Alternative host-searching strategy reported for the ant parasitoid wasp Ghilaromma orientalis

“In our ten years of frantic observation, we were fortunate enough to observe the parasitic behavior of G. orientalis a single time.”

Guest blog post by Yu Hisasue, Kazuhiko Konishi, and Kenji Takashino

Parasitoid wasps have developed behaviors to adapt to the ecology of various hosts and overcome their means of avoiding parasitism. Host searching behavior is a crucial stage for parasitoids, not only for efficient host search, but also for competing with other parasitoids that exploit the same host as a resource. A variety of such behaviors has been reported, including utilizing chemical or sonic cues. Parasitoid wasps select their strategies based on their own morphology, their host, and the host’s habitat.

Parasitoid wasps that challenge the ant society are known to have highly specialized morphologies and behaviors.

Female of Ghilaromma orientalis hanging from the grass above the ant trail with her head facing the trail.

All members of the subfamily Hybrizontinae are ant parasitoids, and specialize to the ant society, representing the third most diverse group of ant parasitoid wasps after Eucharitidae and Neoneurini. The oviposition behavior has been reported for three species belonging to three different genera in Hybrizontinae: Ogkosoma cremieri, Neohybrizon mutus, and Hybrizon buccatus. In these species, the females hover over an ant trail, and when they come across ant larvae carried by workers, they attack and lay eggs on the ant larvae.

No reports have been made for oviposition behavior in the genus Ghilaromma. G. orientalis was suggested to be a specialist of Lasius nipponensis, but its oviposition behavior was unknown.

Female Ghilaromma orientalis using her front legs to contact an ant larva and directing her abdomen towards it, while maintaining a firm grip on the grass with her hind legs.

In our ten years of frantic observation, we were fortunate enough to observe the parasitic behavior of G. orientalis a single time. On 20 October 2015, Kenji Takashino observed the oviposition and took pictures of it on his phone.

He noticed that the female, hung on the grass growing along the ants’ trail on its hind legs with its head down, and when workers with larvae pass by, stretched its abdomen toward the larvae with its hind legs remaining on the grass.

We published these first observations of the oviposition behavior of G. orientalis in a research article in the Journal of Hymenoptera Research.

Female Ghilaromma orientalis maintaining a firm grip on the grass with her hind legs while adjusting her body position to oviposition onto a larva being carried away by an ant.

There are different merits and demerits of the two strategies in Hybrizontinae. The active type has the advantage of covering a wider search area and enabling the movement of the parasitoid to areas where the ants carrying larvae are located. However, this strategy has the drawback that hovering of parasitoid wasps over an ant trail alerts the ants and prevents larva-carrying ants from exiting the nest entrance or covered area. In addition, ants have been observed to open their mandibles to threaten hovering wasps, which then occasionally fail to fly or get captured by worker ants.

Workers of Lasius nipponensis with their mandibles open, alerted to the hovering Ogkosoma cremieri.

The ambush type has the limitation of a narrow search area. As ant larvae are not always conveniently transported by workers close to the wasp, narrowing the search area directly leads to a decrease in parasitic opportunities. However, the ambush type strategy affords G. orientalis the advantage of laying eggs without being noticed by ants and in a narrow environment where wasps cannot fly in active type.

In addition, we report a new host ant (Lasius cf. fuliginosus) for G. orientalis. Some members of this species group are known to transport their larvae outside the nest. Therefore, it is plausible that G. orientalis may use not only a single ant species, but multiple L. fuliginosus-group species that have a habit of transporting larvae outside the nest.

Although we made only one observation of the wasp, we compared and discussed the other ecological information and parasitic behavior of closely related species using observations, literature, and studies on the parasitic behavior of other well-studied parasitoid wasps.

Research article:

Hisasue Y, Konishi K, Takashino K (2023) An alternative host searching strategy found in the subfamily Hybrizontinae (Hymenoptera, Ichneumonidae). Journal of Hymenoptera Research 96: 629-639. https://doi.org/10.3897/jhr.96.106836