3.3. Responsiveness to light (phototaxis)

Phototaxis or the movement of bees towards light can be compared between individuals by measuring the time a bee needs to reach a light source in an otherwise dark arena (Fig. 3). The dark arena (a bucket standing upside down to form a light-proof arena) is illuminated by green light emitting diodes with a maximum light intensity of 5600 mcd. The diodes are driven by 4 V power supply. Different relative light intensities are achieved by using neutral density filters. Twelve LEDs are fixed in the arena with opposite LEDs having the same light intensity. The LEDs are covered by a ground glass. The following logarithmic order of relative light intensities is suggested: 100 %, 50 %, 25 %, 12.5 %, 6.25 % and 3.125 %. At the maximum (100 %) intensity, the 520 nm stimulus has an intensity of 560 lx measured directly at the ground glass.

The phototactic behaviour of bees naturally depends on their locomotor behaviour. Therefore, it is necessary to study this factor independent of illumination. The length of the walking path the bee covers in a specific time in complete darkness can be measured and an average walking speed can be calculated (Erber et al., 2006). These measures should be compared between individuals to test whether differences in phototactic behaviour are causally linked to differences in locomotor behaviour. An example of how to determine the phototaxis of honey bees is given here:

1. Release an individual bee into the dark arena.

2. Videotape its behaviour till you remove the bee from the arena.

3. Measure the walking path length of the bee during 30 seconds in the dark arena.

If you do not possess a computer program for doing this, simply follow the replayed walking path with a pen on the monitor after covering it with transparent film.

4. Switch on one of the light sources with the lowest intensity.

5. As soon as the bee has reached it, switch on the opposite light source of equal intensity

6. Take the time the bee needs to get from the first LED to the second LED of the same intensity.

7. Repeat this procedure three times, so that you can average the walking times of a bee to one light intensity over four trials.

8. When the bee has reached the lowest light intensity for the final time, switch on the next higher light intensity and so forth.

9. Make sure that you switch off one light source before the next one is turned on.

10. To analyse phototaxis of different groups, compare walking times between groups or treatments.

While the walking speed of a bee towards different light sources strongly correlates with its gustatory responsiveness, locomotion was shown to be independent of gustatory responsiveness (Erber et al., 2006; Tsuruda and Page, 2009). Using this assay, Rueppel et al. (2007) demonstrated that bees of different ages do not differ in their phototactic behaviour, if behavioural role was not accounted for. Tsuruda and Page (2009) compared the visual responsiveness of bees of the two genetic strains of Page and Fondrk (1995) which differ in their foraging preferences. This assay is ideally suited to study visual responsiveness separately from locomotor responses in honey bees and will reveal impairments in the processing of visual stimuli. However, one has to bear in mind that using this assay one cannot make statements about the bee's perception of the visual stimuli, because behavioural responses are recorded, not the activity of photoreceptors.

Fig. 3. Phototactic behaviour of honey bees in a dark arena. The arena is illuminated by infrared light, which the bees are blind for. The phototaxis arena can be illuminated by twelve light sources. Two opposing light sources (light emitting diodes, LED) are of the same relative light intensity. The walking paths of a bee (yellow) are displayed for four walks between LEDs of low light intensity (A) and for four walks between LEDs of high intensity (B). The location of the respective light sources is indicated by asterisks. While the bee takes many detours when walking towards an LED of low intensity, its walking paths towards a high light intensity are direct.

1293PN revised Fig 3