Fig 1.
Workflow of laboratory experiments.
A total of 360 I. ricinus nymphs were fed on 12 Balb/c mice. Of those, 50 engorged nymphs were parasitized by I. hookeri wasps. The obtained 96 parasitoids were subsequently screened for the presence of A. nasoniae and rickettsiae by PCR. The DNA of A. nasoniae was found in 27 wasps (28.1%), Rickettsia sp. in 22 wasps (22.9%). Eight wasps were positive for both bacteria—A. nasoniae and Rickettsia sp.
Fig 2.
(A) Habitus of a female after treatment with Marc André solution. (B) Magnified female genitalia. (C) Habitus of a male. (D) Magnified male genitalia. Scale bar represents 0.5 mm.
Fig 3.
Detection of bacteria and wasps in unfed I. ricinus nymphs.
Overall 41 host-seeking I. ricinus nymphs were examined for the presence of A. nasoniae and Rickettsia sp. as well as for I. hookeri by PCR. The DNA of A. nasoniae was successfully amplified in 4 nymphs (9.8%) and the presence of rickettsiae was confirmed in 5 nymphs (12.2%). Six nymphs were parasitized by I. hookeri wasps (14.6%). As shown in the Venn-diagram, all nymphs that were positive for A. nasoniae were simultaneously parasitized by I. hookeri. Co-infection by both bacteria occurred only in one I. ricinus nymph which also contained wasp DNA.
Table 1.
Reports on Ixodiphagus hookeri detected in hard ticks.
Table 2.
Published studies on detection of Arsenophonus nasoniae in ticks.
Fig 4.
The questionable role of parasitic wasps in the transmission of rickettsiae, illustrated on the tick’s life cycle.
Adult female I. hookeri oviposits in larvae and nymphs of ixodid ticks, but the wasp eggs start to develop only in fully engorged nymphs. The immature parasitoid wasps consume the nymph’s tissue and its ingested blood meal, causing nymph death. During the tick’s life cycle (eggs, larvae, nymphs, adults), rickettsiae can pass from stage to stage. In our experiments we successfully amplified rickettsial DNA not just in unfed nymphs but also in emerged adult wasps. More experiments will be needed to demonstrate if I. hookeri may act as a biological vector of A. nasoniae and Rickettsia sp.