Transmission of potato virus diseases. VI. The distribution of the aphid vectors on sampled leaves and shoots

Abstract

Data on the distribution on potato plants of the two aphid vectors Macrosiphum gei Koch and Myzus persicae Sulz., were collected over the six seasons 1941-42 to 1946-47. In the early stages of infestation the counts of alate and adult apterous aphids approximated to a Poisson distribution, indicating random distribution in the crop. Moderate to high counts of nymphal aphids diverged widely from a Poisson distribution. In the later stages of infestation the total population showed an extremely non-random distribution, but examination revealed that this was due largely to the nymphal aphids, the adults both alate and apterous being distributed in a more nearly random way except when influenced by such events as heavy rainfall. A hypothetical example is given to show how part of the heterogeneous distribution of nymphs may be accounted for by multiple deposition, even if this should occur on randomly chosen leaves. Sampled leaves were assigned to ground cover, median or tip zones of the plant according to the known phyllotaxy of the potato plant. The ground cover was divided into basal leaves, close to or touching the ground, and ground canopy, shading the ground. It was shown that Myxus persicae deposits nymphs mainly on the ground canopy, Macrosiphum gei on the basal two or three leaves and the tip. Adults of both species wander over all portions of the foliage. The influences producing non-random distribution of aphid vectors are discussed. It is suggested that the procedure for sampling aphid populations on potato crops be modified to take account of the difference between basal leaves and ground canopy leaves.