Cuticular hydrocarbon cues of immune-challenged workers elicit immune activation in honeybee queens

Recently, evidence has shown that variations in the cuticular hydrocarbons (CHCs) profile allow healthy honeybees to identify diseased nestmates, eliciting agonistic responses in the former. Here, we determined whether these 'immunologic cues' emitted by diseased nestmates were only detected by workers, who consequently took hygienic measures and excluded these individuals from the colony, or whether queens were also able to detect these cues and respond accordingly. Healthy honeybee queens were exposed to (i) healthy, (ii) Ringer-injected and (iii) lipopolysaccharide (LPS)-injected nestmates by allowing direct body contact. Quantitative differences in the CHC profiles of these three groups were measured using GC-MS. The transcript levels of the products of four genes that encode for antimicrobial peptides (AMPs), which are part of the queen's immune response, were measured in bees exposed to direct contact using qPCR. A significant increase in the transcript levels of these AMP genes over baseline levels in queens was observed when body contact was allowed between the queens and the Ringer- and LPS-injected nestmates. These results provide the first evidence that the detection of CHCs contributes to the initiation of an immune response in insects. In an additional experiment, CHCs were extracted from diseased workers and directly presented to queens, which also evoked a similar immune response. A potential mechanism that relied on volatile compounds could be ruled out by conducting a distance experiment. The study helps to expand our knowledge of chemical communication in insects and sheds light on a likely new mechanism of social immunity.