Juvenile hormone and maternal egg care in the European earwig (part I): Manipulation of JH, agonist, and precocene fails to induce the onset of egg care.

Parental care is a crucial behaviour in animals. Yet the mechanisms regulating its initiation remain poorly understood, particularly in insects. Juvenile hormone (JH) is widely recognized as a key hormonal regulator in insects that influences a broad diversity of physiological and behavioural traits, including parental care after oviposition. However, its role in triggering the onset of egg care remains unexplored. To address this, we experimentally manipulated JH levels in 194 pre-ovipositing European earwig females and tested whether they initiated care towards foreign eggs. Contrary to our predictions, topical application or injection of JH, methoprene (a JH analogue), precocene (a JH production inhibitor) or acetone (control) at varying doses failed to induce egg care or to prevent egg cannibalism. This rejection of foreign eggs was not due to experimental stress, as positive control females that had previously laid eggs cared for new foreign eggs, even after acetone exposure. These results demonstrate that neither the presence or absence of JH nor the mere presence of eggs is sufficient to induce maternal care in pre-ovipositing females. Instead, our findings suggest that additional signals beyond JH modulation and/or alternative hormonal pathways may be critical for initiating care behaviours in earwigs. Overall, this study advances our understanding of the complex regulatory mechanisms underlying parental care and the multifaceted role of JH in insect behaviour.