Investigations of the mechanism of the transmission of plant viruses by insect vectors.— I

Abstract

This paper deals with expts. in the mechanical inoculation of insects with plant viruses. The method employed was the introduction of the virus into a puncture of the abdomen or leg made with a finely pointed needle or glass micro-pipette. A large proportion of the insects survived this treatment. The inoculation of active races of Cicadulina mbila, the vector of maize streak, was successful, when the inoculum introduced was the juice of diseased maize seedlings, fresh or kept for 4 but less than 8 days, undiluted or diluted with distilled water by 10-2 (rarely, by 10-3). Inoculation by causing the virus fluid to be drawn up into the rectum per anum failed. By the inoculation of the appropriate fluids, it was shown that the virus was present in infective, active C. mbila, (a) in the contents of the rectum, if the insect had recently fed on a diseased plant, but not otherwise, (b) in the general contents of thorax or abdomen, and (c) in the blood, whether the insect had fed recently on diseased or healthy plants. The virus was not found in the naturally voided feces. The appearance of the virus in the blood preceded in time the development of the power to cause infections. Inactive races of C. mbila[long dash]normally unable to transmit the virus[long dash]were made infective by needle inoculation with the streak virus. The proportion of successes was, however, significantly less than with active races. After feeding on a diseased plant, the inactive insect was found to have the virus in its rectum, but never in its blood. A simple puncture of the abdomen with a sterile needle, either following or followed by feeding on a diseased plant, sometimes caused inactive C. mbila to become infective. By comparison of the efficacy of different positions for the puncture, it was concluded that the treatment was successful only if the needle had penetrated some part of the intestine. Inactive races of Cicadulina zeae proved susceptible to inoculation with the streak virus by the methods successful with C. mbila C. mbila was not successfully inoculated with the viruses of maize stripe and mosaic diseases; nor Peregrinus maidis and Aphis maidis with the virus of streak. In active C. mbila the streak virus, entering the intestine by mouth, passes through the intestinal wall into the blood, and, in the inactive insect, the cells of the intestinal wall resist the passage of the virus. It is recognized that there may be some secondary mechanism of resistance; nevertheless in many inactive individuals, once the barrier of the intestinal wall has been passed, the virus behaves as in an active insect.