Investigations of the mechanism of the transmission of plant viruses by insect vectors III. The insect’s saliva

Abstract

When puncturing through a wax membrane into a fluid (10% sucrose soln.) Cicadulina mbila ejects saliva only when its stylets are in motion, and not when they are at rest and fluid is being drawn into them. This saliva sets to a gel and is moulded internally by the stylets to form a sheath. No other material of insect origin has been observed to Row from the stylets. The salivary glands of infective insects, when inoculated into the abdomens of non-infective ones, caused a few of these to become infective. Comparative expts. with other organs from the insect support the interpretation that the salivary glands may contain virus, either in small quantities or occasionally. Attempts to demonstrate virus in fluid upon which many infective insects had fed were almost always negative. Only when infective and non-infective insects fed simultaneously on a film of fluid held between 2 membranes did a few of the non-infective insects become infective, and these never caused infection more than once in a series of tests. Results were similar with a maize leaf on which infective insects had fed. Simultaneous feeding on a small area of leaf alone caused a few non-infective insects to become infective and these again were of low infective ability. It is suggested that an infective C. mbila ejects virus in very small quantities, so that only rarely can another individual take up enough ejected virus to make it infective and then only weakly so. The manner in which the virus is carried into the plant is still obscure. It is difficult to reconcile the view that the gelling saliva is the vehicle with the evidence of an earlier paper (Proc. Roy. Soc. B 125:455. 1938) in which it was shown that punctures maintained for less than 5 min. cannot cause infection; during the period when the plant is receiving the greatest quantity of gelling saliva it is never infected. Much of this saliva is deposited in the mesophyll, whereas for infection the stylets must penetrate the phloem.