12.3. Tracking individual bees
Measuring the performance of individual bees greatly increases the precision with which various influences on forager activity can be studied. To recognize individuals, they need to be tagged (see section 2.3). Because number tags are not well-suited for automatic identification of bees during hive entrance passage, a combination of optical recognition software and high-resolution video technique is recommended for these applications. Recently, radio-frequency identification (RFID) chips became available which are light and small enough to be used on bees. In the following, a simple method to assess forager performance using "classical" labelling will be described. Afterwards, a detailed account of the RFID method will be provided, which has been employed successfully in several studies.
Flights from individual bees back to the colony are fairly predictable. They can be measured relatively easily, in particular at seasons when feeder trainings are limited to more attractive natural nectar flow. It can be determined reliably whether bees return to the colony and the time they need to return by releasing the bees at some distance (10 m to several 100 m) and registering their arrival time at the colony.
To do this:
1. Catch returning foragers at the hive entrance or from within the hive (see section 2.1) and carry them in glass vials to the release location (Fig. 26).
2. Restrict the observation times to e.g. 30 min after release.
3. Determine returning success to small nucleus colonies late in the evening or early morning.
To be successful, these tests require some precaution. The duration of a return flight is influenced by various variables such:
- Weather condition
- Time of the day
- Age of the bees, and others
In order not to drown the searched-for effects in a vast general variability, one needs to compare carefully matched pairs of individuals or small groups of bees having the same age, and flying at the same time. Accordingly, appropriate statistics for observations on paired samples need to be applied (e.g. Wilcoxon paired sample test, see also the BEEBOOK paper on statistics (Pirk et al., 2013)).
In addition, experimental treatment differences can easily be overrun by any form of general stress applied to the foragers, which requires the experimenter to handle bees with great care, and to release them from the vials without delay. This method may be extended by observing waggle dances of labelled bees in order to determine the danced vector and distance where the foragers found rewarding food sources (see section 10). They may then be released either along the indicated flight path or at certain angles from this vector.
Even low stress levels, due for example to parasitism
by Varroa destructor or Nosema spp., can be detected during
return flight experiments (Kralj and Fuchs, 2006, 2010), because they affect
the duration of return flights and the rate of successful returns. Although
these experiments are laborious, often require two experimenters, and are
restricted to small numbers of bees, they are reliable and very useful when
technically sophisticated methods are unavailable.
Fig. 26. Releasing an individually marked forager bee from a vial to measure its return time and ability to return to its home colony.