Piercing and sucking behaviors monitoring
We monitored the HI lineage’s piercing and sucking behaviors on fresh and pre-infected cucumber leaf using electrical penetration graph system (EPG) (Tjallingii et a., 1988). Cucumber leaf and aphid were made parts of an electrical circuit in this system. An electrode attached to a gold wire (diameter 20 μm) was inserted into leaf petiole, and the other end of the gold wire was attached to aphid dorsum using electric-conducting glue. When the aphid started probing plant cells, the circuit was closed and electrical signals generated. EGP signals (referred to as waveforms) were the result of voltage fluctuations correlating to the stylet locations within leaf mesophyll (Tjallingii and Esch 1993). Piercing and sucking behaviors of 5-d-old aphids were monitored constantly for 6 h under 22±2 °C. Three treatments were set: the HI lineage on fresh cucumber (HI_fre_F1), the HI lineage on pre-infected cucumber (HI_fre_F1) and the CU lineage on fresh cucumber (CU_fre_F1). Each treatment replicated more than 10 successful recordings. Data were analyzed by Stylet+a v1.25 software. Four typical waveforms were analyzed: (1) C waveform, reflecting stylet contacting with the epidermis and penetrating epidermis and mesophyll cells, (2) G waveform, reflecting active sap ingestion from xylem elements. When the stylet reached plant phloem, two types of waveforms occur: (3) E1 and (4) E2 waveform. E1 is often a short phase of stylet secreting saliva into phloem element. E2 is the phloem sap ingestion phase that can last for hours or days on suitable host plants.