Aphid activities during sieve element punctures

Aphid salivation in sieve elements and phloem sap ingestion were linked to waveforms in the Electrical Penetration Graph (EPG). Non-viruliferousRhopalosiphum padi (L.) (Hemiptera, Aphididae) on barley yellow dwarf virus (BYDV) infected wheat could acquire the virus, which was used as an indication for phloem sap ingestion, whereas virus inoculation by viruliferous aphids on healthy plants was associated with salivation in sieve elements or other phloem cells. Probing was monitored and the waveforms recorded were related to ELISA results of test plants. The EPG patterns A, B, and C are indicative of the stylet pathway phase, whereas patterns E1 and E2 reflect the phloem (sieve element) phase with an unknown activity (E1) or with ingestion and concurrent salivation (E2). Aphids showing pathway and E1 rarely acquired virus, suggesting that little or no phloem sap ingestion can occur during these patterns, whereas those showing additionally pattern E2 did so substantially, indicating phloem sap ingestion. The main pattern related to virus inoculation was E1, although some aphids were able to inoculate plants during pathway. Pattern E1 clearly reflects the most important salivation into sieve elements. Pattern E2 had no clear contribution to virus inoculation, supporting the present hypothesis that during this pattern the saliva is mixed with the phloem sap in the single canal at the stylet tips and ingested immediately, without reaching the plant tissue. Sustained sap ingestion did not affect virus inoculation. So, BYDV inoculation mainly occurs during the first period of a sieve element puncture which is always formed by E1. Implications on persistent virus transmission are discussed.