Comparative analysis of immune responses in ovipositing and non‐ovipositing females of European earwig

Reproduction and immunity are essential biological processes that compete for resources and energy. In insects, trade‐offs between immunity and reproduction are often explored before female oviposition. However, the impact of egg laying on immune parameters remains poorly explored. Here, we address this knowledge gap in the European earwig, an insect with extensive maternal care, by comparing immune parameters at different stages of egg production. We examined both the basal immune levels and induced immune responses in 150 females (75 that laid eggs and were providing care and 75 same‐aged females that had not yet laid eggs) by injecting them with heat‐killed Serratia marcescens bacteria and measuring humoral (phenoloxidase activity) and cellular immune responses. The cellular immune response focused on granulocytes, plasmatocytes and spherulocytes, which we morphologically identified using four microscopy techniques. Our results show that females that had produced and cared for eggs exhibited a lower total haemocyte count and a reduced proportion of plasmatocytes but a higher proportion of spherulocytes. In contrast, having produced and cared for eggs had no effect on phenoloxidase activity or the proportion of granulocytes. Interestingly, neither the immune challenge nor female size influenced immune traits alone or in interaction with egg presence. Overall, our findings revealed that oviposition and post‐oviposition maternal investment induce shifts in the composition of immune cell populations. This underscored the importance of oviposition and parental care in shaping the immune response in insect mothers and calls for further research into the specific functions of haemocytes in Dermaptera and the evolutionary implications of these investments for life history traits association.