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。已有实验室研究证据表明，这类兼性共生菌可为宿主提供适应性性状，例如增强对自然天敌的抗性。然而，当宿主遭受自然存在的一系列寄生虫与病原菌侵染时，共生菌如何影响宿主适合度，目前仍不明晰。
2. 本研究针对两种保护性共生菌——雷吉氏菌（Regiella insecticola）与汉密尔顿氏菌（Hamiltonella defensa），检验其在自然环境下是否能提升宿主豌豆蚜（Acyrthosiphon pisum）的适合度。
3. 我们将两组豌豆蚜品系的实验种群（每组均设置带共生菌与不带共生菌两个处理）置于5处湿草甸生境中，使其暴露于自然组合的天敌类群中。随后回收蚜虫，分别评估其因寄生性天敌、真菌病原菌及其他因素导致的死亡率。
4. 研究发现，雷吉氏菌与汉密尔顿氏菌均能降低被实验室及笼养实验中已证实的特异性天敌所击杀的蚜虫比例。但由于其他天敌导致的死亡率上升，以及携带共生菌带来的生存成本，这一优势被抵消（汉密尔顿氏菌）甚至逆转（雷吉氏菌）。因此，共生菌可通过改变不同天敌的相对生存优势，进而影响群落结构。
5. 本研究结果表明，保护性共生菌未必对宿主有利，其行为甚至更接近寄生虫而非互利共生菌。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。