Aphids as a taxonomic group, have been recognised since at least 1758 when Linnaeus coined the genus Aphis and have been cited as important pests for more than 200 years “The Aphis or Blighter, as we now for the first time venture to call it, from its being the most general cause of what are termed blights in plants..” (Curtis, 1802). A detailed understanding of how they fed, was however, longer in being reached, but by 1914 the anatomy of the aphid mouthparts and the process of stylet insertion was fully described (Davidson, 1914). Davidson (1923) also described the role that aphid saliva plays in helping the aphid feed by making it easier for the stylet to move between cells on its convoluted journey to the phloem, made visible as the so-called stylet tracks.
Drawings showing the effects produced by the passage of aphid stylets of three different aphid species through leaf tissue (Davidson, 1923).
Fast forward a couple of years and we have intrepid entomologists producing photographic evidence of aphid stylets in action (Smith, 1926).
Photomicrographs of the stylet of Myzus persicae in situ and the resultant stylet track (Smith, 1926).
One of the reasons that applied entomologists were so interested in aphid feeding was the role that aphids, and other insects, played as vectors of plant viruses, which until the 1920s, was not formally proven (e.g. Kunkel, 1926, Smith, 1926, 1929). You would be forgiven for thinking that once the connection between aphid feeding and plant virus transmission had been demonstrated then that would be it. But no, much wants more, and aphidologists became intrigued about the link between aphid feeding and salivation, in particular when and exactly where these activities occurred in the plant. Those entomologists working on plant viruses wanted to know which part of the feeding process was linked to the acquisition and inoculation of the viruses from and to the aphid host plant. A possible solution to these conundrums, was, however, on the horizon.
In the early 1960s, two entomologists from the Department of Entomology, at the University of California, Davis, Donald McLean and Marvin Kinsey, came up with a system that was to revolutionise the study of the feeding behaviour of aphids and other insects that feed internally on plant using piercing mouthparts (McLean & Kinsey, 1964). In essence, what they did was to make an aphid part of an electrical circuit by attaching a thin copper wire to its back using a quick-drying silver paint. The feeding substrate, a leaf, had a 2.0 Volt, 60-cycle alternating current introduced to it and this was placed on an insulated grid connected to an amplifier connected in parallel with an oscilloscope, a chart-recorder and a speaker. The wire attached to the aphid, was joined to the grid and when the aphid began to feed this completed the circuit, and changes in voltage were able to be observed and recorded. The next step was to identify which chart recordings were associated with sap ingestion and salivation by the aphid. Using an artificial leaf, Parafilm stretched over a well containing a sucrose solution, and watching the aphids under a high power microscope, these innovative entomologists were able to identify four different stages involved in aphid feeding (Mclean & Kinsey, 1965).
The ground-breaking chart recording (Mclean & Kinsey, 1965) and as you might expect it was a pea aphid 🙂
A visual summary of what McLean and Kinsey were watching and recording (from Dixon (1973).
Not satisfied with these findings McLean and Kinsey modified their equipment and intensified their observations, sacrificing a number of aphids in the process. When different waveforms were seen the poor aphids had their stylets amputated and the plant material with the stylet still in place was then examined under a high power microscope. This meant that they were able to definitively correlate their recordings with the position of the stylet in different leaf tissues and during different behaviours (McLean & Kinsey, 1967). As well as trying to understand how, when and where plant viruses were acquired or transmitted, it turns out that using the waveforms generated by the aphid mouthparts as they weave their way through the leaf tissues, is not only a useful way of assessing the resistance mechanism of a plant (e.g. Nielson & Don, 1974; Paul et al., 1996; ten Broeke et al., 2016) but also for detecting resistance to insecticides (e.g. Garzo et al., 2016).
