Juvenile hormone and maternal egg care in the European earwig (part II): Precocene exposure reduces post-oviposition egg care behaviours.

Parental care is a critical behaviour that enhances offspring survival and development across the animal kingdom. In insects, several studies have suggested that juvenile hormone (JH), a key regulator of their development and reproduction, also plays an important role in the regulation of parental care. However, recent findings in the European earwig Forficula auricularia challenge this view. In this study, we experimentally manipulated JH levels in 496 post-oviposition earwig females by exposing them to JHIII, two JH agonists (Methoprene, Pyriproxyfen), or a JH-production inhibitor (Precocene I). We then quantified the effects of these treatments on three key maternal care behaviours: egg gathering, egg guarding, and egg grooming. Consistent with a positive role of JH in promoting maternal care, we found that inhibiting JH synthesis via Precocene I led to a reduction in all three forms of maternal care. In contrast, supplementation with JHIII or exposure to JH agonists had no detectable effect on these care behaviours. The observed effects of Precocene I were specific to maternal care, as we found no effect on non-care behaviours (self-grooming, general activity), female body mass, egg development time, or nymph weight at hatching. These effects were also likely to result from changes in maternal JH titres, as our molecular analyses confirmed that Precocene I reduced the expression of two key genes involved in the JH pathway, JHAMT and Kr-h1, while exposure to JHIII and JH agonists had no effect on gene expression. Together, these results provide both experimental and molecular evidence that JH plays a positive role in regulating maternal care after oviposition in the European earwig. More generally, our findings open new avenues for understanding the hormonal basis and evolutionary diversification of parental care strategies in insects.