Abstract We present the first records of Spalangia crassicornis Bouček, 1963 (Hymenoptera: Spalangiidae) from Japan. This species was observed at three localities in Hokkaido, where it was found in association with nests of Lasius capitatus (Kuznetsov‐Ugamsky, 1928) and Lasius fuji Radchenko, 2005 sensu lato (Hymenoptera: Formicidae). Spalangia crassicornis was frequently observed entering and leaving Lasius ant nests and walking among workers without eliciting aggressive responses, and these interactions were documented using both photographic and video records. These observations suggest that S . crassicornis may have evolved traits that allow it to avoid aggression from host ants. Consistent with these observations, analyses of cuticular hydrocarbons indicated that S . crassicornis does not employ chemical mimicry but instead exhibits a markedly reduced cuticular hydrocarbon profile, which may avoid ant aggression. Furthermore, rearing and DNA analyses revealed that Milichia sp. near ludens represents a new host record for S . crassicornis . Ant workers of L . capitatus briefly antennated a larva of Milichia sp. near ludens upon contact but showed no aggressive behavior. Based on this observation, the larva may possess adaptive traits enabling it to avoid attacks by host ants.
Kajiwara et al. (2026) studied this question.