Abstract A new species Xanthopimpla zherikhini sp. nov. (Hymenoptera, Ichneumonidae, Pimplinae) is described from Late Eocene Baltic amber. The finding indicates a wider geographical distribution than...

From late September to mid-October 2022 in Hokkaido, Japan, females of Ogkosoma cremieri (Romand) (Ichneumonidae: Hybrizontinae) were observed hovering near the nest entrance of Lasius capitatus (Kuznetsov-Ugamsky) (Formicidae), approaching larvae carried by worker ants. Females sometimes aggressively pushed each other in competition over hosts. Individuals that were knocked away were occasionally attacked by ants and carried into the nest. These observations suggest intraspecific competition among females over limited host resources and a high risk of ant attack during oviposition attempts. Nighttime observations revealed active larval transport by ants, but no parasitoidic behavior by O. cremieri, suggesting that females are inactive at night. Parasitoidism of social insects faces high barriers due to their collective defenses and complex communication systems, making understanding parasitoid adaptations essential for elucidating host–parasitoid interactions. Our findings provide insight into the behavioral evolution of parasitoid wasps and the evolutionary arms race between hosts and parasitoids.