Joël MEUNIER

Parental care is a vital behaviour that enhances offspring survival, yet its hormonal regulation in insects remains poorly understood. Using the European earwig Forficula auricularia, we conducted two complementary studies to test the role of juvenile hormone (JH) in the initiation and maintenance of egg care. By experimentally manipulating JH levels before egg laying, we found that neither increasing nor decreasing JH induced females to care for eggs or prevented egg cannibalism, indicating that JH alone cannot trigger the onset of care. However, when we inhibited JH synthesis after egg laying (ie. after egg care has started), females expressed significantly less egg gathering, guarding, and grooming, while JH supplementation had no detectable effect. Molecular analyses confirmed that JH inhibition reduced the expression of key genes in the JH pathway, supporting a positive role of JH in sustaining maternal care once it has begun. Together, our findings demonstrate that JH is not a trigger but rather a maintainer of parental care in F. auricularia, contrary to other earwigs species. These studies are part of Laura Pasquier's PhD thesis.​​

Chemical pollution, particularly from endocrine-disrupting compounds (EDCs), is increasingly recognised for its ability to interfere with key biological processes in vertebrates, such as parental care. However, much less is known about its effects in insects. This review summarises current knowledge on how EDCs affect insect parenting and identifies major research gaps. It highlights that most studies focus on a few species and behaviours, mainly linked to the juvenile hormone, and that evidence for pollutant effects on parental care remains scarce. We propose five priorities for future research: clarifying hormonal mechanisms, applying modern molecular tools, identifying the most harmful pollutants, expanding studies across more species and behaviours, and linking individual effects to population outcomes. Understanding these connections will be crucial to predict and mitigate the impacts of pollutants on insect populations and ecosystems. This review stems from several years of close collaboration with C. Lécureuil and reflects the bridges we have built between our two fields - behavioural ecology and physiology. This study is part of Laura Pasquier's PhD thesis.​​

Animals rely on microbes to stay healthy, and in many species these microbes are passed from parents and shared between siblings. Can chemical pollution disrupts this process? We studied European earwig families to find out. Juveniles were raised either alone or with family members and were fed food with or without cadmium, a toxic heavy metal. We then looked at their gut microbes. We found that both family life and cadmium shaped which microbes juveniles carried. However, living with family did not make them more or less affected by pollution. Instead, family life consistently promoted the sharing of both helpful and harmful microbes. Our results show that family-based microbial sharing is surprisingly resilient, even under environmental stress. This study is part of Romain Honorio and Marie-Charlotte Cheutin projects, while the were postdocs in our group.​​

Birds and mammals are well known for shielding their eggs and young from harmful microbes, but do insects—particularly non-eusocial species—do the same? We addressed this question in the European earwig. First, we exposed pre-ovipositing females and males to organic (presenting a risk of microbioal infection) and plastic items and found that only females selectively covered organic items, beginning at oviposition. We then introduced either an organic item, an organic item with pathogen spores, a plastic item, or no item into female nests post-oviposition. Mothers continued to cover organic items and consistently relocated their eggs away from them, irrespective of the presence of pathogen spores. These findings reveal advanced hygienic behaviours in a non-eusocial insect and demonstrate that certain forms of social immunity can evolve independently of reproductive division of labour. More broadly, they suggest that parental care may have been a pivotal step in the evolutionary trajectory leading to the sophisticated disease defences observed in complex insect societies. This study is part of Simon de Wever's PhD thesis.​​

Living in groups is known to protect young animals from environmental dangers like predators and extreme temperatures, but its role in mitigating toxic stressors such as chemical pollution is unclear. We studied European earwig juveniles, rearing them in different social settings and exposing them to cadmium, and found that while social environment affects juvenile development and survival, it does not lessen cadmium's toxic effects. This highlights the need to explore the mechanisms behind the earwig's resistance to cadmium. This study is part of Romain Honorio's project, while he was still a postdoc (ATER) in our group.​​

In this study, we looked at whether Roundup affects the European earwig. We exposed mothers to different doses of Roundup — including doses much lower and higher than what is normally used — and observed how they cared for their eggs and young. We also looked at how Roundup affected the growth, movement, and immune defences of the juveniles. Somehow surprisingly, we found no clear effects of Roundup on the behaviour or development of either mothers or their young. This suggests that earwigs may be more resilient to this herbicide than previously thought. However, it’s still important to understand why this is the case, and whether other species show similar resilience. Overall, our research highlights the need to study a wide variety of insects to fully understand how pesticides influence ecosystems — and to help develop more sustainable farming practices. This study is part of Laura Pasquier's PhD thesis.​​

Not all species raise their young the same way—even when they look nearly identical. In this study, we uncovered key differences between two look-alike species of European earwigs (Forficula auricularia and Forficula dentata), both important in agriculture and known for their invasive success. We raised 153 females from seven populations under the same lab conditions, measuring 24 traits across their lives and focusing on how they cared for their eggs—how often they moved, protected, or inspected them. Each species followed a different strategy. F. dentata mothers delayed egg-laying to improve offspring quality but had lower hatching success. In contrast, F. auricularia mothers laid eggs earlier, increasing hatching success but producing less robust young. Most maternal behaviours were similar, though F. dentata mothers defended their eggs more strongly. These subtle differences in parenting and reproduction may help explain how both species thrive in new environments. Even among look-alikes, nature finds more than one path to success. This study is part of Laura Pasquier's PhD thesis.​​

We are thrilled to see our special issue of the journal "Insectes Sociaux" out, showcasing the amazing diversity of social lives in insects ! The aim of this special volume is to advocate for this broader definition of insect sociality by shedding light on the diverse insect taxa that exhibit social behaviours. This is achieved through a collection of nine papers that explore social behaviours across several insect orders, including Diptera, Hymenoptera, Dermaptera, Coleoptera, and Hemiptera.

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This special issue has been co-edited with my colleagues J. Quezada-Euan and Miriam Richards.

I am delighted to see our research on earwigs featured in the Knowable magazine. There's a nice overview of the fascinating biology of this insect, our recent results, and behind-the-scenes stories from me and colleagues. A very nice piece of work!

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https://knowablemagazine.org/content/article/living-world/2024/social-life-of-earwigs

We tested whether parental care mitigates the impact of cold and heat waves on eggs and juveniles in the European earwig. We exposed mothers and their eggs to three-day thermal waves of 3°C, 10°C (control), 17°C or 24°C, both 15 and 30 days after oviposition. We then measured four maternal care behaviors, maternal weight variation, as well as eggs’ developmental time, survival, and hatching rate. In the resulting juveniles, we measured weight, developmental time, thermal resistance, and the expression of six heat stress and immunity genes. We found that thermal waves reduced maternal care and induced maternal weight gain. High temperatures also decreased egg hatching success, accelerated egg and nymph development, reduced the upper thermal limit of juveniles and decreased the expression of a heat shock protein (Hsp68), while other traits remained unaffected. Overall, this study suggests that species with maternal care do not necessarily have access to effective thermal protection and may not be better adapted to climate change. This study marks the first collaboration with Irene Villalta, more results to come ...