Is the developmental stage important for the plasticity of transgenerational immune priming?

Parental transfer of immune memory can increase offspring survival, although its benefits may vary across developmental stages. We investigated transgenerational innate immune priming in the fall armyworm (Spodoptera frugiperda) infected with the fungus Metarhizium brunneum. Using a split-design, fathers and mothers were exposed to a low-dose fungal challenge at larval stages L5 or L6, and their offspring were subsequently challenged with a high dose at L5 or L6. We assessed survival, development, immune responses (phenoloxidase, lytic activity), and oxidative stress (total antioxidant capacity, catalase, superoxide dismutase, hydrogen peroxide). Parental immune activation at either instar had no significant effect on offspring survival. However, offspring responses were stage-dependent: L5 larvae in the memory group (homologous challenge) showed higher survival than heterologous or L6-challenged larvae. No L6-challenged larvae reached adulthood. The L5 memory group developed more slowly than controls, while all L6 larvae failed to complete development. Immune response and oxidative stress were higher in L6 larvae, but immune priming effects did not differ. These results suggest that developmental constraints and self-inflicted damage from immune and oxidative responses limit immune priming efficiency at later stages. Our findings highlight that transgenerational immune priming is strongly influenced by the developmental stage at which offspring are challenged.