A fitness cost resulting from Hamiltonella defensa infection is associated with altered probing and feeding behaviour in Rhopalosiphum padi

Aphids frequently associate with facultative endosymbiotic bacteria which influence aphid physiology in myriad ways. Endosymbiont infection can increase aphid resistance against parasitoids and pathogens, modulate plant responses to aphid feeding, and promote aphid virulence. These endosymbiotic relationships can also decrease aphid fitness in the absence of natural enemies or when feeding on less suitable plant types. Here, we use the Electrical Penetration Graph (EPG) technique to monitor feeding behaviour of four genetically-similar clonal lines of a cereal-feeding aphid, Rhopalosiphum padi, differentially infected (+/-) with the facultative protective endosymbiont, Hamiltonella defensa, to understand how physiological processes at the aphid-plant interface are affected by endosymbiont infection. Endosymbiont-infected aphids exhibited altered probing and feeding patterns compared with uninfected aphids, characterised by a two-fold increase in the number of plant cell punctures, a 50% reduction in the duration of each cellular puncture, and a greater probability of achieving sustained ingestion of plant phloem. Feeding behaviour was altered further by host plant identity: endosymbiont-infected aphids spent less time probing into plant tissue, required twice as many probes into plant tissue to reach plant phloem, and showed a 44% reduction in phloem ingestion when feeding on the partially-resistant wild relative of barley, Hordeum spontaneum 5, compared with a commercial barley cultivar. These observations might explain reduced growth of H. defensa-infected aphids on the former host plant. This study is the first to demonstrate a physiological mechanism at the aphid-plant interface contributing to endosymbiont effects on aphid fitness on different quality plants through altered aphid feeding behaviour.\n\nSummaryReduced performance of aphids infected with a common facultative endosymbiont on poor quality plants may be explained by changes in aphid probing behaviour and decreased phloem sap ingestion.