Data from: Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: a correlative analysis

1.Facultative bacterial endosymbionts can protect their aphid hosts from natural enemies such as hymenopteran parasitoids. As such, they have the capability to modulate interactions between aphids, parasitoids and hyperparasitoids. However, the magnitude of these effects in natural aphid populations and their associated parasitoid communities is currently unknown. Moreover, environmental factors such as plant fertilization and landscape complexity are known to affect aphid‐parasitoid interactions but it remains unclear how such environmental factors affect the interplay between aphids, parasitoids and endosymbionts.

2.Here, we tested whether facultative endosymbionts confer protection to parasitoids in natural populations of the English grain aphid, Sitobion avenae, and if this is affected by plant fertilization and landscape complexity. Furthermore, we examined whether the effects of facultative endosymbionts can cascade up to the hyperparasitoid level and increase primary‐hyperparasitoid food web specialization.

3.Living aphids and mummies were collected in fertilized and unfertilized plots within 13 wheat fields in Central Germany. We assessed the occurrence of primary parasitoid, hyperparasitoid and endosymbiont species in aphids and mummies using a newly established molecular approach.

4.Facultative endosymbiont infection rates were high across fields (~80 %), independent of whether aphids were parasitized or un‐parasitized. Aphid mummies exhibited a significantly lower share of facultative endosymbiont infection (~38 %). These findings suggest that facultative endosymbionts do not affect parasitoid oviposition behavior, but decrease parasitoid survival in the host. Facultative endosymbiont infection rates were lower in mummies collected from fertilized compared to unfertilized plants, indicating that plant fertilization boosts the facultative endosymbiont protective effect. Furthermore, we found strong evidence for species‐specific and negative cascading effects of facultative endosymbionts on primary and hyperparasitoids, respectively. Facultative endosymbionts impacted parasitoid assemblages and increased the specialization of primary‐hyperparasitoid food webs: these effects were independent from and much stronger than other environmental factors.

5.The current findings stongly suggest that facultative endosymbionts act as a driving force in aphid‐parasitoid‐hyperparastioid networks: they shape insect community composition at different trophic levels and modulate, directly and indirectly, the interactions between aphids, parasitoids and their environment.

1. 兼性细菌内共生菌（facultative bacterial endosymbionts）可保护其蚜虫宿主免受膜翅目寄生蜂（hymenopteran parasitoids）等天敌的侵害。因此，这类共生菌能够调控蚜虫、寄生蜂与重寄生蜂（hyperparasitoids）之间的互作关系。但目前尚不清楚这类效应在自然蚜虫种群及其伴随的寄生蜂群落中发挥的作用强度。此外，尽管已知植物施肥、景观复杂度等环境因子会影响蚜虫-寄生蜂互作，但这类环境因子如何调控蚜虫、寄生蜂与内共生菌之间的互作关系仍不明确。
2. 本研究以麦长管蚜（English grain aphid, Sitobion avenae）的自然种群为研究对象，探究兼性内共生菌是否能为寄生蜂提供保护，以及该保护效应是否受植物施肥与景观复杂度的调控。此外，本研究还验证了兼性内共生菌的效应是否能向上传递至重寄生蜂层级，并提升初级寄生蜂-重寄生蜂食物网的特化程度。
3. 研究人员在德国中部13个麦田的施肥与未施肥样地中采集了活蚜虫与僵蚜（mummies），并采用新建的分子检测方法，对蚜虫与僵蚜体内的初级寄生蜂、重寄生蜂及内共生菌物种的存在情况进行了检测。
4. 各麦田中的兼性内共生菌感染率均较高（约80%），且与蚜虫是否被寄生无关。蚜虫僵蚜的兼性内共生菌感染率则显著更低（约38%）。上述结果表明，兼性内共生菌不会影响寄生蜂的产卵行为，但会降低宿主内寄生蜂的存活率。从施肥植株上采集的僵蚜，其兼性内共生菌感染率低于未施肥植株上的僵蚜，这表明植物施肥可增强兼性内共生菌的保护效应。此外，本研究发现了充分证据表明，兼性内共生菌分别对初级寄生蜂和重寄生蜂产生物种特异性的负向级联效应。兼性内共生菌会影响寄生蜂群落组成，并提升初级寄生蜂-重寄生蜂食物网的特化程度：这类效应不受其他环境因子的影响，且作用强度远高于其他环境因子。
5. 本研究结果充分表明，兼性内共生菌是蚜虫-寄生蜂-重寄生蜂网络的核心驱动因子：它们能够塑造不同营养级的昆虫群落组成，并直接或间接调控蚜虫、寄生蜂与其生存环境之间的互作关系。