Transmission of Plant-Pathogenic Viruses by Aphids

Abstract

Although many new vectors of plant viruses have been discovered dur­ ing the past decade including nematodes and fungi, aphids stilI remain the taxon that transmits a greater number and variety of viruses than does any other group of organisms. This may partly be explained by their way of life, which is specialized to use food to the full when it is plentiful and suffer the least loss when it is lacking. They reproduce by parthenogenesis, viviparously, and rapidly on susceptible lush vegetation, and then have a short period of sexual activity sufficient to produce eggs that withstand long periods of adverse conditions, and for beneficial mutations to survive. Muta­ tions are many because of the very large populations aphids achieve, and these mutations help them to adapt to changing conditions of climate and host plants. Hence, they have been able to meet changes in agriculture and become ubiquitous in crops of the temperate region. Viruses, which multiply very much faster and attain larger populations than do aphids, have also become adapted efficiently to agricultural condi­ tions and have exploited different kinds of aphid behavior for their trans­ mission. These relationships seem to have evolved around three kinds of aphid behavior: 1. Host selection. Aphids make brief, shallow probes into a plant after migration or other movements between plants. 2. Colony estab­ lishment. Invading alatae of some species deposit a few larvae on each of several instead of establishing a single large colony on one plant. 3. Dispersal of population. After colonies of apterous alienicolae have be­ come dense alatae develop which move within and be. tween crops, establish­ ing new colonies. This insures against extinction of the race by overpopula­ tion and spreads viruses from the parent plants to new hosts. Watson & Roberts (178) and Sylvester (153) separated three categories of viruses which they call nonpersistent, semipersistent, and persistent, ac­ cording to the period during which feeding aphids remain infective after acquiring virus, respectively, one hour, one to three days, and an indefinite number of weeks.