Abstract The innate immune system of insects combines cellular and humoral responses that act together to protect against infection. Humoral defense involves the production of antimicrobial peptides (AMPs), regulated by the Toll and Imd pathways. We performed the first controlled bacterial challenge assays in the stingless bee Frieseomelitta varia to investigate how age and behavior influence humoral immunity. The expression of the AMP genes hymenoptaecin and defensin -1, and the regulatory genes cactus (Toll pathway) and relish (Imd pathway), was quantified at key developmental stages, and in adults (newly emerged, nurse, and forager workers) after 6 and 26 h after aseptic (saline injection) and septic ( Escherichia coli injection) injury. Adults showed higher basal transcript levels than larvae and pupae, suggesting that immune activation is inherent to developmental maturation. At 6 h, all adult groups similarly upregulated the four genes following both injuries. After 26 h, however, newly emerged and nurse bees maintained AMP-biased responses to sepsis, whereas foragers restrained AMP induction but upregulated relish . These age- and behavior-dependent immune profiles suggest trade-offs between susceptibility and resistance, shaped by physiological state and task-specific environmental exposure.
Dalarmi et al. (2026) studied this question.