Effects of experimental warming on competition between Rhopalosiphum padi and Sitobion avenae mediated by plant water content

Global warming is affecting the severity of yield losses due to increased pest abundances. However, few studies have investigated how warming may affect interspecific competitions in multi-pest systems. Here, we investigated the effects of a 2(degrees)C temperature increase on the population growth and distribution on plants of the aphids Rhopalosiphum padi and Sitobion avenae in single versus mixed species releases. We also investigated how the distribution of aphids across plant parts (lower leaves, upper leaves and stems) could be explained by plant tissue content in soluble sugars, free amino acids and flavonoids and by plant tissue microenvironment (leaf temperature and water potential). Populations of R. padi were higher under increased temperature, which caused increased competition on S. avenae. This in turn decreased S. avenae's niche breadth under increased temperature. Contents in soluble sugar, free amino acids and flavonoids were higher in lower and upper leaves compared to stems, but were not affected by temperature. The number of aphids in different plant parts was strongly and negatively correlated with plant tissue water potential, an indicator of reduced content in flavonoids and soluble sugars. Our study shows that even relatively small increases in temperature reflecting climate change may disturb species interactions notably through plant-mediated effects, but consequences for crop productivity are difficult to predict.