conditioning because of low attention, the same method as in previous work was used to ensure learning success (Skiri et al, 2005): if the insect did not extend its proboscis at first encounter with the sucrose, the proboscis was forced out, and the insect was allowed to drink. This was not done in subsequent trials, meaning that the insects that failed to show PER were not rewarded.…”

“…antennae, mouthparts and tarsi, causes the hungry insect to extend its proboscis in order to feed. This response, the proboscis extension response (PER), has been utilised to study classical conditioning, particularly appetitive olfactory learning in several insect species, including the honeybee Apis mellifera (Bitterman et al, 1983;Menzel, 1993;Hammer and Menzel, 1995), the bumblebee Bombus terrestris (Laloi et al, 1999), and several moth species (Hartlieb, 1996;Fan et al, 1997;Daly et al, 2004;Skiri et al, 2005). In all these species, including moths, it was demonstrated that the olfactory conditioning of the PER is associative.…”

“…If an initially neutral odour puff (the conditioned stimulus, CS) is given a few seconds before the sucrose stimulation (the unconditioned stimulus, US), the insects learn to associate the odour with the sucrose reward, and the CS will then trigger a conditioned response (CR), the insects extending the proboscis to the odour. In heliothine moths, previous studies have shown that they will learn to associate odours with an appetitive reward, both in the laboratory and in the field (Cunningham et al, 1999;Hartlieb et al, 1999;Skiri et al, 2005;Cunningham et al, 2006). The olfactory pathways involved in olfactory conditioning have been extensively studied and are well described in several species, including A. mellifera and the moth Heliothis virescens.…”

“…Such experiments in which a decrease in CRs is expected because of the bitter stimuli, have to rely on high learning rates. A previous study of appetitive conditioning in H. virescens analysed the effect of CS quality and concentrations (Skiri et al, 2005). Conditioning with increased CS concentrations increased the learning rate, and odorants activating different receptor neuron types caused different learning performances.…”