Individual bumblebees vary in response to disturbance: a test of the defensive reserve hypothesis

Bees may leave their nest in the event of an attack, but this is not their only response. Here, we examine the behavior of those individuals that remain inside the nest during a disturbance. Specifically, we test the hypothesis that bee workers usually exhibiting high levels of inactivity (i.e., ‘lazy’ bees) may function as defensive reserves that are more likely to respond when the colony is disturbed. We explore this hypothesis by simulating vertebrate attacks by vibrating or blowing carbon dioxide into two colonies on alternating days and measuring the movements and tasks performed by bees inside the nest. Our results show that regardless of the disturbance type, workers increase guarding behavior after a disturbance stops. Although previously inactive bees increased their movement speed inside the nest when the disturbance was vibration, they were not more likely to leave the nest (presumably to attack the simulated attacker) or switch to guarding behavior for any disturbance type. We therefore reject the hypothesis that inactive Bombus impatiens bumblebees act as defensive reserves, and propose alternative hypotheses regarding why many workers remain inactive inside the nest.