What is the password? Female bark beetles (Scolytinae) grant males access to their galleries based on courtship song

• We examined the acoustic signals of bark beetles (Scolytinae) during courtship to test the hypothesis that courtship songs are involved in sexual selection.
• During courtship song performances male bark beetles produced two distinct chirp types: simple and interrupted.
• Many male courtship song characteristics were significantly correlated with body size and may therefore be honest indicators of male condition.
• Females could differentiate between potential mates based on male courtship song performances.
• Males performing courtship songs that included interrupted chirps were significantly more likely to be accepted by a female into her gallery.

Acoustic signals are commonly used by insects in the context of mating, and signals can vary depending on the stage of interaction between a male and female. While calling songs have been studied extensively, particularly in the Orthoptera, much less is known about courtship songs. One outstanding question is how potential mates are differentiated by their courtship signal characteristics. We examined acoustic courtship signals in a new system, bark beetles (Scolytinae). In the red turpentine beetle (Dendroctonus valens) males produce chirp trains upon approaching the entrance of a female's gallery. We tested the hypotheses that acoustic signals are honest indicators of male condition and that females choose males based on signal characteristics. Males generated two distinct chirp types (simple and interrupted), and variability in their prevalence correlated with an indicator of male quality, body size, with larger males producing significantly more interrupted chirps. Females showed a significant preference for males who produced interrupted chirps, suggesting that females distinguish between males on the basis of their chirp performances. We suggest that interrupted chirps during courtship advertise a male’s size and/or motor skills, and function as the proverbial ‘passwords’ that allow him entry to a female's gallery.