M.H. Pham-Delègue

Olfactory information transfer in the honeybee: compared efficiency of classical conditioning and early exposure

We investigated the ability of honeybees, Apis mellifera, to use olfactory information gained in a given experimental context, in other contexts. First, restrained bees were subjected to a Pavlovian associative learning procedure, based on the conditioning of the proboscis extension response (PER), where a floral odour was paired with a sugar reward. We observed the orientation behaviour of conditioned and naïve bees in a four-armed olfactometer with four contiguous fields either scented with the conditioning odour or unscented. Information transfer was clearly shown, conditioned bees orienting towards the conditioning odour, whilst naïve bees shunned it. Second, the effect of passive olfactory exposures during the bees development was assessed in two behavioural contexts: either orientation in the olfactometer or a PER conditioning procedure. Two exposure periods were applied: (1) the pupal stage (9 days before emergence); (2) the early adult stage (8 days after emergence). No effect of preimaginal exposure was recorded, but exposure during the early adult stage induced a higher choice frequency of the odour field in the olfactometer, and lower learning performance in the PER conditioning assay. These observations show that olfactory information gained during development can modify bees later behaviour in different contexts: this is another instance of olfactory information transfer in bees. These results also suggest that nonassociative learning phenomena, taking place at a critical period during development, might be involved in the maturation of the bees olfactory system, and in the organization of odour-mediated behaviours. Copyright 2000 The Association for the Study of Animal Behaviour.

Olfactory conditioning of the proboscis extension in bumble bees

The foraging behaviour of bumble bees is well documented for nectar and/or pollen gathering, but little is known about the learning processes underlying such behaviour. We report olfactory conditioning in worker bumble bees Bombus terrestris L. (Hymenoptera: Apidae) obtained under laboratory conditions on restrained individuals. The protocol was adapted from the proboscis extension conditioning previously described in the honey bee Apis mellifera L. Bumble bees were found to be able to learn a pure odorant when it was presented in paired association with a sugar reward, but not when odour and reward were presented in an explicitly unpaired procedure. This suggests an associative basis for this olfactory learning. Bumble bees showed similar conditioning abilities when stimulated with two different floral odours. An effect of the sugar reward concentration on the learning performances was found.

Asymmetrical generalisation between pheromonal and floral odours in appetitive olfactory conditioning of the honey bee (Apis mellifera L.)

Abstract. The capacity to generalise between similar but not identical olfactory stimuli is crucial for honey bees, allowing them to find rewarding food sources with varying volatile emissions. We studied bees generalisation behaviour with odours having different biological values: typical floral odours or alarm compounds. Bees behavioural and peripheral electrophysiological responses were investigated using a combined proboscis extension response conditioning-electroantennogram assay. Bees were conditioned to pure linalool (floral) or to pure isoamyl acetate (alarm) and were tested with different concentrations of both compounds. Electrophysiological responses were not influenced by conditioning, suggesting that the learning of individual compounds does not rely on modulations of peripheral sensitivity. Behaviourally, generalisation responses of bees conditioned to the alarm compound were much higher than those of bees conditioned to the floral odour. We further demonstrated such asymmetrical generalisation between alarm and floral odours by using differential conditioning procedures. Conditioning to alarm compounds (isoamyl acetate or 2-heptanone) consistently induced more generalisation than conditioning to floral compounds (linalool or phenylacetaldehyde). Interestingly, generalisation between the two alarm compounds, which are otherwise chemically different, was extremely high. These results are discussed in relation to the neural representation of compounds with different biological significance for bees.

Dynamics of odour learning in Leptopilina boulardi, a hymenopterous parasitoid

We investigated the dynamics of odour learning involved in host location by a parasitoid insect, Leptopilina boulardi (Hymenoptera: Figitidae). Females of this species find their Drosophila host larvae by probing fruits with their ovipositor. They can be conditioned to respond to an odour when the odour exposure is associated with oviposition. We investigated the effect of the number of conditioning trials, sensitization tests and extinction tests on the retention of the conditioned response. Results showed that: (1) a single odour-oviposition association produced a strong short-term memory (1-2 h), which rapidly decayed over 24 h; (2) multiple odour-oviposition associations produced a memory trace that was strong in both the short term and the longer term (24 h); (3) sensitization to the odour through mere oviposition experience (without odour) was low after a single trial and high after multiple trials, but was only observed for a short period; (4) all memory traces were erased by three successive extinction tests, regardless of the intertest interval. We conclude that the probing behaviour of a Drosophila parasitoid is characterized by great plasticity shaped in the short term by sensitization, and in the longer term by associative learning. We compare olfactory plasticity in this parasitoid foraging for hosts and that of the honeybee foraging for food, suggesting common underlying processes in the central nervous system. Finally our results may relate to the dynamics of the foraging activity of L. boulardi.

Effects of herbicide‐tolerant transgenic oilseed rape genotypes on honey bees and other pollinating insects under field condtions

Field experiments were carried out to compare the diversity and density of pollinators foraging on two winter oilseed rape varieties, ‘Falcon’ and ‘Artus’, and their respective transgenic counterparts ‘Falcon pat’ and ‘Artus LL’, which are tolerant to glufosinate, a non‐specific herbicide. The number of insects per 1000 available flowers was counted for the four main pollinators: honeybees, bumblebees, solitary bees, and Diptera. Additionally, on ‘Falcon’/‘Falcon pat’ the foraging behaviour of the main pollinating insects, i.e., honeybees and bumblebees, was observed (number of flowers visited per min, foraging postures, intervariety flights). A pleiotropic effect could affect nectar and pollen production, therefore we measured nectar volume, nectar sugar concentration, and composition for the four plant genotypes, and on ‘Falcon’ and ‘Falcon pat’ pollen production was also estimated. The diversity and density of the foraging insect population observed on the transgenic genotypes were similar to that on the non‐transgenic controls. Moreover, the foraging behaviour strategy was similar on ‘Falcon’ and ‘Falcon pat’, and honeybees flew indifferently across these two genotypes. No difference was found in nectar and pollen between the genotypes. Thus, we may assume that the insects do not discriminate between conventional and transgenic oilseed rape specifically resistant to glufosinate. A case‐by‐case approach is recommended for other genetic modifications to plants potentially visited by bees.

Discrimination of oilseed rape volatiles by the honeybee: combined chemical and biological approaches

Honeybees (Apis mellifera L.) were individually subjected to a classical conditioning procedure in order to obtain an olfactory conditioned proboscis extension response. To relate the behavioural response directly to antennal detection abilities, a technique was developped for coupling proboscis extension responses and electroantennogram recordings, with the stimulation being provided by the effluent of a gas chromatograph (GC). Bees were conditioned with a six‐component mixture being part of oilseed rape (Brassica napus L.) floral volatiles, and tested with the individual components separated by GC. Responses of the conditioned bees were compared to those of unconditioned bees. No behavioural response was obtained in the control group, neither to the individual components nor to the mixture. Conditioning induced behavioural responses for three components, and an increase of electroantennogram responses for all components. A second experiment was conducted with an air entrainment extract of oilseed rape flower volatiles. Behavioural responses of conditioned and unconditioned bees were recorded. Responses obtained from conditioned bees tested with the air entrainment extract showed six groups of behaviourally active GC peaks. Unconditioned bees showed the same pattern of responses but at a lower level. The coupled technique described here appears to be a reliable tool for locating active components in a synthetic as well as in a natural mixture of floral volatiles. The effects of conditioning on odour discrimination and on its sensory correlates are discussed.