The electrical penetration graph (EPG) is used to study the interactions between parasitic insects with piercing mouthparts and their host organism. So far, plant feeding aphids (Homoptera) have been used mainly. Insects are attached to a thin (20 mm gold wire) electrode and a second (copper) electrode is inserted into a potted plant or into the soil [1]. The insect should have a low electrical conductivity with its feet to the plant. As soon as the mouthparts (stylets) penetrate, the primary circuit is completed and voltage changes can be recorded. About 10 different EPG waveforms have been distinguished on the basis of amplitude, frequency, voltage level (intra/extracellular), and electrical origin (resistance/electromotive force). Waveforms have been correlated experimentally with the insect’s activities (saliva excretion, ingestion, mechanical stylet propagation) and stylet tip positions in the plant tissues (mesophyll, xylem, phloem). Also, intra- and extracellular positions can be distinguished as the stylets acts as self-penetrating microelectrodes which do not destroy the living plant cells they puncture. Host plant selection, phloem finding and phloem feeding are studied with this method. EPGs show many details that are not detected by any other method. Recently EPGs were able to elucidate the crucial events causing plant virus transmission (acquisition and inoculation) by specific aphid vectors [2]. Insect as well as plant physiological topics are studied.

The EPG uses a DC amplifier, 0-65 Hz, and an input resistor of 1 Gohm (109 ohm), equal to the average resistance of the insect/plant combination. This allows recording of most electrical resistance fluctuations caused by the insect’s penetration activities, the resistance (R) component, and also, it is high enough to record biopotentials generated in the system, the electromotive force - EMF) component. An adjustable voltage source enables to compensate for any offset potentials in the primary circuit. Signals are digitized and recorded on a computer hard disk (and CD-ROM finally). For analysis they are displayed on screen where relevant parameters are retrieved for further data processing. The system is presently used for fundamental and applied studies in about 40 universities, research institutes, and industrial labs world wide. The number of insects and arachnids (mites and ticks) is increasing. EPG recording will be demonstrated (of 4 insects simultaneously) with a poster representing the main measuring principles and some (TEM) micrographs of stylet positions in plant tissues.

References

1. Tjallingii, W.F. (1988). Electrical recording of stylet penetration activities. In: A.K. Minks & P. Harrewijn (eds.). Aphids, their biology, natural enemies and control, 95-108. Amsterdam: Elsevier.
2. Martin, B.; Collar, J.L.; Tjallingii, W.F.; Fereres, A. (1997). Intracellular ingestion and salivation may cause the acquisition and inoculation of non-persistently transmitted plant viruses. Journal of General Virology, 78, 2701-2705.