Measuring Systems

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. The voltage fluctuations appear in a number of distinct patters, referred to as 'waveforms'.

Electrical origin of the signal

The fluctuating voltages are caused by two different sources concurrently: 1) fluctuating electrical resistance of the aphid; and 2) voltages 'generated' in the insect-plant combination. These two sources cause signal components that are referred to as the resistance (R) components and the electromotive force (emf) components of the EPG, respectively, and they are superimposed at the measuring point. The R-components originate mainly from the activity of the valves in the stylet canals, the food and the salivary canal. The emf-components originate mainly from 'membrane potentials' of plant cells - when they are punctured by the stylets - and from 'streaming potentials' caused by the fluid movements in the two capillary stylet canals. Muscle and neural potentials in the insect are outside of the circuit, and appear not to contribute to the EPG signal, therefore. Both, R- and emf-components include important biological information on the insect's activities and the stylet tip position in the plant tissue.

EPG systems

The measuring system that was introduced by McLean and Kinsey (1964) used alternating current (AC) as a voltage source, and the signal was composed of modulations in voltage amplitude caused by resistance fluctuations in the insect, similar to signal processing in AM radio. This AC-system, however, appears to lose the emf-components during signal processing. Subsequently, the voltage source was replaced by a direct current (DC) source, and two DC-system variants were developed (Tjallingii, 1988). In one system, the input resistor (Ri) is very high (>10¹² Ω) so that resistance fluctuations of the insect become negligible and only the emf-components are recorded. The other DC-system has an input resistance of about the same value as the average electrical resistance of the aphid-plant combination, thus providing an optimal 1:1 ratio of resistance, which allows recording of both, the R- and emf-components. In summary, there are in fact 3 EPG systems: 1) the (regular) DC-system that records both signal components, 2) the 'emf-amplifier', the DC-system that only records the emf-components, and 3) the 'R-amplifier', the AC-system that only records the R-components. The EPG from the regular DC-system contains the widest range of biological information in the signal, and therefore is more complicated than the signals from the emf- and R-amplifiers. Some signal details from the regular DC-system are hidden (masked) in the EPG, and they become visible in the EPG from the R-amplifier (Jiang and Walker, 2001) or the emf-amplifier (Tjallingii, 1985. 1988, 2000) but in general, the regular DC-EPG contains the most complete and relevant biological information.

DC EPG system and its parts

We presently distribute 2 main DC-EPG system configurations: The Giga-4 and Giga-8, have 4 and 8 channels, suitable for simultaneous recording of up to 4 and 8 insects, respectively. Giga refers to the 1 GΩ (10⁹ Ω) input resistance in the primary amplifier. All EPG recording should be done in the laboratory using potted plants or detached plant parts. There is no field recording possible with this system but we are developing a system to record EPGs in the field in future (contact us).

System units:
Complete measuring system for 1 channel (figure 2), including:

Plant (detached plant part or artificial diet) and wired insect inside a Faraday cage*.
EPG probe (primary amplifier), separated from the system to mount inside the Faraday cage to shield the input from noise.
Mounting system for the probe with swivel screw clamp and 10mm steel rod to attach further with an ordinary lab stand and screw clamp*.
Main control box - outside the Faraday cage - that provides the further signal processing and a plant electrode connector to supply the adjustable plant voltage. The GND plug needs to be connected to the Faraday cage (alligator clip).
Power adapter, AC(220V)/DC(+/8V). Connected to the control box, it powers the system.
A/D converter. The analog output EPG signal, needs to be converted into a digital signal to feed into the computer. This is a USB device using shielded cable to avoid cross talk between channels.
Computer with Windows platform: 2000 - XP; Vista has shown problems!