3.4. Responsiveness to odours
Gustatory responsiveness correlates with responsiveness to odours, which can be tested in an olfactometer (Fig. 4; Pham-Delègue et al., 1991; Scheiner et al., 2004). In this assay, bees are placed in the centre of a four-armed maze. One of the arms contains an odorant of a certain concentration. If the bee can smell the odorant and if it is attracted by it, the bee will soon walk towards the odorant and remain close to the odour source. Most bees prefer a high odorant intensity over a low intensity. Responsiveness to different odorant intensities can be tested by placing different concentrations of an odorant alternating in the four arms of the olfactometer. An example of an olfactometer experiment is given below:
1. Place an individual bee in the centre of the arena.
2. Add one odour to the end of one arm of the four-armed olfactometer.
3. Determine the time the bee spends in the centre of the arena and in each arm of the olfactometer.
4. Once the bee has reached the odorant source, remove the odorant quickly.
5. Allow a 30-second pause without olfactory stimulation.
6. Place the next odorant or the next higher concentration of the same odorant in a different arm of the olfactometer
7. Measure the time the bee spends in each part of the olfactometer, which corresponds to the preference of the bee for that particular odour.
8. At the end, calculate the time the bee spends in an odorant-containing arm and compare it with the times the bee spent in the other parts of the arena using T Tests (for further details see the BEEBOOK paper on statistical methods (Pirk et al., 2013)).
Individuals with low gustatory responsiveness only walk directly towards high odorant intensities, while bees with high gustatory responsiveness are also attracted by lower odorant concentrations (Scheiner et al., 2003). If a bee is not attracted by an odour or if it is unable to perceive the odour, it spends the time randomly in the four arms of the olfactometer.
Using an olfactometer it was shown,
for example, that bees display an age-dependent preference for queen mandibular
pheromone with five-day-old bees exhibiting the strongest attraction to this
pheromone (Pham-Delègue et al.,
1991). Also, it was tested using an olfactometer how well bees discriminate
between combs based on their odours (Breed and Stiller, 1992). These examples
suggest that the olfactometer assay is also well suited to detect impairments
of olfactory perception. One problem with this assay is that bees might not be
attracted by an odour, even though they may be able to detect differences in
the odour concentration. Therefore, odours to be used in this assay need to be
tested carefully for their behavioural effects on honey bees prior to use in
Fig. 4. Olfactometer for measuring responsiveness to odours. In this arena, the bee is placed in the centre. An odorant of a specific concentration is placed in front of one of the four arms of the olfactometer. While the air is drawn from the arena from its middle and therefore pulls in the odour trough the arm, the bee can smell it. If the bee is attracted by the odour, it will spend a longer time in the odour-containing arm of the arena than in the other arms.