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

Abstract

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.