Abstract
Aphids feed on plant phloem sap, rich in sugars but poor in essential amino acids. However, sugars cause osmotic regulation problems for aphids, which they overcome by hydrolysing the sugars in their gut and polymerising the hydrolysis products into oligosaccharides, excreted with honeydew. Aphids harbour primary bacterial endosymbionts, which supply them with essential amino acids necessary for survival. They also harbour secondary (facultative) endosymbionts (sfS), some of which have a positive impact on life history traits, although it is not yet known whether they also play a role in providing effective tolerance to differing levels of water soluble carbohydrates (WSCs). We investigated the relationship between WSC content of cocksfoot cultivars and performance of clones of the English grain aphid Sitobion avenae F. We evaluated how clone genotype and their sfS modulate performance on these different cultivars. We therefore examined the performance of genetically defined clones of S. avenae, collected from different host plants, harbouring different sfS. The performance was tested on 10 Dactylis glomerata L. cultivars with varying WSC content. D. glomerata is known as a wild host plant for S. avenae and is also commercially planted. We found that high WSCs levels are responsible for the resistance of D. glomerata cultivars to specific S. avenae clones. The minimum level of WSCs conferring resistance to D. glomerata cultivars was 1.7% dw. Cultivars with a WSC content of 2.2% or higher were resistant to S. avenae and did not allow reproduction. Our results further indicate that sfS modulate to some extend host plant cultivar adaptation in S. avenae. This is the first study revealing the importance of WSCs for aphid performance. Cocksfoot cultivars with a high content of WSCs might be therefore considered for aphid control or used for resistance breeding in this and other grass species, including cereals.