Ecology of Sphaerularia vespae, a nematode parasite that sterilizes hornets (Vespidae: Vespa): host range, geographical distribution and prevalence of parasitism

The insect parasitic nematode, Sphaerularia vespae, which causes sterilization of queen hornets, was originally reported in Vespa simillima in Hokkaido and more recently from V. mandarinia and V. ducalis in Kyoto, Japan. However, its parasitism in the field is not fully understood. In this study, six native hornet species (V. simillima, V. mandarinia, V. analis, V. ducalis, V. crabro and V. dybowskii) were collected from numerous sites in the Japanese main islands to investigate the host ranges, geographical distributions, and annual and seasonal changes in prevalence of S. vespae. We determined the hosts of S. vespae to be the queens of five of the six hornet species except V. analis. Workers and males were not hosts. Finding S. vespae from cool to warm regions indicates the likelihood that this nematode’s distribution mirrors that of its host hornets. Parasitism of S. vespae at two study sites where consecutive 10-year observations were conducted persisted in the dominant hornet species, which was V. simillima in a cooler region and V. mandarinia in a warmer region. At these sites, no clear relation was apparent between the prevalence of S. vespae and the number of host hornets captured. A rising prevalence during the season was observed in V. simillima but not in V. mandarinia, possibly caused by the species-dependent characteristics of where the colony was founded. Persistence of S. vespae parasitism in host hornets and the potential of S. vespae as a biological control agent against exotic hornets were discussed.