Transmission of the bacterium through horizontal transfer has been observed within the same species and among different species of "Trichogramma", including "T. kaykai", "T. deion, T. pretiosum," and" T. atopovirilia"; however, limitations to transmission exist. "In vitro" successful horizontal transfer is uncommon within "Trichogramma", which suggests that the density of "Wolbachia" must be relatively high inside of the hosts' ovaries. Cytoplasmic incompatibility of the host and bacterium can also be the source of this unsuccessful transfer in-vitro. These limitations "in vitro" suggest that in nature, horizontal transfer by parthenogenesis-inducing "Wolbachia" may be a difficult and rare phenomenon. However, when looking at the "Wolbachia"-host associations, the "Trichogramma-Wolbachia" form a monophyletic group based on several "Wolbachia"-specific genes, which may be explained by horizontal transfer of "Wolbachia" between different species. Therefore, although interspecific horizontal transfer of "Wolbachia" is limited "in vitro", it is likely to occur quite frequently in nature and is not well understood yet.