Earwig mothers employ a suite of hygienic behaviors to defend their nest against microbial threats

Abstract Hygienic behavior is a key defense against pathogens in mammals and birds, yet it remains surprisingly understudied in insects outside eusocial species. This gap raises the questions of whether such behaviors are unique to eusocial insects or instead reflect widespread forms of social immunity that may have shaped the evolution of insect sociality. Here, we investigated hygienic behavior in the European earwig, a non-eusocial insect exhibiting maternal egg care. First, we exposed pre-oviposition females and males to organic (microbial risk) and plastic items, and found that only females selectively covered organic items, starting 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, regardless of the presence of pathogen spores. Nest architecture remained unaffected by item type. Altogether, these findings reveal advanced hygienic behaviors in a non-eusocial insect, confirming that certain forms of social immunity can evolve independently of eusociality and reproductive division of labor. These findings highlight how parental care could have been a pivotal step in the evolutionary trajectory toward the sophisticated disease defenses seen in more complex insect societies.