Possible multiplication in the aphid vector of sowthistle yellow vein virus, a virus with an extremely long insect latent period

Abstract

Abstract Sowthistle yellow vein virus (SYVV) causes striking veinclearing and veinbanding in sowthistle (Sonchus oleraceus L.) and has transmission characteristics many of which are very similar or identical to those first described for plant-insect viruses transmitted by leafhoppers. An increase in aphid numbers increased transmission percentages and decreased the apparent latent period range, the average minimum latent period within the insects, and the average incubation period in all plants infected in series. The sowthistle aphid, Amphorophora lactucae (L.), may become viruliferous in a 2-hour acquisition feeding. Transmission, except for the percentage of insects that become viruliferous, was independent of the length of the acquisition feeding period. Efficiency of transmission following the latent period in the vector is higher than with any other vector known to the author. If one considers the latent period as starting with the beginning of the acquisition period then the minimum latent period in the aphid vector, as well as the time of retention of the virus by the aphid, were independent of the length of the acquisition feeding. Insects reared on diseased plants indicated that the entire latent period could be spent on diseased plants; such insects transmitted immediately after removal. The latent period of sowthistle yellow vein virus in its vector at various temperatures seemed to be negatively correlated with the activity of the insects as measured by longevity and nymph production. The shortest insect latent period recorded was 8 days at 25°C. The longest latent period was 46 days at 5°C. The first two or three infections induced by an aphid resulted in long incubation periods in infected plants. Later infections induced by the same aphid resulted in shorter incubation perods, but as an insect continued to transmit, the incubation periods again increased. The shortest plant incubation period observed was 7 days and the longest 55. The influence of various temperatures on incubation period in the plant was similar to the effects of temperature on the latent period of the virus in aphids. Nonviruliferous aphids acquired virus from inoculated plants as early as 5 days before symptom expression but never earlier than 6 days after inoculation. Individual insects transmitted SYVV 52 days after leaving diseased plants and probably retained the virus for the remainder of their life. Multiplication of SYVV in its aphid vector seems the most likely explanation for the observed relationships of the virus to the vector.