Simultaneous percussion by the larvae of a stem-nesting solitary bee – a collaborative defence strategy against parasitoid wasps?

Abstract

Disturbance sounds to deter antagonists are widespread among insects but have never been recorded for the larvae of bees. Here, we report on the production of disturbance sounds by the postdefecating larva (“prepupa”) of the Palaearctic osmiine bee Hoplitis (Alcidamea) tridentata, which constructs linear series of brood cells in excavated burrows in pithy plant stems. Upon disturbance, the prepupa produces two types of sounds, one of which can be heard up to a distance of 2–3 m (“stroking sounds”), whereas the other is scarcely audible by bare ear (“tapping sounds”). To produce the stroking sounds, the prepupa rapidly pulls a horseshoe-shaped callosity around the anus one to five times in quick succession over the cocoon wall before it starts to produce tapping sounds by knocking a triangularly shaped callosity on the clypeus against the cocoon wall in long uninterrupted series of one to four knocks per second. Sound analysis revealed that the stroking sounds consist of several syllables, which are very similar to the single syllables of the tapping sounds: both last about 0.5 ms and spread over 40 kHz bandwidth from the audible far into the ultrasonic range. The production of stroking sounds by a prepupa induces other prepupae of the same nest to stroke and/or to tap resulting in a long-lasting and simultaneous albeit unsynchronized percussion by numerous prepupae along the whole nest stem. We hypothesize that these disturbance sounds serve an anti-antagonist function and that they have evolved to disturb the reflectance signals that parasitoid wasps use to localize concealed hosts during vibrational sounding.