HOW APHIDES FEED

Press, Volume CV, Issue 31047, 30 April 1966, Page 10

HOW APHIDES FEED

't’ARMERS frequently find themselves puzzled by various questions arising out of the spread of viruses among farm crops. It is usually known that aphides are referred to as vectors—a word defined by the Oxford diction- » ary as “carriers of disease or infection.” The more puzzling aspect comes when we ask just how this occurs. Although scientists themselves cannot answer this question fully, some light is thrown on it by understanding how aphides feed. A common distinction made by home gardeners is that between biting and sucking insects. This is an important difference. Insects like caterpillars, grass grub and earwigs have mouthparts rather like sharpened pincers—they literally bite pieces out of the plants leaving obvious holes in foliage. Sucking insects —aphides and plant bugs of various kinds—have mouth-parts that are often likened to a hypodermic needle. This is an apt metaphor when we are considering the spread of viruses because it is quite correct that in most of these diseases the aphis gives the plant an injection of infective material. An aphis’s mouthpait is correctly referred to as the rostrum. This consists of a beak-like structure with a hardened exterior. But within the rostrum are some finer structures that actually penetrate the plant. These are the

stylets which are thread-like in appearance and when not in use for feeding, lie in a channel on the surface of the protective rostrum. When an aphis wants to feed it chooses a suitable site on the plant and thrusts its stylets into the plant. There are two pairs of these, the

The accompanying article was written by Mr A. D. LOWE, of the Entomology Division of the Department of Scientific and Industrial Research, for the aphis and virus committee.

outer pair acting as a kind of brace to assist the inner pair in their penetration of the plant surface. Stylets are about the thickness of human hair and it is the inner pair that is used to pierce the plant tissue and through which the food in the form of plant sap flows into the digestive organs. On the inner face of the penetrating stylets are two longitudinal grooves which, when the stylets enter the leaf and are pressed together, form two channels. One of these is used for the downward passage of saliva, and the other for the upward passage of sap. An aphis is thus feeding by virtually pumping saliva down one stylet to force sap from the plant cells up the other stylet and into its digestive tract.

With this explanation it is easier to see how viruses are spread from plant to plant. Virus diseases of plants are

broadly divided into two groups. One is the styletborne viruses, of limited infectiveness, carried as contamination on the outside of the stylet They may be infective from a few minutes until some hours after the aphis has fed on diseased materia). The second group—the circulative viruses contains most of the virus diseases that affect farm crops. In these the infective material is able to pass up the receiving stylet with the plant sap, whenever an aphis feeds on an infected plant. The virus then remains with the aphis for its whole life. Whenever it feeds on a fresh plant infective saliva makes a return journey down the salivary stylet into that plant It is easy to see why these viruses are sometimes called the persistent viruses. Most of our serious virus diseases in farm crops in Canterbury are of this type—pea leaf-roll, barley yellow dwarf, and potato leaf-roll—and all are fairly difficult to control. The problem is added to by the winged aphis’s habit of probing several times in different parts of the plant and on different plants when they first land before finally settling down to feed in one spot.