Investigation of an expanded, lumped-element model of electropenetrography (EPG) and the accuracy of the traditional R and emf model

Electropenetrography (EPG) allows researchers to measure feeding behaviors of piercing-sucking pests such as aphids and mosquitoes. The presently accepted electrical circuit model of an EPG monitor shows excitation voltage (Vin) applied to a feeding substrate (such as a plant), then interacting with a variable resistor (Ra) representing the feeding insect, plus biopotentials generated in the plant-insect interface (Vemf). Together, Ra and Vemf then interact with the input resistor (Ri) of the first amplifier in the monitor. After signal processing, variable voltage output (Vout) results in waveforms that represent insect feeding. The present work performed a lumped-element circuit analysis to determine whether the above simple model was correct by today's electrical engineering standards. This work showed that the above model was too simple to explain EPG signals. The newly developed model adds valuable information on parasitic effects such as capacitance in the amplifier (Ci) and from poor insect tethering (Cai), as well as effects of filtering due to induction (Lp) and capacitance (Cp) in the plant. This work recommends specific steps to avoid these problems. In addition, it was found that the insect is a complex electrical system, now symbolized by Za for impedance. Yet, despite the findings of both parasitic and complex, new elements in the EPG model, this work fundamentally supports the foundation of EPG science. That is, it confirms that the emf and still-named R components exist and can provide the basis for interpretation of EPG waveforms. Nonetheless, designers and researchers must be alert to potential problems with parasitic elements in the EPG circuit and take active steps to reduce them. These findings provide important benchmark data for future studies of the nature of Za, especially its importance in interpreting waveforms in the context of the crucial emf and R responsiveness curves. Together with the present study, such analysis will enable improvements in future EPG monitor designs.