Data from: The outcome of competition between two parasitoid species is influenced by a facultative symbiont of their aphid host

1. Symbiotic bacteria can act to protect their host against natural enemies. Where this protection is asymmetric against different natural enemies protection conferred by symbionts has the potential to mediate interactions between natural enemies, as well as between enemies and the host. 2. In pea aphids (Acyrthosiphon pisum), resistance against parasitoid wasps can be conferred by facultative symbiotic bacteria. We investigated whether the outcome of competition between two parasitoid species can be influenced by the presence of a defensive symbiont in the host. 3. We exposed pea aphids from a single clonal line, with and without a strain of the protective endosymbiont Hamiltonella defensa, to multiparasitism by the parasitoid wasps Aphelinus abdominalis (Aphelinidae) and Aphidius ervi (Braconidae), and recorded the outcome. The symbiont strain is known to impact A. abdominalis more strongly than A. ervi. 4. We found that the presence of a strain of the protective endosymbiont Hamiltonella defensa can reverse the outcome of competition between the wasps. In the absence of the symbiont, A. ervi gains very little success when attacking an aphid previously parasitized by A. abdominalis. However, where the aphids possessed the symbiont, A. abdominalis did not develop successfully, and the success rate of A. ervi was significantly increased. 5. Our results show that defensive facultative symbionts are able to influence community interactions at the trophic level above their host.1. 共生细菌（symbiotic bacteria）可发挥保护宿主抵御天敌的功能。若该保护作用对不同天敌存在不对称性，那么共生菌所赋予的保护便有可能介导天敌之间，以及天敌与宿主之间的相互作用。 2. 在豌豆蚜（Acyrthosiphon pisum）中，兼性共生细菌（facultative symbiotic bacteria）可赋予其对抗寄生蜂的抗性。本研究旨在探究宿主内防御性共生菌的存在，是否会影响两种寄生蜂之间的竞争结果。 3. 我们将来自单一克隆株系的豌豆蚜分为两组，一组携带保护性内共生菌汉密尔顿氏防御菌（Hamiltonella defensa）菌株，另一组则不携带该菌株；随后让两组蚜虫接受阿氏蚜小蜂（Aphelinus abdominalis，蚜小蜂科Aphelinidae）与麦蚜茧蜂（Aphidius ervi，茧蜂科Braconidae）的多重寄生，并记录实验结果。已知该共生菌株对阿氏蚜小蜂的影响强于对麦蚜茧蜂。 4. 研究发现，保护性内共生菌汉密尔顿氏防御菌（Hamiltonella defensa）菌株的存在，可逆转两种寄生蜂的竞争结果。在无该共生菌的情况下，麦蚜茧蜂攻击此前已被阿氏蚜小蜂寄生的蚜虫时，成功率极低。而当蚜虫携带该共生菌时，阿氏蚜小蜂无法顺利完成发育，麦蚜茧蜂的成功率则显著提升。 5. 本研究结果表明，防御性兼性共生菌能够影响宿主营养级之上的群落相互作用。