Many insects harbor facultative microbial symbionts which affect the ecology of their hosts in diverse ways. Most symbionts are transmitted vertically with high fidelity, whereas horizontal transmission occurs rarely. Parasitoid larvae feed on a single host and are in close physical contact with it, providing an ecological opportunity for symbionts’ horizontal transmission, but there is little empirical evidence documenting this. Here we studied horizontal transmission of three bacterial symbionts—Rickettsia, Sodalis, and Wolbachia—between three fly pupal ectoparasitoid species: Spalangia cameroni, S. endius, and Muscidifurax raptor. Muscidifurax raptor readily parasitized and successfully developed on the Spalangia spp., while the inverse did not happen. The two Spalangia spp. attacked each other and conspecifics in very low rates. Symbiont horizontal transmissions followed by stable vertical transmission in the recipient species were achieved, in low percentages, only between conspecifics: Wolbachia from infected to uninfected M. raptor, Rickettsia in S. endius, and Sodalis in S. cameroni. Low frequency of horizontal transmissions occurred in the interspecific combinations, but none of them persisted in the recipient species beyond F₄, at most. Our study is one of few to demonstrate symbionts’ horizontal transmission between hosts within the same trophic level and guild and highlights the rarity of such events.

许多昆虫具有兼性微生物共生体，它们以多种方式影响宿主的生态。大多数共生体以高保真度垂直传播，而水平传播很少发生。寄生性幼虫以单一宿主为食，并与其有密切的身体接触，为共生体的水平传播提供了生态机会，但几乎没有经验证据证明这一点。在这里，我们研究了三种细菌共生体——立克次体、Sodalis和Wolbachia——在三种苍蝇蛹外寄生物物种之间的水平传播：Spalangia cameroni、S. endius和Muscidifurax raptor。猛禽很容易寄生并在Spalangia spp.上成功开发，而相反的情况并没有发生。两个Spalangia spp。以非常低的速度互相攻击和同种。共生体水平传输随后稳定垂直传播在受体物种中实现的，在低百分比，只有同种之间：沃尔巴克氏体从感染的未感染M.猛禽，立克次氏体在S.恩迪斯，和Sodalis在S. cameroni。在种间组合中发生了低频率的水平传播，但在 F ₄之后的受体物种中没有一个持续存在， 最多。我们的研究是少数证明共生体在同一营养级和行会内的宿主之间水平传播的研究之一，并突出了此类事件的罕见性。