Salience of multimodal flower cues manipulates initial responses and facilitates learning performance of bumblebees

Visual and olfactory cues are the first interface between flowers and their visitors and are adaptations to facilitate successful pollination. Initial responses to and associative learning of multimodal cues by flower visitors are based on the perception of colours and volatiles. In this study, we tested how visual, olfactory and multimodal stimuli affect the behaviour of bumblebees (Bombus terrestris) and correlated the properties of 28 stimuli in relation to the insects’ sensory equipment to these behaviours. Initial responses and associative learning were recorded using a radio frequency identification system, tracking the visitation sequences of individual bumblebees to artificial flowers treated with naturally occurring pigments and/or volatiles. The salience of the stimuli was evaluated as the colour contrast to the background and as electroantennogram responses. The main finding was that both initial responses and learning performance were positively correlated to the salience of the stimuli, suggesting that salience is a key feature of flower cues in the interactions with insects. The salience of compound stimuli consisting of two or more pigments and/or volatiles was largely additively determined by the saliences of individual compounds. Potentially, the valence of the stimuli may interfere with the positive relationship between salience and behaviour, which is indicated by our results, too. The salience of multimodal cues depends on the species-specific equipment of visual and olfactory receptors and thus enables flowers to be advertising for some but rather inconspicuous for other flower visitors.