Mountain Agriculture Research Unit, Institute of Ecology, University of Innsbruck, Innsbruck, Austria.
Agroecology, Department of Crop Sciences, Georg-August-University Göttingen, Göttingen, Germany.
J Anim Ecol. 2018 Sep;87(5):1440-1451. doi: 10.1111/1365-2656.12875. Epub 2018 Jul 16.
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. 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. 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. Facultative endosymbiont infection rates were high across fields (80%), independent of whether aphids were parasitized or unparasitized. Aphid mummies exhibited a significantly lower share of facultative endosymbiont infection (38%). These findings suggest that facultative endosymbionts do not affect parasitoid oviposition behaviour, 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. The current findings strongly suggest that facultative endosymbionts act as a driving force in aphid-parasitoid-hyperparasitoid networks: they shape insect community composition at different trophic levels and modulate, directly and indirectly, the interactions between aphids, parasitoids and their environment.
兼性细菌内共生体可以保护它们的蚜虫宿主免受膜翅目寄生蜂等天敌的侵害。因此，它们能够调节蚜虫、寄生蜂和重寄生蜂之间的相互作用。然而，目前尚不清楚这些效应在自然蚜虫种群及其相关寄生蜂群落中的程度，以及环境因素（如植物施肥和景观复杂性）如何影响蚜虫-寄生蜂相互作用。在这里，我们测试了兼性内共生体是否能为自然种群中的英国谷物蚜虫 Sitobion avenae 提供保护，以及植物施肥和景观复杂性是否会影响这一保护作用。此外，我们还研究了兼性内共生体的作用是否会级联到重寄生蜂水平，并增加初级-重寄生蜂食物网的专业化。在德国中部的 13 个麦田中，我们在施肥和未施肥的地块中收集活蚜虫和蛹。我们使用新建立的分子方法评估了蚜虫和蛹中初级寄生蜂、重寄生蜂和内共生体物种的发生情况。无论蚜虫是否被寄生，田间的兼性内共生体感染率都很高（约 80%）。蚜虫蛹的兼性内共生体感染率明显较低（约 38%）。这些发现表明，兼性内共生体不会影响寄生蜂的产卵行为，但会降低寄生蜂在宿主中的存活率。与从未施肥植物中收集的蛹相比，从施肥植物中收集的蛹的兼性内共生体感染率较低，这表明植物施肥增强了兼性内共生体的保护作用。此外，我们还发现了强有力的证据表明，兼性内共生体对初级和重寄生蜂分别具有物种特异性和负级联效应。兼性内共生体影响寄生蜂组合，并增加了初级-重寄生蜂食物网的专业化：这些效应独立于其他环境因素，且比其他环境因素更强。目前的研究结果强烈表明，兼性内共生体是蚜虫-寄生蜂-重寄生蜂网络中的驱动因素：它们塑造了不同营养水平的昆虫群落组成，并直接和间接调节了蚜虫、寄生蜂及其环境之间的相互作用。
