Mature Vespa mandarinia larvae continue calling at night.

Animals produce and perceive vibroacoustic signals. Larvae of Vespa hornets generate a rhythmic "rasping" signal by rubbing their mandibles against the cell wall (hereafter calling), a behaviour proposed to function as a hunger signal to workers. Detailed observations, however, exist for only a few Vespa species, and whether calling varies with time of day is unknown. Using workerless laboratory observations of Vespa mandarinia, we recorded group-level calling and found that fifth-instar larvae called throughout both day and night, with a higher calling rate at night than during the day. Only fifth-instar larvae called; earlier instars did not. We also characterise the airborne acoustic component of the rasping signal by its duration and dominant frequency. To our knowledge, this is the first report of nocturnal larval calling in any Vespa species, providing a baseline for future work on larval signaling under natural colony conditions.