-- this section is under revision --
In the EPG technique, an aphid (or another insect with piercing mouthparts) and a plant are made part of an electrical circuit by inserting a wire into the soil of a potted plant, and attaching a very thin wire to the insect. The circuit also incorporates an electrical resistor (Ri) and a voltage source (V), as illustrated below. As soon as the aphid stylets penetrate the plant, the circuit is completed and a fluctuating voltage, called the 'EPG signal', occurs at the measuring point which is then amplified and recorded on a computer hard disk. The voltage fluctuations appear in a number of distinct patters with respect to amplitude, frequency, and voltage level, which are referred to as 'EPG waveforms'. For a number of insects thesewaveforms have been correlated experimentally with the insect´s stylet penetration (probing) activities and stylet tip positions in the plant tissues and cells.
Fig. 1. The primary EPG circuit
The electrical origin of the EPG signal
The fluctuating voltages oringinate from two different physical sources in the insect-plant combination: 1) fluctuating electrical resistance (R) and 2) fluctuating 'generated' voltages, called electromotive force (emf). The signal components of these sources are referred to as the R-components and emf-components, respectively. The two components concurrently occur and are mixed at the measuring point in the circuit. The R-components are mainly due to 'valve' activities and positions in the food and salivary stylet canals and resistance changes at the stylet tips inside the plant. The emf-components originate mainly from 'membrane potentials' of plant cells when punctured by the stylet tips and also, from 'streaming potentials' caused by the fluid movements in the two capillary stylet canals. Muscle and neural potentials in the insect do to contribute to the EPG signal. Both, R- and emf-components include biological information on the insect's activities and the stylet tip positions in the plant. Both, R and emf components are measured as voltage fluctuations. The insect resistance fluctuations modulate the stable DC voltage in the primary voltage as well as the amplitude of the emf component voltage, which makes the EPG signal not a simple sum of the two componenets
The first EPG system was introduced by McLean and Kinsey (1964) using an alternating current (60Hz AC) as a voltage source, the voltage amplitude of which was modulated by resistance fluctuations in the insect, similar to signal processing in AM radio circuits. However, this design appeared to be not sensitive to the emf-components in the primary circuit. Therefore, it is better to indicate this as the 'R-EPG system': it measured only resistance fluctuations. In the later 'DC-EPG system' (Tjallingii, 1978, 1988) the voltage source was replaced by a direct current (DC) source, which appeared to be sensitive to R and emf origin. Since it records both signal components, the DC-EPG system - regularly used now - can be indicated therefore as the 'full-', 'regular-', or 'normal-EPG system'. In addition to the R-EPG and normal-EPG systems, a third EPG system has been constructed: the emf-EPG system, based on the DC design too. This system is using a much higher input resistor value (with Ri ≥ 1013 Ohm [≥10 TΩ]*, Tjallingii, 1988) making resistance fluctuations of the insect or plant negligible, thus only recording the emf-components.
Originally, aphids were mainly used in both systems.The input resistor (Fig. 1, Ri) of the DC system has a value of 109 Ohm (1 GΩ), which was selected because the aphid average resistance fluctuates around this resistance value. There is a discussion going It has been discussed that This results in a system sensitivity to changes in resistance as well as to emf changes of the insect An input resistance of the approximate the same value as the resistance of the used insect is indicated as matching (1:1 ratio).
The newest Giga-8dd (2018) has a digitally operated switch function between normal EPG (R+emf) and emf-EPG recording mode. In emf-mode the system is especially suitable in plant physiology interaction related experiments to accurately measure plant cell membrane potentials and depolarisations in stylet punctured plant cells (Salvador-Recatalá et al, 2014).
In summary, there are in fact 3 EPG systems: 1) the full or normal EPG system (DC based) that records both signal components, 2) the R-EPG system (AC based) that only records the R-components, and 3) the emf-EPG system (DC based) that only records the emf-components. The EPG from the normal EPG system is more ´complicated´ but contains the widest range of biological information.
The complete experimental setup (essentially the same in the new Giga-8dd)
Fig. 2. Experimental EPG configuration. Only one of the 8 possible probes is shown here.