4.1. Introduction

The honey bee colony can be considered as a superorganism including numerous bees of different castes, ages and sex acting together to develop the nest. The evolutionary success of honey bee colonies is based on social organization between the workers and the queen for colony growth and development. The social organization is based on division of labour that depends on individual endogenous biotic factors like hormonal, genetic, immune and neurobiological backgrounds and on exogenous biotic factors like chemical communications, social immunity and behavioural interactions, with all of these factors capable of being modulated by the external environment.

Bearing in mind the complexity of the functioning colony, when significant variability in the response to toxic substances of bees is demonstrated using cage experiments, it is reasonable to expect that the difference in response will be even greater between bees in cages and in natural conditions. Depending on the questions to be addressed, it may be necessary to consider working either at individual (cage) or colony level. Thus, for studying the molecular effects of a toxin on bees, cage experiments using very controlled environment may be the best choice. However, in the end, the effect of the toxin in the real life of the bee, i.e. in natural conditions, should be addressed, even if it is much more difficult to manage honey bee colonies than cages.

Ideally, studies on the effects of toxic substances at the colony level require contiguous treated and non-treated areas of a field where colonies can be placed. Unfortunately, these protocols are not easy to use as the bees will forage in both non-treated and treated areas. Moreover, the sites at which bees can forage in field conditions are not controlled at all, even when colonies are placed close to the observation areas. Thus, it is proposed to observe the behaviour of foragers directly on the target crops, in addition to overall colony development or in semi-field trials (in tunnels), to determine the effect of treated crops on honey bee colonies in semi-controlled conditions. These semi-field trials are informative, but with the bias that usually the colonies do not develop as well as colonies placed in natural conditions. Another approach consists of mimicking the exposure to a substance on the field crop by forced in-hive feeding with syrup or pollen patties and observing the colony development and the impacts on individuals using various investigation methods. This approach can be used to test the effects of acute, chronic, lethal or sub-lethal exposures to different substances. Different parameters can be studied using those methods for testing the toxins on bees at the colony level: individual adult and brood mortality, clinical symptoms or colony development. However, individual observations on behaviour are particularly interesting for gathering information on sublethal effects of the toxins. Different technologies such as honey bee counters, RFID labelling or harmonic radars have been proposed for this purpose.

This section gives information on techniques used to study the effects of toxic substances, including dusts dispersed during sowing and systemic substances distributed in plant matrices, at the colony level. Different field or semi-field protocols are described and in the future could be the basis of procedures used in the risk assessment of pesticides.