Apis mellifera Worker Bees Selected for Varroa -sensitive Hygiene Show Higher Specific Sensitivity and Perception Speed Towards Low Concentrations of Chemical Cues Emitted by the Brood

Varroa -sensitive hygiene (VSH) is highly influenced by the worker bee’s olfactory ability. Workers bred for VSH and non-selected control line workers were tested for differences in their speed and perception ability when presented with highly diluted stimuli. Four different substances (citral – dilution 1:1300, linalool dilution 1:1300, Varroa -parasitized brood extract, isopropanol) were used as tactile stimuli for differential conditioning with the proboscis extension response (PER). Discrimination ability and generalization were assessed. In a second set of conditioning experiments differences in sensitivity to the highly diluted citral and the Varroa -parasitized brood extract as reinforced stimuli (Cs +) were explored between workers from both lines. The worker bees were classified into three groups ( Time points ) depending on how long before they started correctly extending their proboscis to the Cs + , and results were examined separately for each of the two stimuli and group. While the VSH-selected line exhibited a significantly higher perception ability for the parasitized brood extract than the non-selected line, the two lines showed no differences when conditioned with the floral stimulus citral as Cs + . Furthermore, the VSH-selected line displayed a significantly higher number of worker bees that perceived the complex bouquet of the Varroa -parasitized brood extract at the earliest time grouping ( Time point 1 ). The odds of perception at the earliest possible time point were 2.6-times higher for the VSH-selected line. Although no comparison was made between healthy and parasitized brood, the results indicate an enhanced specific sensitivity in VSH-selected workers towards chemical cues emitted by the brood, which might play a role in the detection of Varroa destructor .