Tritrophic interaction influenced by warming and tillage: A field study on winter wheat, aphids and parasitoids

Global warming is expected to affect tritrophic interactions between plants, herbivores, and their natural enemies. Tillage, as a pervasive anthropogenic perturbation, also affects agricultural pests and their natural enemies in cropland systems. The effect of warming combined with tillage on tritrophic interactions is poorly known. A field experiment using infrared warming devices was conducted in conventional and no-tillage wheat fields in Northern China to examine the effects of warming and tillage on tritrophic interactions between winter wheat plants, aphids, and parasitoids. The results show that warming increased plant biomass and advanced plant phenology from re-greening to maturity by 6–11 days. No effects of tillage on plant phenology or biomass were found. Warming significantly increased the numbers of the aphid Sitobion avenae in 2010, when the parasitoid was scarce. Populations of S. avenae were 57.2% larger in warmed than in control plots. In 2011, aphid populations did not differ between warmed and control plots, but parasitoids were abundant, with approximately three times as many in warmed plots than in control ones. The rate of parasitism was also significantly increased in the warmed plots. Tillage had no significant effects on aphid and parasitoid populations in both years. These results indicate that the temperature-induced acceleration of winter wheat phenology resulted in increased aphid abundance. Warming strengthens the bottom-up and top-down effects. The response of parasitoids to warming varied according to their yearly population fluctuations, highlighting the need for a greater basic understanding of parasitoids and conservation of natural enemies.