The present study reports the oviposition behavior of the ant parasitoid wasp, Ghilaromma orientalis, on an undescribed ant species from Lasius fuliginosus species group in Japan, illustrated by clear photographs. Previously, the oviposition behavior in the subfamily Hybrizontinae had been limited to species hovering on an ant trail and attacking larvae carried by worker ants. In contrast, in G. orientalis, whose oviposition behavior had not been reported to date, the wasp hung on the grass growing along the ants’ trail by its hind legs with its head down, and when workers with larvae pass by, directed its abdomen toward the larvae with its hind legs remaining on the grass. Our findings suggest that the subfamily Hybrizontinae employs two host-searching strategies—an active strategy previously known and the ambush-type host-searching strategy employed by G. orientalis. The ambush-type strategy affords G. orientalis the advantage of laying eggs in a narrow environment where wasps cannot fly without being noticed by ants. Moreover, by avoiding detection through ambush tactics, wasps are increasing their chance for attack, as ants continue to transport their larvae. However, the search range of wasps is reduced, which may limit the opportunities for parasitization. Additionally, while Lasius nipponensis has been observed as the sole known host of G. orientalis, the oviposition behavior has now been observed in an undescribed species of L. fuliginosus group, suggesting that G. orientalis may have a subgenus-specific host range.
