New structure in cell puncture activities by aphid stylets: a dual-mode EPG study

Intracellular punctures by aphid stylets appear as potential drop (pd) waveforms in DC electrical penetration graph (EPG) recordings. We used a dual-EPG device that recorded in one channel the ‘full EPG’ with R-plus emf-components (i.e., the usual DC EPG) and concurrently in a second channel the ‘R-EPG’ with R-components only. The circuit of the latter channel was an optimised amplitude modulation (AM) version derived from early (before 1990) AC systems. We also made some ‘emfEPG’ recordings using a separate high input resistance ‘emf-amplifier’ sensitive to emf-components only. The intracellular pd waveforms have previously been divided into three subphases, and we aimed to distinguish and separate these subphases more accurately by the dual-EPG recordings than with the normal full EPG only. In this study, we temporarily distinguished five subphases (a–e), but unequivocal distinction of only a few of these appeared possible, in spite of the information coming from the two signals. The lack of clearly separable features in R-EPG signals often provided serious difficulties in pd recognition without the concurrent full EPG, but once located, only subphase II-2 features were clear and supported the II-2 data from the full EPG. Consequently, we could not distinguish subphases of complete pd waveforms better with additional R-EPG information during cell punctures by Aphis gossypii Glover (Hemiptera: Aphididae). In Brevicoryne brassicae (L.) (Hemiptera: Aphididae), however, distinguishing II-2 subphases in the full EPG was sometimes a problem. Our detailed dual-EPG observations showed some waveform continuity from halfway into the II-1 subphase (start of the newly recognised subphase b) until the end of the pd, with a strong but variable emf origin. This waveform tended to overrule other subphase waveforms in B. brassicae more than in A. gossypii and Myzus persicae (Sulzer) (Hemiptera: Aphididae). Subphase waveforms in full EPGs were especially difficult to recognise when pd periods had been interrupted in a virus inoculation experiment and additional R-EPG information could then be useful. This inoculation experiment showed again that only the first subphase (II-1) contributes to virus (Cucumber mosaic virus) inoculation by A. gossypii. In B. brassicae, the benefit of concurrent R-EPG information in such virus experiments is presently under further investigation. Apart from this special application to virus experiments, we do not recommend the routine use of the dual-EPG device. Furthermore, we do not advocate the distinction of more than the previously recognised three intracellular pd subphases as a feasible option in future studies. Analysis of EPGs with concurrent R-EPGs requires substantially more analysis work without yielding consistently useful additional insights. This confirms earlier dual-EPG results from thrips.