Principle of EPG technique and plant factor localization modified after Tjallingii [53].

A) Principle of EPG technique: the aphid is connected to the EPG device using conductive glue and a thin gold wire. The plant is connected to the EPG device by inserting an electrode into the soil next to the plant. As soon as the aphid starts plant penetration the electrical circuit is closed and EPG waveforms can be observed and recorded. Amp = amplifier, Ri = input resistor. B) Simplified illustration of various stages in the penetration of plant tissue by the aphid stylets correlated with a schematic representation of EPG recordings below. The parameters derived from the EPG recordings (listed in Table S1) indicate the tissue location of plant factors influencing stylet penetration and feeding behavior. (1) As long as the aphid does not penetrate the plant a flat line, called non-probing waveform (np) is visible. EPG parameters from this stage are influenced by volatile or surface plant factors influencing aphid feeding behavior (Table S1, parameters #1–#4). (1)/(2) When the aphid starts penetration, short probes can often be observed, some with cell punctures (potential drops (pd) marked by asterisks) and separated by non-probing periods. The number and duration of short probes are influenced by factors in the epidermis (parameter #5) or the mesophyll (#8). (2) During the pathway phase (#15) the aphid navigates its stylet bundle through the plant apoplast towards the sieve elements (SEs). Almost every single cell along the stylet pathway is punctured (pd, marked by asterisks) by the aphid’s stylet (#16). Aphid activities during epidermis and mesophyll penetration are reflected in parameters #5–#24. (3) Parameters #25–#51 reflect SE factors, important as the SEs are the aphid’s ultimate feeding target. Before SE salivation (E1) and ingestion (E2), A. pisum often carries out extended and repetitive cell punctures (r-pd).