A Mechanical Signal Biases Caste Development in a Social Wasp

Understanding the proximate mechanisms of caste development in eusocial taxa can reveal how social species evolved from solitary ancestors [1]. In Polistes wasps, the current paradigm holds that differential amounts of nutrition during the larval stage cause the divergence of worker and gyne (potential queen) castes [2]. But nutrition level alone cannot explain how the first few females to be produced in a colony develop rapidly yet have small body sizes and worker phenotypes [3, 4]. Here, we provide evidence that a mechanical signal biases caste toward a worker phenotype. In Polistes fuscatus, the signal takes the form of antennal drumming (AD), wherein a female trills her antennae synchronously on the rims of nest cells while feeding prey-liquid to larvae [5]. The frequency of AD occurrence is high early in the colony cycle, when larvae destined to become workers are being reared, and low late in the cycle, when gynes are being reared [6]. Subjecting gyne-destined brood to simulated AD-frequency vibrations caused them to emerge as adults with reduced fat stores, a worker trait [7]. This suggests that AD influences the larval developmental trajectory by inhibiting a physiological element that is necessary to trigger diapause, a gyne trait [8]. ► Gyne-destined larvae given AD-like vibrations developed low fat, a worker trait ► Gyne-destined larvae given control vibrations had high fat, similar to field gynes ► The effect of simulated AD was stronger on 3rd-instar larvae than on 4th and older ► Treatment had no effect on the rate at which adults fed larvae or performed AD