Multiplication of the Rice Stripe Virus in its Insect Vector, Laodelphax striatellus Fallen

Abstract

The present paper describes the results of experiments carried out with the aim of obtaining evidences other than the transovarial virus transmission, of multiplication of the rice stripe virus in its insect vector, the smaller brown planthopper Laodelphax striatellus Fallén, and information about the possible site of virus multiplication and pathological effects of the virus on the insect vector.
Serial transfers of the rice stripe virus from insect to insect were made using micro-injection method described by Maramorosch (1952). The virus was successfully carried in 4 serial passages in the smaller brown planthopper. The final dilution of the inoclum used to inject the last group of the series is calculated to be at least 1.25×10^-6 of the original inoculum, while the virus derived from viruliferous insects was shown to have a dilution end-point between 10^-2 and 10^-3. Groups of planthoppers which had been injected with the juice from viruliferous insects, were macerated 2, 5, 10 and 15 days after the injection, and the preparations were tested for infectivity by inoculating to virus free insects. The inocula prepared from the planthoppers 2 and 5 days after injection were found noninfective, in coincidence with the incubation period in the vector. A 1:10 dilution of the macerate from planthoppers 10 days after injection and a 1:50 dilution of those from planthoppers 15 days after injection proved infective. This suggests that the virus may have multiplied in its vector at least fivehold during the period from 5th to 15th day after injection.
Blood and various organs from viruliferous planthoppers were tested for the presence of virus by the injection method. Virus was recovered from eggs, male reproductive organs, and blood and fat body, while no virus was recovered from alimentary canal.
Male planthoppers following 2 days of acquisition feeding on diseased rice plants were dissected to remove legs, wings, alimentary canal, Malpighian tubules and reproductive organ, and the remaining part was separated into head, thorax, and abdomen. These body parts were incubated for 13 days at 25-27°C in hanging drops of a medium consisting of Takami's salt solution, D-glucose and extract from their respective part. The virus was not recovered from fresh material of these parts, but was recovered from the abdominal part incubated in vitro for 13 days. Female and male reproductive organs excised from planthoppers which had fed for 3 days on diseased plants, were incubated for 12 days in vitro. From both organs virus was recovered after having been incubated for 12 days, but not before incubation.
Cytological examinations of blood corpsucles, reproductive cells, salivary glands, alimentary canal, dermis, and fat body of viruliferous planthoppers showed no consistent, conspicuous changes in these tissues. Histochemical tests for hyaluronic acid, RNA, DNA, lipid, and phospholipid, and phosphatase activity showed no distinct difference between viruliferous and virus-free planthoppers. Relative amount of glycogen and of polysaccharides in the fat body and of mycetome, as determined by Bauer-Feulgen stain and by Lillie method, appeared to be somewhat less in viruliferous planthoppers than in virusfree ones.