The presence of the intracellular bacterium Wolbachia in the tick Ixodes ricinus is in fact due to the cryptic presence of an endoparasitoid hymenoptera.

National and/or international context/issues/issues

The identification of microorganisms carried by ticks is an essential issue in human or animal health. In addition to their role as pathogen vectors, ticks also harbour symbiotic bacteria. However, the consequences of the presence of these bacteria on tick biology (impact on tick fitness, their vectorial competence, their mode of reproduction, etc.) are not known.

Results

Our investigations have shown that the endoparasitoid Ixodiphagus hookeri (Hymenoptera, Chalcidoidea, Encyrtidae), is almost 100% infected by W. pipientis. We developed specific PCR primers for the hymenoptera and for W. pipientis which showed that all nymphs of ticks parasitized by I. hookeri also carried Wolbachia, whereas ticks not parasitized by the insect were free from Wolbachia. We also showed experimentally that pupae obtained from larvae exposed to I. hookeri during their gorgement on a vertebrate host also carried Wolbachia. We suggest that the presence of W. pipientis in ticks, already reported in different publications, was in fact due to the cryptic presence of the endoparasitoid hymenoptera I. hookeri. This association had remained unsuspected until now because the parasitoids cannot be detected before the tick nymphs become blood-soaked, thus raising the diapause of the hymenoptera eggs. 

Perspectives/eventual impact

From a practical point of view, the detection of the presence of the parasitoid in a tick finally proved to be more sensitive by the search for Wolbachia than that of the hymenoptera itself. This tool can therefore be useful to estimate parasitism rates within natural tick populations (which frequently reach 20% and are the origin of biological control projects against ticks via this specific parasitoid).

This study shows the importance of experimental approaches to identify the origin and function of certain bacteria in a given sample and underlines the danger of abusive extrapolations that can result from certain descriptive approaches (such as those involving metagenomics).

Partners

This work is the result of a collaboration with a laboratory of the EFPA department (UR CEFS in Toulouse) and colleagues from the ONCFS (Chizé), which made it possible to collect a large number of ticks from deer in areas with a high density of this mite (an area also favourable to parasitism by I. hookeri).

Valorisation

Plantard, O., A. Bouju-Albert, M. A. Malard, A. Hermouet, G. Capron, and H. Verheyden. 2012. Detection of Wolbachia in the Tick Ixodes ricinus is Due to the Presence of the Hymenoptera Endoparasitoid Ixodiphagus hookeri. Plos One 7: 8.