Aphids are sap feeding, most of the time they feed from the phloem, or sieve elements, that part of the plant responsible for transporting the food made in the leaves by photosynthesis, around the plant. Aphids face three problems arising from their phloem feeding habit. First, the phloem sap is largely composed of sugars, with a few trace elements and nitrogen in the form of soluble amino acids. The aphids are mainly interested in the nitrogen and that poses the second problem, the amino acids are mainly non-essential ones. Thirdly, the phloem is under pressure, figures range from 2 to 40 Bars* (about twice to forty times atmospheric pressure) (e.g. Mittler, 1957; Rogers & Peel, 1975; Barlow & Randall, 1978; Wright & Fisher, 1980). Imagine that you are trapped in an air-tight room and your only source of air is an inflated tractor tyre. You have a sharp metal straw which you can stick into the tyre to release the air into your mouth. If you put one end of the straw in your mouth and then pierced the tyre wall, your head would explode.
Sadly I couldn’t find a picture of an exploding aphid and my cartoon version was a failure, so this is it 🙂
Aphids face the same sort of pressure. Fortunately evolution has provided them with a very strong pharyngeal pump and incorporated a series of valves in their mouth-parts (stylets = straw) with which they are able to control the flow of the phloem into their bodies. The last thing they want to do when plugged into the phloem is suck, it would be the last thing they did 🙂 and that’s why aphidologists get upset when people describe aphids as sap-suckers!
Aphid feeding apparatus – adapted from McLean & Kinsey (1984)
To be fair, we are being somewhat pedantic, the fluid transported in the xylem tubes, largely water, is also colloquially known as plant sap. The xylem, unlike the phloem is not under pressure (Sperry et al., 1996), so on those rare occasions when the aphid does need to drink water, they do have to suck sap (Spiller et al., 1990). The other occasion on which aphids need to suck rather than regulate the flow of sap is when they are feeding in very artificial laboratory situations, on leaf discs or on artificial diets where the nutrient solution is between two pieces of Parafilm™. In both these cases there is negative pressure and the cibarial pump does then come into operation. Interestingly, it is sometimes quite difficult to get aphids to feed on artificial diets unless a phagostimulant is included to overcome their reluctance to feed on sap that is not under pressure (Mittler & Dadd, 1963), but that’s a story for a future post.
Aphids feeding on leaf discs, in this case for insecticide assays at Rothamsted Research
Aphids feeding on artificial diet through Parafilm™. Photo Meena Haribal https://www.sciencedaily.com/releases/2015/12/151216151742.htm
Barlow, C.A. & Randolph, P. A. (1978) Quality and quantity of plant sap available to the pea aphid. Annals of the Entomological Society of America, 71, 46-48.
McLean, D.L. & Kinsey, M.G. (1984) The precibarial valve and its role in the feeding behavior of the pea aphid, Acyrthosiphon pisum. Bulletin of the Entomological Society of America, 30, 26-31.
Mittler, T.E. (1957) Studies on the feeding and nutrition of Tuberolachnus salignus (Gmelin) (Homoptera, Aphididae) I. The uptake of phloem sap. Journal of Experimental Biology, 34, 334-341.
Mittler, T.E. & Dadd, R.H. (1963) Studies on the artificial feeding of the aphid Myzus perslcae (Sulzer) – I. Relative uptake of water and sucrose solutions. Journal of Insect Physiology, 9, 623-645.
Sperry, J.S., Saliendra, N.Z., Pockman, W.T., Cochard, H., Cruiziat, P., Davis, S.D., Ewers, F.W. & Tyree, M.T. (1996) New evidence for large negative xylem pressures and their measurement by the pressure chamber method. Plant, Cell & Environment, 19, 427-436.
Rogers, S. & Peel, A.J. (1975) Some evidence for the existence of turgor pressure gradients in the sieve tubes of willow Planta (Berl.) 126, 259-267.
Spiller, N.J., Koenders, L. & Tjallingii, W.F. (1990) Xylem ingestion by aphid – a strategy for maintaining water balance. Entomologia experimentalis et applicata, 55, 101-104.
Wright, J.P. & Fisher, D.P. (1980) Direct measurement of sieve tube turgor pressure using severed aphid stylets. Plant Physiology, 65, 1133-1135.
*Phloem pressure is most often calculated by letting large aphids insert their stylets into their host plant and then cutting off the aphid’s head leaving the stylets inserted in the plant and allows the flow rate to be measured directly L