TEXT HERE IS UNDER REVISION
In the EPG technique, an insect with piercing mouthparts and a plant are made part of an electrical circuit by inserting an electrode into the soil of a potted plant and attaching a very thin wire to the insect. In general, it can be stated that the insect may be any 'organism' with piercing mouthparts and the plant can be any substrate. This electrical circuit is called the primary circuit (chapter 1). Essential is that legs or any other body parts are not electrically short circuiting the electrical potentials (voltages) generated by a the organism or the substrate or the electrical resistance in the circuit. These two properties, the generated voltages and the fluctuation resistance of the organism and the substrate are the signal sources of the EPG (chapter 2). In addition to the organism and the substrate the primary circuit also includes a voltage source: either an AC or a DC voltage source, used in different EPG systems (chapter 3) to measure and record EPG signals. These signals are electronically processed by the EPG systems in a circuit that does not affect the primary circuit properties. Signal processing differs between EPG systems. Also, the primary circuit should be protected against external signal sources such as noise from the environmental or the the electricity grid. In lab conditions this requires a Faraday cage and careful grid connections.
The Basic EPG system as provided by EPG Systems is under increasing criticism by Backus and followers. The main claims are:
1) For larger insects, Heteroptera and Homptera larger than aphids and whiteflies would be the DC Giga systems would be unsuitable since an input resistor value should be of the same value as the 'intrinsic lower resistance' of these insect in order to get a proper recording of R-components in their waveforms.
2) The larger insects would need an AC system since they are sensitive to DC voltages, in contrast to smaller insects being more sensitive to AC voltages supplied in the primary circuit.
3) The electrinic components used in the Basic EPG system provided on this website, would be inferrior and cause disturbing interferences.
4) The Basic EPG system would supply inaccurate DC voltages
- - all these allegations will be refuted here - -
the alternative device supported Backus et al. will be evaluated here
1. Primary circuit
The primary circuit properies determine what is recorded and what is not. It incorporates a voltage supply Vs, the plant electrode in the plant itself, in potting or outside soil, or as decised part in water, the insect its electrode, and the input resistor Ri shown in the figure below. The input resistor is positioned between the pre-amplifier input and ground. The pre-amplifier is connected to the circuit at measuring point (M) but is not a part of the primary circuit as such, and does not affect the voltages in the primary circuit. As soon as the insect stylets penetrate the plant, the circuit is completed and a fluctuating voltage at the measuring point - the EPG signal - is 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 or EPG signal sources
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
Different EPG measuring systems
The first EPG system - called feeding monitor - was introduced by McLean and Kinsey (1964) supplying an alternating current (AC) voltage source (Vs). This source was in fact the carrier wave of which the amplitude was modulated by resistance fluctuations of the insect. This fluctuating amplitude was processed in the same way as AM radio signals. The demodulated signal reflected exactly the resistance fluctuations of the insect. This first monitor has been called an AC system but as this AM signal was only sensitive to resistance fluctuations it can also be frerred to as an 'R-EPG system'. Later, using a different system design with a DC voltage supply it appeared that the EPG signal also contains electromotive force (emf) originated components, i.e. fluctuating voltages at the mesuring point that are generated by the insect activities and active electrical components in the plant, mainly cell membrane potentials (Tjallingii, 1978, 1988). Similar to the carrier wave amplitude modulation in the R-EPG system here the DC voltage was amplitude modulated. In this EPG system desing thus there is a DC AM signal and an emf originated signal. This system was called a DC EPG system, but as its signal is sensitive to both R and emf components it can also be referred to as an ´R+emf EPG system. This has become more or less the standard EPG system as shown in this website. In addition to the R-EPG and the standard R+emf EPG systems, a third EPG system has been constructed: which can be referred to as and emf-EPG system; except for the much higher Ri value (with Ri ≥ 1013 Ohm, = 10 TΩ) the design is identical to the standard DC system (Tjallingii, 1988. In the present Giga-8dd version a remote controlled switch the device can switched between normal mode (standard R+emf system) and emf mode (emf EPG sysyem). The much higher input resistor value is making the device insensitive to resistance fluctuations of the insect or plant.
Originally, aphids were mainly used in both systems.The input resistor (Fig. 1, Ri) of the standard DC system has a value of 109 Ohm (1 GΩ) since 1988, This was selected because the aphid average resistance seemed to fluctuate around this 1 GΩ resistance value.
The newest Giga-8dd (2020) has a digitally operated (remote) switch between the standard Ri of 1 GΩ normal mode and the 10 TΩ for emf mode. In emf-mode the system is especially suitable in plant physiology to measure plant cell membrane potentials and their de- and re-polarisations of stylet punctured plant cells, phloem cells in prticular (Salvador-Recatalá et al, 2014).
Summary There are in fact 3 basic EPG systems: 1) the AC R+EPG system recording R-components only, 2) the DC R+emf EPG system recording both R- and emf-components, and 3) the DC emf EPG system recording emf-components only. The standard DC EPG system contains the widest range of biological information.
The complete experimental setup
In the complete experimental setup the pre-amplifier is incorporated in a separate unit, the EPG probe. The probe is the most sensitive part of the setup and located close to the insect in a Faraday cage to shielding it from external electro-magnetic noise, mainly coming from 110 or 220VAC power cables at 60 or 50Hz. It comprises the input (measuring point) and the first stage 50x amplifier. The plant and insect should be located in the Faraday cage too. The probe is enveloped by conductive material that is grounded as the Fararday cage is as well. The control box can stay outside the cage.
Fig. 2. Experimental EPG configuration. Only one of the 8 probes shown
in free the manuals for hard and software on the Downloads page.