Data from: Symbionts modify interactions between insects and natural enemies in the field

1. Eukaryotes commonly host communities of heritable symbiotic bacteria, many of which are not essential for their hosts’ survival and reproduction. There is laboratory evidence that these facultative symbionts can provide useful adaptations, such as increased resistance to natural enemies. However, we do not know how symbionts affect host fitness when the latter are subject to attack by a natural suite of parasites and pathogens. 2. Here we test whether two protective symbionts, Regiella insecticola and Hamiltonella defensa, increase the fitness of their host, the pea aphid (Acyrthosiphon pisum), under natural conditions. 3. We placed experimental populations of two pea aphid lines, each with and without symbionts, in five wet meadow sites to expose them to a natural assembly of enemy species. The aphids were then retrieved and mortality from parasitoids, fungal pathogens and other causes assessed. 4. We found that both Regiella and Hamiltonella reduce the proportion of aphids killed by the specific natural enemies against which they have been shown to protect in laboratory and cage experiments. However, this advantage was nullified (Hamiltonella) or reversed (Regiella) by an increase in mortality from other natural enemies and by the cost of carrying the symbiont. Symbionts therefore affect community structure by altering the relative success of different natural enemies. 5. Our results show that protective symbionts are not necessarily advantageous to their hosts, and may even behave more like parasites than mutualists. Nevertheless, bacterial symbionts may play an important role in determining food web structure and dynamics.1. 真核生物（Eukaryotes）通常携带有可遗传共生细菌群落，其中多数对宿主的生存与繁殖并非必需。已有实验室证据表明，这类兼性共生菌（facultative symbionts）可为宿主提供有益的适应性性状，例如增强对天敌的抗性。然而，当宿主遭遇自然群落的寄生虫与病原体攻击时，共生菌如何影响宿主适合度（host fitness），目前仍不明晰。 2. 本研究旨在探究两种保护性共生菌——烟晶雷杰氏菌（Regiella insecticola）与防御汉密尔顿菌（Hamiltonella defensa）——在自然条件下是否能够提升其宿主豌豆蚜（Acyrthosiphon pisum）的适合度。 3. 我们将两组豌豆蚜品系的实验种群（每组分别携带与不携带共生菌）放置于5处湿草甸生境中，使其暴露于自然组成的天敌群落中。随后回收蚜虫，并评估其因寄生性天敌、真菌病原体及其他因素导致的死亡率。 4. 研究发现，烟晶雷杰氏菌与防御汉密尔顿菌均可降低因特定天敌致死的蚜虫比例——这类天敌正是此前实验室与笼养实验中证实的该共生菌的防护对象。然而，其他天敌导致的死亡率上升以及携带共生菌的生存代价，抵消了防御汉密尔顿菌的生存优势，甚至逆转了烟晶雷杰氏菌带来的益处。因此，共生菌可通过改变不同天敌的相对生存竞争力，进而影响群落结构。 5. 本研究结果表明，保护性共生菌并非总能为宿主带来益处，其甚至可能更偏向于寄生者而非互利共生者。尽管如此，细菌共生菌或许在决定食物网结构与动态方面发挥着重要作用。