Modifications to the original equipment happened very quickly; by 1966, a more compact and easier to use version using Direct Current had been developed (Schaefers, 1966). That said, the first correlation of a specific waveform and virus acquisition by the pea aphid, was shown using the original AC equipment (Hodges & Mclean, 1969). A further modification of the Schaefers DC equipment was developed during the 1970s, such that test aphids were able to live and reproduce for up to ten days whilst attached to the set-up, thus allowing very detailed investigation of the correlations between the electrical signal patterns produced and the feeding behaviours of the aphids (Tjallingii, 1978).
Those of you who take note of such things, will have noticed, that so far, some 14-years after its invention, the term electrical penetration graph has not yet appeared, either here or in the scientific literature. Earlier references to recordings using the technique use the term actograph which was somewhat non-specific, as it refers to any graphical representation of behavioural activity. So when did the term Electrical Penetration Graph (EPG) first appear in the literature. Google Scholar gave me a date of 1984 from a paper looking at the resistance of lettuce to the cabbage aphid Brevicoryne brassicae, a paper that includes Freddy Tjallingii in the authorship list (Mentink et al., 1984). In this paper the authors refer to a conference proceedings paper (Tjallingii, 1982) as being the source of the name. On tracking down that paper I found that it doesn’t actually mention the term EPG. The first paper that specifically mentions and defines the term as “the recorded graph as a result of an overall electrical signal caused by stylet penetration activities” is Tjallingii (1985). Strangely the author introduces the term thus “Here we introduce the term ‘electrical penetration graph (EPG)”, which I found slightly odd as it is a single author paper 😊 Inputting EPG or electrical penetration graph into Web of Science shows an increasing number of papers using and mentioning the technique, but surprisingly the first paper recorded is from 1999.
NGram finds the first mention slightly earlier, 1981. A puzzle waiting to be solved for anyone with the time or inlcination.
The frequency of the occurrence of the phrase “Electrical penetration graph” according to Ngram Viewer (accessed and downloaded May 1st 2018).
The technique is now very well established and used around the world. The equipment is commercially available through EPG Systems, which is where we got ours from and just in case you were wondering, this is what it looks like.
Faraday Cage (an earthed metal screen) surrounding the equipment to exclude electrostatic and electromagnetic influences
Our test plants in situ connected up to the electrical supply, recording equipment and amplifier.
Close up of the plants and EPG electrodes
Aphids connected up to the EPG. Photo courtesy of https://sites.google.com/site/ezwear1/epgIMG_0903.jpg
A simple guide to interpreting the waveforms
For Open Days and public displays it is not unknown for mischievous entomologists to link particular waveforms to recordings of sucking and spitting sounds and to play these back when the equipment is being demonstrated 🙂
Curtis, W.L. (1802) IV. Observations on aphides, chiefly intended to show that they are the principal cause of blights in plants, and the sole cause of the honeydew. Transactions of the Linnaean Society of London, 6, 75-94.
Davidson, J. (1914) On the mouth-parts and mechanism of suction in Schizoneura lanigera, Hausmann. Zoological Journal of the Linnaean Society, 32, 307-330.
Davidson, J. (1923) Biological studies of Aphis rumicis Linn. The penetration of plant tissues and the source of the food supply of aphids. Annals of Applied Biology, 15, 35-54.
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Garzo, E., Moreno, A., Hernando, S., Marino, V., Torne, M., Santamaria, E., Diaz, I. & Fereres, A. (2016) Electrical penetration graph technique as a tool to monitor the early stages of aphid resistance to insecticides. Pest Management Science, 72, 707-718.
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Mentink, P.J.M., Kimmins, F.M., Harrewijn , P., Dieleman, F.L., Tjallingii, W.F., van Rheenen, B. & Eenink, A.H. (1984) Electrical penetration graphs combined with stylet cutting in the study of host plant resistance to aphids. Entomologia experimentalis et applicata, 35, 210-213.
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Smith, K.M. (1929) Studies on potato virus diseases, V. Insect transmission of potato leaf roll. Annals of Applied Biology, 16, 209-229