It’s complicated: the role of false brood cells as an anti-parasite strategy in Hymenoptera is not simple despite their ubiquity

Abstract False cells are empty chambers in the nests of many mass-provisioning Hymenoptera and are thought to be an adaptive parental strategy to limit the access of parasites to the host brood. The presence of false cells across diverse bee and wasp clades suggests evolutionary convergence, but the selective pressures driving their use remain unclear. False cells might represent a trade-off between reducing nesting space and providing anti-parasite defence, but different parasites can vary in how they attack their hosts. This could lead to a complex dynamic involving this trade-off and the relative value of false cells as a defence against different parasite strategies. Using field data from a facultatively social stem-nesting bee, Amphylaeus morosus, we explore whether false cells trade off with provisioned cells and whether false cells deter parasites of different types. We find that false cells limit available space for brood production, suggesting that a trade-off does operate. False cells provide effective parasite defence in some cases, but efficacy needs to be measured against trade-offs with brood production and defence against a broader range of alternative parasite strategies. This trade-off can be circumvented by cooperative nesting, which provides a more effective form of parasite defence compared with false cells.